First published online November 22, 2006
British Journal of Radiology (2007) 80, 209-215
© 2007 British Institute of Radiology
doi: 10.1259/bjr/61306844
Craniospinal irradiation using a forward planned segmented field technique
J M Wilkinson, DCR(T), HDCR(T), IIPEM
1
J Lewis, MRCP, FRCR
2
G P Lawrence, MSc, BSc, MIPEM
1
H H Lucraft, FRCP, FRCR
2 and
E Murphy, DCR(T)
2
1 Regional Medical Physics Department, 2 Northern Centre for Cancer Treatment, Newcastle General Hospital, Newcastle upon Tyne NE4 6BE, UK
Correspondence: Mrs Gill Lawrence, Regional Medical Physics Department, Newcastle General Hospital, Newcastle NE4 6BE, UK. E-mail: gill.lawrence{at}nuth.northy.nhs.uk.
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Abstract
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Craniospinal irradiation is technically demanding due to the complex shape of the planning target volume (PTV). Radiotherapy treatment techniques have evolved over time as imaging and radiotherapy treatment technology have improved. However, most are variations on a class solution utilizing a prone patient position with two shaped lateral cranial portals and a matched posterior spinal portal with moving junctions. Major areas of difficulty remain with the accurate definition of the PTV and achieving a homogeneous dose within it, especially at the junctions. We describe a three-dimensionally (3D) planned craniospinal radiation technique that permits rapid image acquisition with reduced localization time, simplified spinal PTV definition and standardized cranial PTV definition. Improved dose homogeneity within the PTV is achieved by use of a segmented "field-in-field" technique (forward planned intensity-modulated radiotherapy (IMRT)) in place of customized compensators. This has negated the requirement for constructing physical compensators. Autosequencing for field delivery enables the junction to be "moved" during a single fraction and reduces the overall treatment time, an important consideration when treating very young patients.
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Introduction
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Craniospinal radiotherapy (CSRT) encompassing the entire subarachnoid space is an essential component in the management of patients with a number of central nervous system malignancies, notably primitive neuroectodermal tumours (PNET) and intracranial germ cell tumours. Modern imaging, planning and radiotherapy treatment technology offer potentially improved accuracy, but must be considered in the context of potential increased treatment complexity and planning time.
The original technique at The Northern Centre for Cancer Treatment (NCCT) evolved for use on a GE Saturne 41/800 linear accelerator, and utilized lateral opposed cranial fields and a spinal field. A moving junction ensured smoothing of any dose inhomogeneities over a greater length of spinal cord [1]. Whilst there is no evidence that failure to treat in this way leads to any clinical consequences, it was deemed prudent to exercise as much care as possible in these areas of known dose variations [2]. Localization involved a combination of conventional simulator radiographs and, since late 2000, a CT study. The junctions between the spinal field and the cranial fields were achieved by matching the divergence of the spinal field at its cranial aspect by collimator rotation of the cranial fields and direct apposition of the field edges as seen on the patient's skin. Gap junctions were not used as these have been shown to amplify inhomogeneities risking overdose or underdose [3]. All fields were defined and marked on the simulator which could take up to 1 h depending on the cooperation of the patient. A planning CT scan was carried out through the marked limits and introduced into the treatment planning system (TPS) Helax-TMS (Nucletron BV, AXVeenendaal, The Netherlands) where the data were used to plan the spinal field. The isodose distribution for the open spine field was generated and point depth doses at regular intervals along the length of the anterior cord were determined. The point doses were used to calculate the thickness of lead at each point required to attenuate the beam to achieve a homogeneous dose along the spine length and were used in the construction of a customized compensator from sheets of lead mounted onto a Perspex tray. The compensator factors were imported into the TPS and the isodose distribution regenerated to validate the compensator design. Missing tissue compensators were constructed for the skull fields using data acquired from the plaster cast used in the manufacture of the beam directional shell (BDS) and a Huestis Compuformer (Heutis Medical, Taunton, MA). These data could not be imported into the TPS and the dosimetry for the cranial fields was determined by direct measurement. Facial shielding was defined by the clinician on the lateral skull radiographs, which were then used to construct individual divergent cerrobend blocks. The blocks and the cranial compensators were mounted on individual Perspex trays.
