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CT simulation for radiotherapy treatment planning

¹ Medical Physics Department, Mount Vernon Hospital, Rickmansworth Road, Northwood, Middlesex HA6 2RN and ² Department of Radiotherapy Physics, Weston Park Hospital, Whitham Road, Sheffield S10 2SJ, UK

	Abstract

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

 
The present status of CT simulation (CT sim) hardware, software and practice is reviewed, particularly with regard to the changes that have taken place over the last 5 years. The latest technology is discussed together with some recently developed techniques. The article concludes with a discussion of virtual simulation vs physical (conventional) simulation; in particular there is a review of the changes that have been made to the "Disadvantages table" presented by Conway and Robinson [1], which now make CT sim an attractive system for any radiotherapy department.

	Introduction

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

 
The demands of modern radiotherapy planning are quite different from those 20 years ago. Clinicians now require to define the target volume more precisely, not just in two dimensions, but also in three dimensions. It has therefore become necessary to visualize anatomy in three dimensions to enable planning to conform the dose around the target volume in order to irradiate the tumour to as high a dose as possible, whilst saving the normal tissues. In order to achieve this the following tools are necessary:

• Identification of critical structures using advanced anatomical and functional imaging methods.
  
• Visualization of treatment targets with respect to other structures in three dimensions.
  
• Efficient and accurate outlining of tumour using contouring tools.
  
• Addition of symmetrical or asymmetrical volumetric margins.
  
• Beam's eye view (BEV) of targets and structures.
  
• Shaping fields around the target.
  
• Adding beams together.
  
• Dose volume histogram (DVH) generation.
  
• Tools for optimizing plans using forward or inverse interactive techniques.
  
• Export of plan to linear accelerator.
  
• Monitor unit calculations.
  
• Export of digitally reconstructed radiographs, (DDRs — see below) to an image database for on-line assessment of treatment accuracy.
  

It is the intention of this article to review the place of CT simulation (CT sim) in radiotherapy planning as it has developed since the article by Conway and Robinson in 1997 [1].

	CT planning development

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

 
When CT became available to radiotherapy patients in the 1970s, its role in treatment planning was very quickly recognized [2] since the transverse cross-sections produced are exactly the sections required for isodose charting. However, it has taken many years of development to realise the full impact that CT can have in treatment planning, since it has been necessary to wait for rapid scanning and very rapid computing power to implement the most important aspects of CT planning and develop these into CT sim.

The transfer of planning information (reference marks, field entry point etc.) from CT sim to the patient prior to treatment is the most critical step; without an accurate and reliable method of doing this, the usefulness of CT planning is greatly reduced and, indeed, may introduce error. The practice of virtual simulation (VSIM) relies on this concept being realisable. The two main elements of VSIM essential to its accuracy and verification of an individual patient's treatment are: transfer of coordinates (marks identifying beam centres, field edges, block positions etc. as necessary); and the construction of DDRs [3].

Goitein and Abrams [4] and Goitein et al [5] discussed the development of CT planning from a system performing "almost none of the functions associated with a treatment simulator" to a system where "the simulation of treatment by the computer can be much more comprehensive and valuable". Goitein et al [5] developed the concept of beam's eye view (BEV) following the idea of McShan et al [6], and recognized the importance of projecting through CT sections to produce an image for verification purposes. It was, however, Sherouse et al [7] and Sherouse and Chaney [8] who first used the terms virtual simulation and virtual simulator, and the concept of the DRR was further developed by Sherouse et al [9]. The DRR "traces rays from the X-ray source through a 3-dimensional model of the patient made up of voxels determined from CT scans" [9]. This particular DRR software also separated photoelectron and Compton components in order to compute either a DRR similar to a verification image on the simulator or a DRR that looked more like the high-energy portal radiograph taken on the linear accelerator. These different images are produced using different image processing techniques in the modern virtual simulator.

