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British Journal of Radiology (2003) 76, 850-856
© 2003 British Institute of Radiology
doi: 10.1259/bjr/19737738
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Review article

Implementation of IMRT in the radiotherapy department

H A McNair, DCR(T), MSc 1 E J Adams, MSc 2 C H Clark, PhD 2 E A Miles, BSc 1 and C M Nutting, MRCP, FRCR, MD 1

Departments of 1 Radiotherapy and 2 Physics, Royal Marsden NHS Trust and Institute of Cancer Research, Downs Road, Sutton, Surrey SM2 5PT and Fulham Road, London, UK


   Abstract
Top
 Abstract
Introduction
 Requirements for IMRT delivery
 Treatment planning
 Treatment delivery
 Staff acceptance
 Maintaining the IMRT programme
 Conclusion
 References
 
This paper describes the implementation of intensity modulated radiotherapy (IMRT) in a radiotherapy department. When preparing to set-up an IMRT programme, it is important to review departmental protocols with regard to immobilization, CT planning, treatment planning and verification. Any additional quality assurance steps also need to be fully understood. A new IMRT programme is most likely to be successful if it builds on established clinical experience with three-dimensional conformal radiotherapy (CRT). Training of radiographers, clinicians and physicists is critical, and both team-work and communication are vital to ensure a smooth transition from 3DCRT to IMRT delivery in the clinic.


   Introduction
Top
 Abstract
 Introduction
Requirements for IMRT delivery
 Treatment planning
 Treatment delivery
 Staff acceptance
 Maintaining the IMRT programme
 Conclusion
 References
 
Intensity modulated radiotherapy (IMRT) has the capability of sculpting the isodose distributions closely to the tumour volume, thereby reducing normal tissue irradiation and potentially allowing dose escalation. Clinical trials are currently underway to assess whether these characteristics lead to an improved clinical outcome. The increase in interest in IMRT and the potential clinical benefits for patients have encouraged many departments to commence an IMRT programme. This paper outlines the requirements for the implementation of IMRT into the routine radiotherapy department.


   Requirements for IMRT delivery
Top
 Abstract
 Introduction
 Requirements for IMRT delivery
Treatment planning
 Treatment delivery
 Staff acceptance
 Maintaining the IMRT programme
 Conclusion
 References
 
The formation of the National Institute of Clinical Excellence (NICE), and introduction of clinical governance has meant a greater accountability in terms of best practice in the UK when using new treatments and technologies. It has been recommended that all new technologies are compared with standard treatments before widespread implementation occurs [1]. The implementation of IMRT should follow an already existing programme of conformal radiotherapy (CRT). This is because many of the skills and technical requirements for IMRT are also required for CRT and valuable experience can be gained in target volume definition, immobilization and verification of CRT. Furthermore it allows the comparison of IMRT with CRT and should also result in a smoother implementation process.

Before commencing an IMRT programme there should be a clear goal. A department should identify a particular tumour site for implementation. Most commonly world-wide this has been head and neck or prostate cancer. Departments may wish to make this choice based on evidence of the improved benefit, experience of staff, resources or simplicity of technique. There are many planning studies in the literature which demonstrate improved dose distributions [2–7]. Each study is particular to the planning system used, site treated and method of treatment planning/delivery. It is therefore advisable that departments perform introductory planning exercises to try to reproduce these dose distributions, and seek the advice of centres treating similar cases. Hands-on training courses are particularly useful in this regard where clinicians and physicists can gain experience of outlining and treatment planning for IMRT using a variety of treatment planning systems (TPS). This early experience can significantly speed up implementation into the clinic.

IMRT delivery requires a linear accelerator with computer controlled multileaf collimator (MLC). Although custom-made compensators can be used, the time and resources required to manufacture and use them tends to be prohibitive. The two most common methods of MLC IMRT delivery are "step and shoot" also known as static MLC (SMLC) and dynamic (dMLC). The SMLC typically involves 5–15 segments per gantry angle where the beam is switched off between each segment. The dynamic mode involves continual movement of the MLC, generally in the form of a "window" sliding from one side of the field to the other. Varying the speeds of the leading and trailing leaves then modulates the intensity of the beam.