There were a number of perceived limitations of this technique. The overall timescale from the initial Mould Room session to first treatment appointment could be up to 2 weeks, which was of major concern due to the established adverse prognostic effects of delay to start of radiotherapy and overall radiotherapy duration [4, 5]. Although three junctions were planned between the spine and the skull only one junction was treated per fraction, the junction being moved on sequential days. This enabled the NCCT to participate in international trials, but led to complexity in the use of the record and verify system due to the limitations of the scheduling facility on that software. The treatment staff were required to enter the treatment room between each of the cranial fields and the spinal field to set up the radiation fields with the appropriate compensators and blocks. The repeated staff entries into the treatment room required 30 min appointments to be scheduled even for younger patients. Finally, there were concerns about the lifting and handling of the blocks and compensators by the treatment staff.
An opportunity to re-evaluate the technique arose with the installation of a dedicated CT scanner for treatment planning and a Siemens Primus (Siemens AG, Erlangen, Germany) linear accelerator with autosequence capability and an electronic interface to a modern record and verify system The linear accelerator which has multileaf collimators (MLCs) with low transmission and interleaf leakage [6] was commissioned for both conventional and small segmented low dose treatment delivery in preparation for delivering intensity modulated treatments [7]. The three-dimensionally (3D) planned craniospinal radiation technique described below has been developed to enable the routine planning and check systems in place within the dose planning section at NCCT to be used when developing the treatment plan.
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Methods and materials
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Patient position and immobilization
The standard patient position is prone lying over a polystyrene block, 5–7 cm thick to straighten the spine, and with the head immobilized in a BDS. The immobilization device is registered on the CT scanner, simulator and linear accelerator couch. The head position is extended and the shoulders pulled down to maximize the distance between the chin and the shoulder. This allows the spinal field to be positioned to avoid beam divergence into the mandibular region and dentition superiorly. It also facilitates the use of a moving junction between the spine and the cranial fields.
Image acquisition
A CT study is acquired on a Siemens Emotion Duo CT scanner, scanning from the base of the spine to the top of the skull with the patient in the treatment position. A slice thickness of 0.5 cm is routinely used, the limiting factor being the height of the patient, the permitted scanning length of approximately 1 m and the number of CT slices that the TPS accepts (160 slices). Sagittal and lateral reference marks are placed on the BDS and also in the mid-thoracic region.
Dose planning technique: spine
The CT data are introduced into the TPS. The clinician outlines a "simple" spinal planning target volume (PTV) consisting of a square box 4–6 cm wide (depending on patient size) placed over the spinal cord/vertebral bodies with 1 cm margin beyond the pedicles, to cover the spinal cord and nerve roots (Figure 1
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The caudal aspect of the volume must extend to 1 cm below the termination of the thecal sac. This may be as low as the third sacral vertebra and should be defined with reference to a sagittal spinal MR scan [8]. Placement too high may lead to treatment failure, too low to an increased dose to the pelvis (especially the gonadal tissue). A broader "sacral spade" caudally is not used, as there is no evidence that omitting it compromises control, and its inclusion will increase the pelvic dose. Volumes of interest (VOIs) outlined for dose statistics are the spinal cord, kidneys, lens, optic nerves, optic chiasm, pituitary and thyroid gland.
Set-up parameters
Two spinal techniques are used, the first where the length of the spine is <45 cm, the second where the length is 
45 cm. The set-up parameters are designed to be consistent for both spinal field techniques. The treatment is planned for 6 MV photons with a source–skin distance (SSD) of 130 cm. This facilitates the treatment of large spinal fields and the use of small asymmetric radiation fields. The couch is positioned at 90°, to avoid collision with the portal imaging device mounted on the linac gantry when the SSD is 130 cm. A collimator angle of 90° is required to facilitate the use of wedged asymmetric fields.
Technique 1: spinal technique where the length of spine is <45.0 cm
Planning is undertaken in the sagittal plane on multiplanar reconstruction (MPR) through the spinal cord. A single asymmetric field is selected with a maximum collimator position of 20 cm (100 cm SSD) caudally and 14 cm (100 cm SSD) in the cephalic direction. These values are determined by the maximum overtravel beyond midline permitted by the Y jaw and are intended to limit the minimum segment size to 4 cm (100 cm SSD). The actual field dimensions are determined so that the superior edge exits below the level of the lower dentition/mandible and the inferior edge includes the whole of the PTV. The normalization point is in the mid-thoracic region in the centre of the spinal cord, except for PNET 4 when the normalization point is on the anterior border of the cord. Multileaf collimators (MLCs) are used to conform the field to the shape of the PTV, avoiding where possible the kidneys. A moving junction at three positions is achieved by copying the initial field and reducing the superior collimator edge by 0.75 cm (100 cm) and 1.5 cm (100 cm), respectively, to achieve 1 cm between each junction on the surface of the patient (Figure 2).