Processing the DRR, particularly the use of various types of filter to change the appearance of the image, is now considered to be a major asset of VSIM. More information can be visualized than in conventional radiography, even if some detail is lost in the digital nature of the image with its finite number of pixels (typically 512 x 512). Standard filters include low energy, to simulate a 60–80 kV radiograph (simulator film filter), high energy, to simulate 6 MV portal image (port film filter), customizable filters (window/level mapping) and special techniques, e.g. depth control/depth shading. Depth control or depth shading is the reconstruction of a DRR for a limited range of depths (a region of interest defined by the user) in which, say, the target lies. This produces an image that is very useful for checking margins. It is superior to a conventional radiograph, particularly if bone overlies the region of interest.

A key feature to the efficient use of CT sim is the speed of reconstruction of DRR. This used to take a minute or more, however, it is now possible to move a beam and have the new DRR recalculated and displayed almost in real time.

Another feature of CT planning and VSIM recognized by early workers in this field was the use of non-coplanar beams [10]. These beams were already in use to treat patients, but verification using imaging could not generally be achieved. The size of the image intensifier on the simulator often prevented positioning of the beam with the correct geometry with respect to the target and the patient. The image was also difficult to interpret. Both of these problems could be overcome in VSIM. In particular, interpretation of the image became possible since not only could it be processed to improve the quality of the image, but by looking at the set of transverse sections, it was possible to see the various organs and structures covered by the beam.

	Specification of a CT simulator (Figure 1)

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

 
The term CT sim is associated with "virtual simulation", a term coined by Sherouse et al [7] to refer to the processes on a computer, using a 3D CT patient data set, that allow full simulation and verification of radiotherapy treatment. CT Sim=physical CT scanning (patient required)+VSIM (patient not required). The main item of equipment needed is a CT scanner connected to a computer containing a suite of programmes that allow all the processes outlined above to be performed, including virtual modelling of the radiotherapy simulator process together with advanced DRR production (many of these features of VSIM are now built into the treatment planning system), with the addition of moveable lasers driven under computer control.

View larger version (18K): [in this window] [in a new window]   Figure 1. Schematic of CT simulator and associated systems. DRR, digitally reconstructed radiograph; EPI, electronic portal imaging; PET, positron emmission tomography; RTP, radiotherapy treatment planning.

View larger version (90K): [in this window] [in a new window]   Figure 2. Large bore (85 cm physical aperture) oncology CT scanner showing breast patient positioning (photo reproduced by permission of Phillips Medical Systems).

Virtual simulator software
The most important features of the virtual simulator are fast CT scanning and reconstruction of transverse slices, fast reconstruction of any section, automatic skin outlining, automatic lung/bone outlining, semi-automatic outlining of critical structures/vital organs, user friendly target outlining (accurate interpolation/ease of editing) and volumetric growing of margins using a true 3D volume growing algorithm.

It should not be necessary to outline all features on all slices; interpolation is possible provided that the user does not leave too many gaps for the computer to fill in. Outlining the tumour volume, usually the gross tumour volume (GTV), is the clinician's responsibility. Again, some degree of interpolation is possible provided that the contours on the interpolated slices are checked for accuracy. Methods of linear and non-linear contour interpolation are combined with manual slice-by-slice checking and editing. The treatment planner then grows the GTV to the planning target volume (PTV) by a true 3D volume growing algorithm [13]; many planning systems now allow for different margins to be added in different directions.

3D display systems are continually improving. These are vital features to any virtual simulator since internal anatomy, beam geometry and dose distributions need to be easily and accurately displayed and manipulated quickly. It is especially important that the PTV is seen by the planner in three dimensions to be covered by the high dose region and, conversely, that critical structures and vital organs are in low dose regions. Of course, other tools within the planning system, e.g. dose–volume histograms (DVHs), assist this process, but generally these do not contain the geometrical and anatomical information given in the image display. Several commercial systems offer additional features within their VSIM package, such as a virtual light field, which illuminates the skin surface of the 3D image of the patient. Most systems also have an image of the treatment machine together with a picture of the patient on the treatment table. The gantry and table will move to show the position of the beam chosen and particularly whether there is any possibility of a collision.