TPS for IMRT must be capable of either forward or inverse treatment planning. Forward planning can provide treatment plans that can be delivered using SMLC. This is often only a small step beyond CRT and is therefore less labour intensive and easier to deliver and verify. However, for complex planning problems manual forward planning is too time consuming and so for most departments inverse planning will be chosen. The majority of commercial planning systems now offer inverse planning [8]. The time taken to commission these systems and complete adequate quality assurance (QA) of the delivery is not insignificant and must be considered before embarking on a full programme [9–11]. To verify treatment set-up accurately and efficiently, electronic portal imaging (EPI) is essential. EPI can also be used to verify the individual intensity maps and in the future, the dose delivered.


   Treatment planning
Top
 Abstract
 Introduction
 Requirements for IMRT delivery
 Treatment planning
Treatment delivery
 Staff acceptance
 Maintaining the IMRT programme
 Conclusion
 References
 
Patient position
To take advantage of, and fully utilize, the capabilities of IMRT the ideal patient position and methods of reproducibility should be re-examined. For example, patients receiving head and neck radiotherapy no longer need to be positioned with a straight spinal cord since matched photon/electron fields are not required. This allows the neck to be fully extended, facilitating the selection of field orientations which avoid sensitive normal tissue, such as the oral cavity (Figure 1Go).



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  		Figure 1. Lateral simulator film showing extended head and neck position and isocentre verification field.

 
Using IMRT it is common to treat the primary and nodal volumes with different doses-per-fraction. This has been termed simultaneous modulated accelerated radiotherapy (SMART, [12]) or simultaneous integrated boost (SIB, [13, 14]). This often produces a large overall target volume and the obvious reference set up position, at the geometric centre of this volume, may be unstable or not easily accessible for set-up. In this case it is better to select an appropriate anatomical reference point to ensure patient reproducibility. In head and neck patients, for example, it may be more pragmatic to place the set-up point superior to the shoulders and close to the spinal cord. This provides both useful anatomy for the treatment verification, and allows easier patient set-up. In patients receiving radiotherapy to the prostate gland and pelvic nodes the pubic symphysis remains the preferred set-up point, as for patients treated for prostate gland alone, since it is a more stable site to tattoo than the geometric centre of the volume [15]. Asymmetric jaws can then be used to create the treatment field size required (Figure 2Go). This may reduce the need for a couch shift during treatment set-up reducing the risk of error. If the linear accelerator has an MLC with different leaf widths, then it may be preferable to use a couch shift to maximize the use of the narrower MLC leaves. In this case it is advisable to have an immobilization system indexed to the couch [16].



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  		Figure 2. Anterior digitally reconstructed radiograph of prostate and node planning target volume and field parameters with position of set-up point shown.

 
Immobilization and reproducibility
The general principles of immobilization, relevant to CRT, are also applicable to IMRT. Indeed, reducing patient movement and internal organ motion may be even more critical for IMRT where the dose distribution can be sculpted closer to the tumour volume or organ at risk (OR) with rapid fall off outside the volume. It is therefore important that immobilization and reproducibility of patient position are re-evaluated prior to the implementation of IMRT. The method of immobilization can be improved if necessary and information gained will help determine margins for planning target volumes (PTV).

Ideally immobilization devices should be indexed to the couch providing greater stability and allowing the use of record and verify systems to check daily reproducibility. Improvements in immobilization and reproducibility need not be complex, for example the use of ankle stocks for patients receiving radiotherapy to the prostate can significantly improve set-up accuracy [17].

The stereotactic frame has been shown to be the most accurate immobilization system for patients receiving cranial irradiation [18–20]. Whether this degree of accuracy is necessary for head and neck or cranial IMRT will depend on the proximity of the high dose region to OR, and the fall off of dose around the planning target volume. In this regard CRT and IMRT are similar. Stereotactic frames are under development for skull-base tumours to provide increased accuracy in set up for IMRT. As with conventional radiotherapy, it is important for head and neck cancer patients that the immobilization system allows maximum skin-sparing and if shell systems are used they should maintain stability when cut-out.

Intrafraction movement may be a more significant problem for IMRT compared with CRT. The potentially longer treatment times, as well as the combination of moving anatomy and time varying beam shapes, may result in unexpected dose distributions. The magnitude of this potential difference has been investigated and IMRT treatments to sites where internal organ motion is significant should proceed with caution [21–23].

The effects of internal organ motion can be reduced by two approaches: tumour immobilization or tumour tracking. The second approach includes image-guided radiotherapy (IGRT) and is the subject of intense investigation at present.

Prostate motion can be reduced by maintaining a constant bladder and rectal volume [24–26]. This can be achieved by using devices such as a rectal balloon [27]. However, appropriate patient instruction regarding rectal filling can also help to reproduce the rectal volume without the discomfort of a balloon [28].