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Figure 2. Multiplanar reconstruction(MPR) showing the uncompensated isodose distribution achieved with three large spinal fields intended to permit a moving gap. The 105% isodose line is orange, the 100% isodose line is green and the 90% isodose blue.
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A dose distribution is calculated on the MPR. In general a variation of greater than 5% in dose is noted with a significant decrease in the upper cervical and mid-lumbar region (Figure 2
). Maintaining the SSD and gantry angle, asymmetric radiation beams (segments) are then added to achieve a more homogeneous dose, wherever the dose varies by more than 4% from the dose at the normalization point. The minimum segment size permitted routinely is 4.0 cm (100 cm) and the beam weight for each segment is of the order of 5%. The number and magnitude of these segments depends on the position and anatomy of the patient. However, an additional segment is usually required to increase the dose in the upper cervical region, and another single segment in the lower lumber region (Figure 3). The addition of segments to the upper cervical region is most complex as they have to be added to each of the three large beams, maintaining the superior edge of each and with the inferior edge positioned at the 96% isodose as seen on the MPR (Figure 3). It is not necessary to achieve an isodose of 100% at the superior edge, as the absolute dose is supplemented by scatter from the cranial fields. A typical dose distribution with all the added segments is shown in Figure 4.
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Figure 3. A typical field arrangement for a spinal field<45 cm in length. The central axis is indicated by the dashed green line. The extent of the maximum radiation field is indicated by the solid green lines. The extent of the additional segment in the upper cervicothoracic region is indicated by blue lines, that in the lower lumber region by the yellow lines and the three segments added to the upper cervical region are defined by the red lines. The solid red line indicates the position of the lower limit of the three segments, the dotted lines indicate the position of the upper extent of the fields which also corresponds to the positions of the moving junctions.
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Figure 4. Multiplanar reconstruction(MPR) showing the compensated isodose distribution with the 110% isodose line in yellow, 100% isodose in green and the 90% isodose blue.
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Technique 2: spinal technique where the length of spine is 45.0 cm
If the length of the spine is greater than 45 cm then it is not possible to use the asymmetric segments in the cervical cord region because of the limited overtravel of the Y jaws and only two junctions are employed. Planning is again undertaken in the sagittal plane on a MPR. In this case the initial field is positioned with a maximum collimator setting of 40 cm (100 cm SSD) giving a maximum field size of 52 cm at 130 cm SSD (Figure 5). The second junction is achieved by copying the initial field and reducing the superior edge by 0.75 cm (100 cm) to achieve 1 cm between each junction on the surface of the patient. Instead of asymmetric segments, small wedged asymmetric fields termed "beamlets" are used. To achieve the correct position of the beamlets, the couch is moved longitudinally from the initial position and the gantry angle adjusted to ensure the divergence of the beamlets coincides with the divergence of the large open spinal fields. It is accepted that the SSD of the beamlets differs from 130 cm SSD. The normalization point is positioned as for technique 1 and the MLCs used to modify the shape of the field. A dose distribution on the MPR is calculated. Areas of low dose in the thoracolumbar region can be increased by additional segments in a similar way to technique 1 and in the cervical region using beamlets (Figure 6).
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Figure 5. For a spinal field45 cm a multiplanar reconstruction (MPR) showing the uncompensated isodose distribution, with the 115% isodose line in dull yellow, 100% isodose in green, the 90% in blue and the 85% isodose in grey.
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Figure 6. A typical field arrangement for a spinal field45 cm. The central axis of the open fields is indicated by the solid green line. The extent of the open radiation fields is indicated by the dashed green lines, the two junctions in the upper cervicothoracic region. The extent of the two additional segments, one in the thoracolumbar region and the other in the lumbosacral region is indicated by the dotted yellow lines. The central axis of the asymmetric beamlets is indicated by the solid blue line. The limits of the three pairs of beamlets in the cervicothoracic region are indicated by the pairs of dotted blue lines.
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Whilst three junctions would be considered optimal for this technique, the treatment delivery time is significantly increased by the use of wedged "beamlets", which are delivered at a lower dose rate than the open field segments. The use of two junctions is considered a reasonable compromise between the requirement to reduce dose inhomogeneities in the region of the junctions and the time of treatment delivery which requires cooperation from the patient to minimize movement during treatment.