	Other aspects of CT sim

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

 
Multislice CT scanners
As multislice CT scanners become more common in diagnostic radiology it will be important for the radiotherapy community to assess their role for VSIM. One issue is that of speed. An 8-slice scanner can scan 48 cm in 3–6 s at 1 cm slice width, or 48 cm in 15–30 s at 2 mm slice width. With older systems these times would nominally be approximately eight times longer (the latter time, at least 240 s, would require pauses for anode cooling, or would not be attempted).

Fast scan times are advantageous in reducing motion artefacts, but there will be a question as to which phase of breathing has been scanned. With the older single slice systems there is some blurring of images owing to patient movement. This has been accepted since the patient is treated with beam-on times similar to CT scanning times, and it has been assumed that any effects of movement would be averaged out. However, now that more centres are beginning to address the problem of motion, both on the CT scanner and on treatment, the faster scanners may be an enormous advantage.

Multislice scanners will also need to be assessed for their accuracy since the off-axis slices need to be reconstructed from ray paths travelling obliquely through the patient.

Dedicated CT?
The special requirements of a CT simulator suggest that a dedicated CT scanner designed to fit the demands of radiotherapy planning on CT is required. However, many centres will also wish to use their CT simulator for diagnostic work. As diagnostic scanners are developed for purposes other than radiotherapy, we may see a divergence in development between the two types of equipment. Each centre will need to specify its own requirements, provided that the computer network connections can be made so that any CT scanner can be linked to a computer with its virtual simulator package (see below), with the need also to add a laser marking-up system.

Immobilization
The importance of effective positioning of the patient to facilitate optimum treatment design and the ability to re-establish this position on a daily basis are recognized as essential to accurate radiotherapy. Some of the early work [9] with CT sim emphasized the difficulties of emulating the patient support and accessory attachments of a linear accelerator on a CT scanner. After 10 years of CT sim use, manufacturers have recognized the need to provide a table top that is identical to the top used on the treatment machine (previously it was standard practice to provide a flat-top couch insert to the conventional curved CT diagnostic couch that could easily rotate slightly so that the patient was no longer on a horizontal surface). The CT therapy couch top should also be designed to take the usual accessories needed to position the patient, for example breast boards and head rests. These accessories significantly enhance positioning accuracy and patient comfort and reduce patient set-up time.

Aperture
The constraints of a 70 cm aperture on radiotherapy patient positioning are obvious for some treatments, such as breast and mantle techniques. The move to more dedicated oncology CT scanners has led to designs that can accommodate these set ups using a larger aperture. One commercial system is available, the Marconi AcQsim CT Scanner (Philips Medical Systems Ltd, Stevenage, UK), with an aperture of 85 cm (Figure 2). At the present time, potential purchasers wishing to decide which system to choose will need to explore the compromise that has to be made between aperture size and image quality and the possible need to modify set-up techniques.

Display
A modern virtual simulator system will have many options to display all the required features in colour/colour wash/line drawing, or to remove features as required. It is usually possible to view all sections, namely axial, coronal or sagittal, in multiple windows on the same computer page. Other features will include 3D views with appropriate CT slices superimposed and rotation of the 3D view.

CT sim to radiotherapy treatment planning system connectivity
The importance of efficient and accurate connectivity between CT sim and radiotherapy treatment planning system (RTPS) radiotherapy treatment planning systen cannot be overemphasized. Many of the problems associated with having two separate computer systems, one providing the function of a simulator and the other providing a dose calculation engine, are due to theincompatibilities between common parameter transfer protocols. Standards such as Digital Imaging and Communications in Medicine DICOMv3, and the standard image transfer protocol for radiotherapy (DICOM-RT) can be highly complex to implement and can vary in interpretation. The advent of DICOM-RT enables export of radiotherapy images, treatment plans and structure sets (contours). However, this standard is not always fully implemented and can have exclusions, e.g. dynamic treatment data, that can limit functionality. Problems may be encountered when transferring data, even between systems from the same manufacturer. Transfer protocols should be fully tested for all conditions and any inconsistencies reported.