The accuracy of set-up for patients receiving radiotherapy for lung cancer is affected by breathing. The tumour motion created by the normal breathing cycle can be in the order of 2 cm [29]. To reduce this tumour motion the patient can either be trained to self breath hold [30, 31] or use a device such as the active breathing co-ordinator (ABC), to assist breath hold [32]. Either method requires patient and "operator" training and it is vital that intrafraction and interfraction reproducibility of the method is established before implementation.

IGRT offers the possibility of real-time tumour tracking. The use of implanted markers, such as gold grains have been shown to improve the accuracy of prostate cancer treatment [33–37]. Implanted markers have also been used in patients treated for lung cancer to synchronise or "gate" delivery in time with the patient's respiratory cycle [38]. Another approach is to gate the delivery with reference to external markers [39–43]. However, to be effective, the relationship between the external markers and tumour position must be known. The complexity and increased time required for gating procedures may limit the applicability for some tumour sites. Repeatedly interrupting the machine during treatment requires the radiation beam to reach stability quickly and may also cause additional problems, particularly when using dynamic IMRT.

Systems that provide soft tissue imaging, by CT or ultrasound, prior to radiotherapy treatment are also being evaluated [44–46]. Although these systems will inevitably initially increase set-up times and demand greater resources, detailed information can be gained regarding daily internal motion in different anatomical sites. It may be possible to develop models to improve set-up accuracy in a particular group of patients, or identify patients prior to treatment who are predicted to have an excessive degree of internal organ motion and who are unsuitable for highly conformal radiotherapy techniques.

Imaging
The requirements of radiotherapy imaging for IMRT are similar to CRT. CT scanning provides accurate anatomical detail and tissue density information for radiotherapy planning. The new generation of multislice scanners can also scan using small slice thickness and intervals to generate high resolution digitally reconstructed radiographs (DRR) for reference images.

It is essential that the patient position during CT scanning can be easily and accurately reproduced throughout the treatment process. Different procedures can be used to accomplish this. Permanent marks can be made at the time of CT with shifts to the set-up point used for daily treatment set-up. An alternative is to mark the patient semi-permanently at the time of CT and then permanently at the isocentre during verification, or first treatment visit if virtual simulation is used.

The use of contrast agents will enhance quality of the images and assist in better lymph node definition. However, the TPS can interpret contrast-bearing tissues as high-density tissue which may affect the dose calculation and the magnitude of this effect on the treatment plan should therefore be established. Multimodality imaging is being investigated in many centres as an aid to define tumour volumes. These modalities can provide more information but the question remains as to which is the most accurate. Problems such as geometric distortion in MRI must also be overcome.

Computer planning
As with immobilization, defining the clinical target volume (CTV) and the gross target volume (GTV) accurately is more crucial in IMRT. Interclinician variability in defining the target volume makes a significant contribution to the errors in the treatment planning process [47–49]. Training and previous experience with outlining target volumes from CT images is therefore highly desirable before implementing IMRT. The time taken to outline for IMRT, though initially longer, does decrease with experience. In addition, IMRT may require more extensive normal tissue anatomy to be outlined as structures to be avoided. This is particularly important for inverse planning. Different TPS vary both in the required inputs and the way in which they are used; hence planning studies are useful to establish a starting set of constraints, which usually can then be successfully applied, to other similar patients, with minimal alteration. Sometimes artificially strict constraints are required to steer the optimization algorithm towards a desired goal such as OR sparing. Many TPS perform leaf sequencing after optimization; this will affect the dose distribution due to transmission and head scatter. It is important that the final dose distribution includes the calculation of these factors. Most TPS allow a different priority to be assigned to different organs and these can also be used to guide the optimization algorithm towards the desired result.

Careful choice of the isocentre position may be important to optimize the dose distribution, particularly to allow avoidance of OR and external structures such as the couch or immobilization systems. In some cases the isocentre may lie outside the PTV.

One advantage of inverse planning is that in using the SIB technique, all the treatment planning can also be completed in a single session. This compares with two to three sessions conventionally used for multiphase CRT planning.

Machine quality assurance
Additional machine checks are likely to be needed for IMRT since the MLC is operated differently from conventional treatments; the nature of these checks will depend on whether the IMRT is being delivered in dMLC or SMLC mode.