Cranial fields
The technique used for the cranial fields is isocentric, with gantry angles 90° and 270°, couch 0° and an initial collimator setting of 22.0 cmx22.0 cm. The opposed cranial fields are defined using the MPR. The collimator rotation is adjusted to match the angle of beam divergence of the cephalic half of the spinal field. The field is then modified such that the inferior edge of the cranial field abuts the superior edge of the spine field. The collimator setting is adjusted asymmetrically, if appropriate, to encompass the whole skull with a 1.5 cm margin.
All meningeal surfaces are covered by the cranial fields and particular care is required with coverage of the temporal fossae anteriorly. A 1 cm margin below the meninges with a 0.5 cm margin at the cribriform plate is standard acceptable practice.
Using a beam's eye view (BEV) the clinician defines the shape of the area to be shielded using a "help contour" and an initial fit of the MLCs is made to the contour. The treatment area and dose distribution are assessed on each axial slice to ensure that the field coverage is optimal and that there are no areas of underdosage. Where necessary the MLCs are manually adjusted to achieve the required dose distribution.
Once the position of the facial shielding has been defined for spinal technique 1, each field is copied twice and the inferior edge of each pair adjusted asymmetrically to match the superior edge of the maximum and minimum field length at the centre of the spinal cord. Spinal technique two requires the fields to be copied once as only two junctions are accommodated.
The MPR distribution will show an inhomogeneous dose distribution, with an area 110% in the periphery of the skull, a large volume of skull receiving 105–110%, and an area around the isocentre receiving 100% (Figure 7). Additional segments are added to achieve a homogeneous dose. The MLC shaped segments are designed to match the 110% isodose and the 105% isodose, respectively (Figure 8). Beam weights for each segment are between 5% and 10%. The isodose distribution is assessed in the sagittal MPR and also on each transverse slice.
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Figure 7. The uncompensated isodose distribution for the cranial fields, with the 110% isodose in yellow around the periphery of the skull, 105% isodose in orange covering most of the cranium and the 95% isodose in pale green.
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Figure 8. The segmented cranial fields used to achieve a homogeneous dose distribution. One pair of segments closely follows the 110% isodose line and the second pair closely follows the shape of the 105% isodose line as shown in Figure 7.
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Once a satisfactory cranial dose distribution has been achieved in the MPR the cranial and spinal plans are summated and the overall dose distribution assessed to ensure no areas of under- or overdosage (Figure 9). It is important to ensure the maximum dose to the lower dentition/mandible does not exceed 3 Gy. The dose to the lens should be below 6 Gy, if possible, but coverage of the cribriform plate may not permit this. Coverage of the PTV takes priority over lens tolerance as cataract is treatable whereas recurrent PNET is usually fatal.
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Figure 9. A multiplanar reconstruction(MPR) demonstrating for a spinal field <45 cm the summated compensated cranial and spinal fields with the 100% isodose in green and the 105% in orange.
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Posterior fossa boost for medulloblastoma
The posterior fossa remains the most likely site for relapse, hence PTV definition and adequate treatment is extremely important. Target defining errors may occur superiorly as the tentorium must be included and this is not easily visible on either lateral orthogonal radiographs or CT images. MR studies have refined the anatomical landmarks for the tentorium and the superior PTV edge but remain in part subjective [9, 10]. Our practice of CT based posterior fossa PTV definition depends on direct comparison with coronal MR images and permits the definition of the cochlea bilaterally. Persuasive evidence for radiation induced hearing damage exists [11] and cis-platinum ototoxicity is well recognized. Planning studies have shown that cochlear sparing can be achieved, but we acknowledge that there may be some risk as a result of tight margins on the meninges of the adjacent petrous temporal bones [12]. Since the Packer regimen with post-radiotherapy, relatively high dose cis-platin has been adopted as standard chemotherapy and cochlea avoidance has become an important issue. Additional work investigating field arrangements suggests that the optimal field arrangement may be conformed coplanar posterior oblique fields as this minimizes dose to the cochlear, pituitary and supratentorial brain [13]. It is our practice to use posterior oblique fields, provided PTV coverage is not compromised. Although there are no large-scale prospective data on the benefits of cochlear sparing in a lateralized primary it is the oncologist's discretion as to whether to practice contralateral cochlear avoidance (assuming a good margin from the primary can be achieved).
Multisegmented field delivery
Data preparation prior to the first treatment fraction
The data are checked and prepared for treatment delivery following standard protocols. It is important to minimize the time from the initiation of the radiation exposure to completion of the delivery of all the segments. Two factors were considered: the removal of segments with a small number of monitor units and the most time efficient sequence for the delivery of the segmented fields. When tested, the removal of single segments with 2 MU made less than 8 s difference as the most significant proportion of time was that required to move the collimators to the correct position. Resequencing all the segments to minimize the number of large collimator settings between segments reduced the treatment time by 15 s whilst maintaining the integrity of the planned treatment.