Most CT sim systems are configured as single virtual simulator workstations interfaced to a CT scanner. Problems arise when additional VSIM stations are added to accommodate increased workload, with multiple copies of patient data and lack of synchronization between these files. Future systems must incorporate patient images, structures and treatment files in a database that enables multi-user access with full data protection, e.g. file locked while in use to avoid secondary access.

Treatment charting (dosimetry)
Modern VSIM software packages also contain many of the features of a treatment planning system, with the exception of the calculation of dose distribution. Correspondingly, modern treatment planning systems are now available with VSIM software. For VSIM or treatment planning systems the typical features for conventional and conformal planning include:

• beam position/rapid editing of position, size, wedge, weight;
  
• adding further beams using copy/position facilities;
  
• auto-beam positions according to a stored protocol (beam configuration library);
  
• auto-shaping for the multileaf collimator/blocks including optimization of collimator angle;
  
• accurate 3D calculation of dose from each beam using a complex dose calculation algorithm, taking 3D scatter and inhomogeneities into account;
  
• display of complete dose distribution; and
  
• calculation of monitor units for each beam.
  

	CT sim/VSIM process

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

 
General
This process may vary depending on local procedures adopted to suit the working conditions of a particular department. The ultimate aim is to achieve the same level of treatment simulation as conventional physical methods but with the added features that are available from 3D visualization. Significant advantage is gained through the reduced visits required by the patient and the flexibility offered through tasks, such as contour marking, that can be undertaken after the patient has left.

A general discussion of the CT VSIM process follows, with an indication of alternative methods and options where appropriate.

The patient is positioned on the flat-top couch of a CT scanner in the treatment position. Alignment of the patient is made with lateral wall lasers and sagittal laser. Opaque catheters may be used as visual markers. A prior simulator visit is not usually required. A pilot (scout view) scan is made to determine the region over which axial slices will be scanned. These slices are then made according to the particular protocol for the site to be treated, e.g. prostate. A single visit to the CT simulator is usually preferred. Two methods can be adopted. The first requires the oncologist to be present to identify the target volume and isocentre from the scan information while the patient remains in the treatment position. In the second method the operator identifies a reference slice containing a reference point from the scan study and target definition is then undertaken when the oncologist is available. In both methods the patient is "marked" where the laser projection illuminates the skin and finally the patient is removed from the couch. In the second method the isocentre is eventually defined in terms of "shift coordinates from the reference point" (Figure 3).

View larger version (71K): [in this window] [in a new window]   Figure 3. Localization: a suitable "patient origin" (isocentre) is marked as the centre of the purple triangle (mark-up). These coordinates are sent to the laser system and the patient marked. All plan isocentres are related to this mark in terms of "shift coordinates".

View larger version (69K): [in this window] [in a new window]   Figure 4. Verification: the "shift coordinates" represent the relationship between the isocentre and the "patient marked origin" to be used for treatment setup. All field digitally reconstructed radiographs are exported for portal verification on the linear accelerator.

Examples of CT sim practice
The CT sim process depends on defining a relationship between the CT image coordinates (patient) and the treatment coordinates (machine) that allows a precise transformation from the localization setup to radiotherapy treatment coordinate space. The methods of achieving this are dependent on local equipment and working practices. Inherent in all successful CT sim techniques is the appropriate immobilization of the patient that is compatible with the constraints of the CT scanner. For some sites radiotherapy techniques will have to be adapted to accommodate these constraints.

Successful CT sim practice will require changes to working practice that will allow similar patient throughput to a conventional simulator. This may require flexible working of the oncologists involved in defining treatment volumes. The advantages of CT sim over conventional simulation, such as one planning session visit for the patient, volume mark-up without the patient present and minimal patient wait, can only be realised if working practice is tailored to the system.

The following is a discussion about some site-specific CT sim procedures.