Whichever mode is used for delivering IMRT, tests must be carefully designed and carried out to ensure that the leaves are in the required positions at the required times. Even small errors in the positions of the individual leaves can cause unacceptable inaccuracies in the dose distributions delivered to the patient (further details are given in Prof. Williams paper in this series [50]). As well as standard MLC QA, the testing of the motion and repositioning of the leaves should be carried out if dynamic IMRT is used. These tests should include the positioning ability of the leaves, the speeds of the leaves, including acceleration and deceleration and any effect that gravity might have on the movements of the leaves [51]. A step-and-shoot delivery may include multiple segments with small numbers of monitor units (MU) and therefore it is important to check the beam characteristics under these conditions; in particular, dose per MU and beam flatness/symmetry.

Verification of the treatment plan
It is important to verify both the calculated MU and individual field intensity maps (analogous to conformal field shapes). It may also be desirable to verify the dose distribution. A variety of methods exist for performing these checks. For "simple" IMRT techniques, that use a small number of beams and segments, it may not be necessary to carry out additional QA checks as such treatments can be considered as a simple extension of conformal techniques with each segment equivalent to a conformal field. For more complex IMRT treatments, this becomes impractical and more elaborate QA measures are therefore necessary. When carrying out QA measurements for IMRT fields, it is preferable to use the MU that will be delivered for the patient treatment, since this will affect the leaf speeds (dynamic) or MU per segment (step-and-shoot).

Individual field checks may be either quantitative or qualitative and are generally performed using film, EPI device (EPID) or diode arrays. Quantitative checks ensure that the planned intensity map is correctly delivered and are analogous to performing radiation field checks for conformal fields. Comparison of the measured and predicted dose distributions can be achieved by overlaying printed isodoses (Figure 3Go). However, a more thorough comparison can be carried out if the measured and predicted dose distributions are available in electronic format using the "gamma index", which examines how well the distributions agree within specified acceptance criteria in terms of both dose difference and distance-to-agreement [52]. Qualitative checks may be useful to verify that the correct field is delivered, or to monitor the reproducibility of the delivery over time.



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  		Figure 3. Overlay of measured (----) and predicted (—) isodoses.

 
Verification of the entire treatment delivery can be performed using a variety of phantoms. It is advantageous if the phantom has the flexibility to perform a number of simultaneous measurements, with different measuring devices, since this reduces the time required for QA. Thermoluminescent dosimeter measurements have the advantage of allowing a large number of absolute dose measurements to be performed simultaneously throughout the treatment volume. Ion chambers allow the results to be available immediately and therefore errors in delivery can be quickly identified. Film can also be used to verify the entire treatment by sandwiching it between the slices of a suitable phantom and verifying the dose distribution in a plane. It should be noted that if the record and verify system adds dose delivered in each treatment session it will be necessary to schedule an extra treatment session for verification.

Comparisons with planning system predictions can be carried out as for single fields; ideally, any calculation of the gamma index should use the 3D distribution from the planning system, since apparent discrepancies in the 2D distributions may be attributable to high dose gradients perpendicular to the measurement plane. Gel dosimetry also allows verification of the entire treatment plan. However gels can be complicated to use (some gels are toxic to handle) and require the availability of an MRI scanner. More recently Monte Carlo codes for entire treatment verification have been developed in various university hospitals [53], however these are still generally used in parallel with measurements. There are also several independent monitor unit calculation programs on the market. However, it is advised that these are first used in parallel with measurements until sufficient confidence levels are developed.

The exact details of the QA program implemented in any department will depend on the type and complexity of treatment delivery, and also on the available equipment for making measurements. Regardless of the method used, this process is time consuming and access to the linear accelcrator is necessary. The verification is often carried on outside routine clinical hours and it is important to allow sufficient time for service days in planning the QA process times. However, as experience with the planning and delivery systems increases, it should be possible to reduce QA procedures, particularly as new tools become available.


   Treatment delivery
Top
 Abstract
 Introduction
 Requirements for IMRT delivery
 Treatment planning
 Treatment delivery
Staff acceptance
 Maintaining the IMRT programme
 Conclusion
 References
 
Patient position verification
To verify the isocentre position orthogonal images are necessary, as for conformal plans. The reference images can be simulator images or digitally reconstructed radiographs (DRRs). Field sizes are chosen to include appropriate anatomy to perform accurate field matching. Additional fields are generally required since the modulations may obscure the anatomy; however a method has been devised to remove the modulations from the images enabling IMRT fields to be used for positioning verification (Figure 4Go) [54].



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  		Figure 4. Intensity-modulated radiotherapy fluence images (a) before and (b) after calibration.