Treatment verification
Prior to treatment, verification is carried out on the simulator as for the original technique. The extent of the radiation fields and the position of the mid-junction of the cranial and spinal field are marked on the beam directional shell. At the first treatment fraction portal imaging of the cranial fields is undertaken. At an SSD of 130 cm it is not possible to verify the position of the spine using the current portal imaging facilities. A new verification procedure is under development using an in-room CT scanner.
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Treatment delivery
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At the start of each treatment fraction the patient is positioned on the couch and the matchplane is verified visually. The mid-spine field and the mid-cranial fields are downloaded sequentially and the position of the edges of the light fields checked against the marks on the beam directional shell before commencing treatment delivery. The absolute couch coordinates are verified using the record and verify system. The two parallel opposed cranial fields are autosequenced, including the gantry movement, and provided the patient is cooperative the treatment staff do not enter the treatment room. On completion of the cranial fields, the treatment staff re-enter the room to rotate the couch through 90°, set up the spinal fields to 130 cm SSD and recheck the position of the junction. The second autosequence of radiation fields for the spinal fields is then delivered. The appointment times for cooperative patients, who require minimal treatment staff entry, are scheduled for 15 min. Young children who are not anaesthetized may require longer.
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Results
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The following advantages of the new technique have been demonstrated: (1) All junctions between the skull and spinal fields are treated each fraction, so that there is no daily variation in the treatment delivered. (2) The overall appointment time is reduced for the majority of patients and does not, except in exceptional circumstances, exceed that of the previous technique. (3) Whilst the treatment staff enter the room once during the treatment, they only do so to reposition the patient; there is no manipulation of blocks or compensators. (4) The reduction in number of treatment staff visits to the treatment room during a treatment fraction means that the patient is not distracted and is considered to remain in the treatment position with no problems. (5) There are no lifting and handling issues for treatment radiographers who previously had to manoeuvre blocks and compensators safely whilst the patient remained in the treatment position. (6) The shorter appointment time is particularly advantageous for patients who are anaesthetized as they require a shorter anaesthetic time compared with the previous technique. (7) Whilst the irradiation time for the new spinal technique is longer than that employed for the single junction technique, this increase in time is considered to be compensated for by the improved treatment technique using three junctions daily and the autosequencing which facilitates the reduced overall treatment appointment time.
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Discussion
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The use of CT based planning has resulted in improved organ definition. CT/MRI matching allows accurate delineation of clinical and planning target volumes which is particularly important for the posterior fossa PTV definition for medulloblastoma patients.
CT based localization instead of simulator films has greatly reduced the amount of time spent by the patient in the department during the planning stage, as planning can be carried out "virtually". The CT scan takes approximately 20 min whereas conventional simulation often took over 1 h to acquire all relevant radiographs and patient external outlines.
A further advantage is that VOI dose data are available at an early stage in the planning process. Modifications to the position of an MLC, for example, to produce an acceptable lens dose can be achieved without the need to have to decide either to alter customized blocks or to accept inferior treatment rather than delay the start of treatment.
Although the time on the treatment planning system has increased, approximately 1.5 h overall, the use of segmented fields has eliminated the time and personnel resources required to manufacture, check and undertake dosimetry measurements for individual compensators for the spine and skull, which used to be carried out by physicists and clinical technologists over a number of days. Additionally, the use of MLCs to produce facial shielding eliminates the requirement to manufacture individual shielding blocks. There is a positive benefit for treatment staff in the reduction in manual handling of heavy blocks and compensators.
Potential gains have been made in reducing systematic and stochastic set-up variation with the use of MLCs and segments, moving junctions intrafraction and the use of autosequencing for treatment delivery.
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Conclusion
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A technique has been developed for craniospinal radiotherapy which complies with the requirements of international studies, improves the patient experience during the planning and treatment processes by reducing the initial treatment preparation time and appointment times, has the potential to reduce the time from initial planning to start of treatment and reduces manual handling for the treatment staff.
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Acknowledgments
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The authors would like to acknowledge the contribution of Dr Rob Turner (now at Cookridge Hospital, Leeds) and the dosimetrists in the dose planning section of RMPD in the preparation of this paper.
Received for publication February 9, 2006.
Revision received June 2, 2006.
Accepted for publication June 23, 2006.
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References
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Abstract
Introduction