Breast (Figure 5a)
Stage 1. Localization is usually undertaken with the patient positioned on a purpose designed "breast board". The patient's arms must not impede free movement of the CT couch and therefore careful thought must be given to the design of the board. Use of large aperture CT will allow more flexibility in patient positioning.

View larger version (82K): [in this window] [in a new window]   Figure 5. (a) Virtual simulation planning of tangential breast fields. Collimator and table angle to provide matching borders are obtained from multiple window views.

Radio-opaque catheters can be used to mark superior, inferior, medial and lateral extents of the volume. The patient is scanned to include superior and inferior extents (from the pilot scan) and external contouring of those slices containing the catheters are performed (purple lines in Figure 5a).

A reference mark is set to the medial catheter on the central slice, midway between the superior and inferior marked slices, and this is defined as the "patient origin". The patient is marked using the patient origin coordinates transferred to the CT sim couch and lasers. The patient session is now finished.

Stage 2. VSIM planning requires the glancing fields to be positioned in BEV so that the posterior field edges pass through the medial and lateral catheters. Adjustments are made to minimize encompassed lung, this can be visualized by altering the CT window and level for lung and soft tissues. Field parameters are selected according to the breast protocol to be used and the plan is passed to the RTPS for calculation and dose optimization. The plan, including the final isocentre coordinates, which may have changed during plan optimization, is exported back to the virtual simulator for verification using DRRs. The shift coordinates are printed from the relationship between the plan isocentre and patient origin coordinates. These are transferred to the treatment machine with the plan details. Worksheets and DRRs are printed [14, 15].

Head and neck (Figure 5b)
Stage 1. Localization requires the immobilization shell to be attached to the flat CT couch in order to emulate exactly the patient positioning on the treatment machine. Careful consideration should be given to the design of the head fixation device to enable compatibility between the CT and accelerator table supports.

View larger version (80K): [in this window] [in a new window]   Figure 5. (b) Virtual verification of a carcinoma of the tongue. The purple triangle represents the plane containing the "patient origin" reference point from which the position of the isocentre gives the "shift coordinates".

A reference slice plane is selected (purple contour in Figure 5b) and the patient origin coordinates created and transferred to the CT couch and laser. The CT longitudinal couch, vertical couch and sagittal laser positions are set to define the patient origin, and the patient is marked. The patient session is now finished.

Stage 2. VSIM requires marking of the GTV, CTV, PTV and organs at risk. The isocentre and field parameters can then be defined using the virtual simulator or the RTPS. The plan is sent for calculation and optimization to the RTPS and exported back to the virtual simulator for verification. The DRRs for all fields are printed (laser imager) and approved by the oncologist.

Shift coordinates are printed from the relationship between the isocentre and patient origin coordinates. These are transferred to the treatment machine with the plan details. Worksheets and DRRs are printed (Figure 5b) [16].

Bronchus (Figure 5c)
Stage 1. Localization of the patient is in the supine position with arms overhead clasping arm-poles attached to an indexed radiotherapy couch top. The scan length is customized for each patient by visual inspection within the CT aperture and from the pilot (scout) view, but generally covers the whole chest. The scan protocol is the same as for the breast. Localization and planning procedures are similar to those used for head and neck with the exception of palliative bronchus treatments. For these cases the definition of field size position and shielding can be performed by direct marking of the DRRs, being analogous to conventional physical simulation. This technique has been termed "virtual fluoroscopy" [17]. The effect of diaphragm movement in these cases, which cannot be easily assessed by CT sim, must be allowed for in the target margins, if a breath hold protocol is not used.

View larger version (96K): [in this window] [in a new window]   Figure 5. (c) Localization of a chest lesion using virtual fluoroscopy. Anteroposterior and lateral virtual radiographs (digitally reconstructed radiographs) show the isocentre in simulator and CT views.

The quality control procedures can be split into daily and monthly procedures and those performed at acceptance and then yearly. Acceptance tests are shown in Table 2 [18]. Special phantoms to perform these tests have been designed [19].