 
The treatment verification protocol is department and site dependant. Tolerances and frequency of imaging should be set after consultation within the multidisciplinary team. An example of a protocol used for CRT that is easily applied to IMRT is the protocol used for the RTO1 MRC prostate trial. EPIs are taken on the first 3–5 days, assessed for systematic error and corrected if necessary. Once an accurate set-up has been established, images can be taken once a week.

Delivery verification
EPI or films attached to the gantry head can be used to verify the delivery of the intensity map to the patient and to provide a record of the intensity map delivered during treatment. These can be compared with films or EPID images taken whilst irradiating the phantom. It must be noted, as mentioned earlier, that the treatment fields may include the primary and nodal volumes in one field. The maximum extent of the field length can be close to the maximum field size able to be captured by the EPI system, particularly when collimator twists are added. Caution must be used when using the EPI system in this way, to ensure to record the intensity map that the electronics of the imaging system are not irradiated. Some MLC systems allow the MLC to be withdrawn leaving the maximum field size visible and allowing the imager to be aligned, or a warning message may be given if the imager detects that the electronics will be irradiated. Alternatively, the most offset segments can be used to check the MLC shapes against the imager.

Treatment times
Before commencing IMRT it is recommended to carry out trial runs on the treatment unit with the full radiographer team. This will establish approximate treatment times and confirm clearance of all beam segments and shapes with external apparatus.

For the majority of treatment sites IMRT will continue to demonstrate an increase in daily treatment time. This is a result the complexity of delivery and the use of more treatment fields. However for patients with head and neck cancer single-phase IMRT has been shown to be more efficient than a multiple phase conventional treatment (2 or 3 phase requiring matched posterior electron fields) [55]. Building up the IMRT workload gradually can instil confidence and familiarity with the process and continuing developments in linear accelerator hardware and software are reducing treatment times. The manageable patient workload with IMRT will depend on department waiting times though is more likely to be limited by planning and QA, rather than treatment times.


   Staff acceptance
Top
 Abstract
 Introduction
 Requirements for IMRT delivery
 Treatment planning
 Treatment delivery
 Staff acceptance
Maintaining the IMRT programme
 Conclusion
 References
 
The success of the introduction of a new technology can be influenced by user response. In a study conducted in our centre, when IMRT was initially introduced, radiographers were invited to be interviewed regarding their perceptions of IMRT. The study was based on semi-structured interviews with 16 radiographers. The majority of radiographers (12/16) demonstrated positive attitudes regarding technology. The introduction of IMRT was seen to be stimulating and motivating. Negative aspects were associated with increased stress from learning new skills and the additional pressure of the increased workload. Though there were contradictory views regarding the effect of the increased use of technology on the patient–radiographer relationship; technological skills and patient care were not found to be mutually exclusive. With the current problems of recruitment and retention of radiographers in the UK, full exploitation of modern technology could be used to improve job satisfaction. However, careful integration is required to balance training needs with service demands.


   Maintaining the IMRT programme
Top
 Abstract
 Introduction
 Requirements for IMRT delivery
 Treatment planning
 Treatment delivery
 Staff acceptance
 Maintaining the IMRT programme
Conclusion
 References
 
The implementation of IMRT is inevitably time consuming. However, once a department has gained experience with the TPS, the time taken to plan decreases. Quality assurance can also be reduced once confidence has been gained with the TPS and delivery system. It is advisable to have regular multidisciplinary team meetings to review continually the IMRT programme. This ensures the system is streamlined, removing any unnecessary procedures and also highlighting any need for additional staff and training.


   Conclusion
Top
 Abstract
 Introduction
 Requirements for IMRT delivery
 Treatment planning
 Treatment delivery
 Staff acceptance
 Maintaining the IMRT programme
 Conclusion
References
 
For IMRT to be implemented into the routine workload of the radiotherapy department it is important for all members of the team to embrace the challenges faced. This will be facilitated by multidisciplinary communication and training. Changes in immobilization, localization, planning, verification and delivery may be required. These changes should be seen as a logical progression from CRT. Careful consideration of all of these factors is essential for the clinical introduction of IMRT to be successful.

Received for publication July 9, 2003. Revision received July 21, 2003. Accepted for publication July 28, 2003.


   References
Top
 Abstract
 Introduction
 Requirements for IMRT delivery
 Treatment planning
 Treatment delivery
 Staff acceptance
 Maintaining the IMRT programme
 Conclusion
 References
 

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