• Alignment of external vertical and horizontal lasers and their position with respect to the virtual isocentre of the CT simulator.
  
• Accuracy and linearity of the sagittal laser, driven by computer or manually set.
  
• Alignment of the internal laser within CT aperture with respect to the scan plane.
  
• Couch position, vertical and longitudinal, registration.
  

• Distance between known points in the image plane.
  
• Left/right registration.
  
• CT number/electron density verification.
  
• Noise on CT number in uniform phantom.
  
• Reconstructed slice location.
  
• Image transfer protocols, e.g. Dicom-RT, using standard plans.
  

	Which to choose: CT sim or physical simulation?

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

 
Although attempting to replicate the same task, conventional and virtual simulators are very different systems with major differences in hardware. Most significantly, VSIM has a different approach to providing the clinician with information to define the target volume, which can result in significantly different treatments (beam arrangements and target volumes).

A comparison of virtual vs physical simulation aims at answering a number of questions. The answers to these questions are fundamental to decisions on equipment selection when either replacing an existing simulator or providing additional resources. Each question will be addressed based on published investigations and according to the authors' own experiences and opinions.

(a) Do VSIM methods lead to the same level of treatment accuracy as physical simulation? Two recent randomised trials have compared simulation techniques. 75 patients undergoing four-field conformal prostate treatment in a study by Valicenti et al [20] had CT sim, with one group having physical simulation prior to treatment. Both patient groups had their port films reviewed to quantify the differences between the two techniques. Results indicated no significant difference in set-up errors between the two techniques and concluded that physical verification could be omitted from the CT-based planning process. McJury et al [21] considered 86 patients undergoing palliative radiotherapy using parallel-opposed fields to the chest, all patients had CT VSIM and physical simulation but patients in each group of the study received treatment using either the CT sim or physical simulation plan. Results indicated that setup errors were typically 2–3 mm for both patient groups and there were no significant differences in terms of accuracy.

(b) Do VSIM methods result in significant differences in target volume definition compared with physical simulation? The primary objective of this double-blind randomized trial by McJury et al [21] was to determine the differences in target volumes contoured using both techniques. Comparing fields defined in each study arm, there was a major or complete mismatch in coverage between fields in 70% of cases. The use of VSIM resulted in field sizes on average 25% smaller than physical simulation. Senan et al [22] also found that the use of CT sim allowed for smaller planning target volumes in radical lung cancer.

(c) Does VSIM cause problems with regard to patient throughput owing to changes in length of procedure times? Comparing the relative time expended for CT sim and physical simulation requires an assessment of procedure time involving the patient and radiotherapy staff. A number of centres have published data on time comparisons. Buchali et al [23] have reported a study of 23 patients having tangential breast irradiation. The use of CT sim resulted in a mean saving of 22 min in the whole treatment planning process compared with physical simulation. This reduced the time interval between CT and first treatment by 31%, mainly due to the omission of conventional simulator verification from the 3D planning process. For those centres with increasing patient workloads, this economy can have a significant effect on patient throughput. However, a check by the physician is still required.

Raga et al [24] have reported that the physician's time involved in the planning process can be significantly reduced using CT sim, typically from 25 min to 5 min per patient (brain and prostate).

Mah et al [25] used CT sim for craniospinal paediatric patients, where time efficiency can improve patient comfort and increase accuracy. On average patient involvement and immobilization time during simulation could be reduced from 45 min to 20 min when using CT sim instead of physical simulation.

These results suggest that the use of CT sim with omission of conventional simulation may improve the efficiency of the treatment planning process without compromizing accuracy. Raga et al [24] report that 60% of their planned patients were suitable for CT sim, whereas some early work by Nagata et al [26] indicated that this figure could be as high as 70%. One author's (JC) own experience indicates that 65% of planned patients are selected for CT sim.

	Advantages and disadvantages

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

 
Many of the advantages of CT sim have been discussed in the preceding sections but can be summarized as:

• full 3D simulation allowing unique verification of beam coverage and avoidance in three dimensions,
  
• beams can be simulated and verified that are not possible with conventional simulation, e.g. vertex fields,
  
• the verification images, DRRs can contain more information than conventional simulation and can be manipulated to enhance tumour visualization, and
  
• there is a much closer connection to diagnostic information with CT sim, allowing integration of multimodality images.
  

	Conclusion

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

The entire simulation of the patient, ensuring all beams are achievable and safe, makes use of room's eye view and anti-collision software algorithms.

CT sim enables doctors and dosimetrists to work at their convenience while minimizing patient attendance. However, for some palliative treatments the planning process using CT sim might be prolonged compared with physical simulation.

The adoption of CT VSIM in favour of conventional simulation is recommended where small oncology departments have a requirement for only one simulator while expanding their 3D treatment planning methods. For larger departments the retention of conventional simulation would seem advantageous, and a ratio of two CT sim units to one physical simulation unit would provide the balance of resources for the precision required in a modern radiotherapy department.

Received for publication September 24, 2001. Revision received April 2, 2002. Accepted for publication June 27, 2002.

	References

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

 

1. Conway J, Robinson MH. CT virtual simulation. Br J Radiol 1997;70(Suppl.):S106–18.
2. Dobbs HJ, Parker RP. The respective roles of the simulator and computed tomography in radiotherapy planning: a review. Clin Radiol 1984;35:433–9.[Medline]
3. Cheng CW, Chin LM, Kijewski PK. A coordinate transfer of anatomical information from CT to treatment simulation. Int J Radiat Oncol Biol Phys 1987;13:1559–69.[Medline]
4. Goitein M, Abrams M. Multi-dimensional treatment planning: 1. Delineation of anatomy. Int J Radiat Oncol Biol Phys 1983;9:777–87.[Medline]
5. Goitein M, Abrams M, Rowell D, Pollari H, Wiles J. Multi-dimensional treatment planning: II. Beam's eye view, back projection, and projection through CT sections. Int J Radiat Oncol Biol Phys 1983;9:789–97.[Medline]
6. McShan DL, Silverman A, Lanza D, Reinstein L, Glicksman A. Three dimensional radiation treatment planning and dose display utilizing interactive colorgraphics. Br J Radiol 1979;52:478–81.[Abstract]
7. Sherouse GW, Mosher CE, Novins KL, Rosenmann JG, Chaney EL. Virtual simulation: concept and implementation. In: Bruinvis IAD, van der Giessen PH, van Kleffens HJ, Wittkamper FW (editors). Ninth International Conference on the Use of Computers in Radiation Therapy. Amsterdam, The Netherlands: North Holland Publishing Co., 1987:433–6.
8. Sherouse GW, Chaney EL. The portable virtual simulator. Int J Radiat Oncol Biol Phys 1991;21:475–81.[Medline]
9. Sherouse GW, Novins KL, Chaney EL. Computation of digitally reconstructed radiographs for use in radiotherapy treatment design. Int J Radiat Oncol Biol Phys 1990;18:651–8.[Medline]
10. Mohan R. 3-D radiation treatment planning. Int J Radiat Oncol Biol Phys 1988;15:481–95.[Medline]
11. Galvin J, Heidtman B, Cheng E, Block P, Goodman R. The use of a CT scanner specially designed to perform the function of a radiation therapy treatment unit simulator. Med Phys 1982;9:615.
12. Nishidai T, Nagata Y, Takahashi M, Abe M, Yamaoka N, Ishoihara H, et al. CT simulator: a new 3-D planning and simulating system for radiotherapy: Part 1. Description of system. Int J Radiat Oncol Biol Phys 1990;18:499–504.[Medline]
13. Neal AJ, Sivewright G, Bently R. Evaluation of a region growing algorithm for segmenting pelvic CT images during radiotherapy planning. Br J Radiol 1994;67:392–5.[Abstract]
14. Butker EK, Helton DJ, Keller JW, Hughes LL, Crenshaw T, Davis LW. A totally integrated simulation technique for three-field breast treatment using a CT simulator. Med Phys 1996;23:1809–14.[Medline]
15. Das IJ, Cheng EC, Freedman G, Fowble B. Lung and heart dose volume analyses with CT simulator in radiation treatment of breast cancer. Int J Radiat Oncol Biol Phys 1998;42:11–9.[Medline]
16. Lohr F, Schramm O, Schraube P, et al. Simulation of 3D-treatment plans in head and neck tumor aided by matching of digitally reconstructed radiographs (DRR) and on-line distortion corrected simulator images. Radiother Oncol 1997;39:1131–5.
17. Forster KM. The use of spiral CT to access internal-motion target volume margins for thoracic and abdominal tumour. 7th annual Oncology Symposium, Boston 2000.
18. McGee KP, Das IJ. Commissioning, acceptance testing and quality assurance of a CT simulator. In: Coia LR, Schultheiss TE, Hanks GE. A Practical Guide to CT Simulation. Madison, WI: Advanced Medical Publishing, 1995:5–23.
19. Craig TC, Brochu D, Van Dyk J. A quality assurance phantom for three-dimensional radiation treatment planning. Int J radiat Oncol Biol Phys 1999;44:955–66.[Medline]
20. Valicenti RK, Waterman FM, Corn BW, Curran WJ. A prospective, randomized study addressing the need for physical simulation following virtual simulation. Int J Radiat Oncol Biol Phys 1997;39:1131–5.[Medline]
21. McJury M, Fisher PM, Pledge S, Brown G, Anthony C, Hatton MQ, et al. The impact of virtual simulation in palliative radiotherapy for non-small cell lung cancer. Radiother Oncol 2001;59:311–8.[Medline]
22. Senan S, van Sornsen de Kose J, de Boer J, et al. The use of CT simulation in digitally reconstructed radiographs (DRRs) in set-up verification allows for smaller planning target volumes in lung cancer. Lung Cancer 2000;20(Suppl 1):162.
23. Buchali A, Geismar D, Hinkelbein M, Schlenger L, Zinner K, Budach V. Virtual simulation in patients with breast cancer. Radiother Oncol 2001;59:267–72.[Medline]
24. Raga DP, Forman JD, He T, Mesina CF. Clinical results of computerised tomography-based simulation with laser patient marking. Int J Radiat Oncol Biol Phys 1996;34:691–5.[Medline]
25. Mah K, Danjoux CE, Manship S, Makhani N, Cardosos M, Sixel KE. Computed tomographic simulation of craniospinal fields in paediatric patients: improved treatment accuracy and patient comfort. Int J Radiat Oncol Biol Phys 1998;41:997–1003.[Medline]
26. Nagata Y, Nishidai T, Abe M, Takahashi M, Okajima K, Yamaoka N, et al. CT simulator: a new 3-D planning and simulating system for radiotherapy: Part 2. Clinical application. Int J Radiat Oncol Biol Phys 1990;18:505–13.[Medline]

	Futher reading

Top Abstract Introduction CT planning development Specification of a CT... Other aspects of CT... CT sim/VSIM process Which to choose: CT... Advantages and disadvantages Conclusion References Futher reading

1. Van Dyk J, Taylor JS. CT simulators. Van Dyk J (editor). The Modern Technology of Radiation Oncology. Madison, WI: Medical Physics Publishing, 1999.
   
2. Galvin JM. The CT-simulator and the Simulator-CT: advantages, disadvantages and future developments. Smith AR (editor). Radiation Therapy Physics. Berlin, Heidelberg, New York: Springer Verlag, 1995:19–32.
   
3. Petschen I, Perez-Calatayud J, Tormo A, Lliso F, Badel MD, Caromona V, et al. Virtual Simulation in radiation therapy planning. Report of five-year experience. Revista de Oncologia 2000;2:213–22.
   
4. Coia LW, Schultheiss TE, Hanks GE (editors). A Practical Guide to CT Simulation. Madison, WI: Advanced Medical Publishing, 1995.
   

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