|Year : 2018 | Volume
| Issue : 1 | Page : 16-22
Craniospinal irradiation by rapid Arc® technique in supine position: A dosimetric and clinical analysis
Vijayaprabhu Neelakandan1, S Sherly Christy2, Ashutosh Mukherji1, K Sathyanarayana Reddy3
1 Department of Radiotherapy, Regional Cancer Centre, JIPMER, Puducherry, India
2 Department of Physics, Anna University, Chennai, Tamil Nadu, India
3 Department of Oncology, Mahatma Gandhi Institute of Medical Sciences and Research Centre, Puducherry, India
|Date of Web Publication||18-Jun-2018|
Dr. Ashutosh Mukherji
Department of Radiotherapy, Regional Cancer Centre, JIPMER, Puducherry - 605 006
Source of Support: None, Conflict of Interest: None
Introduction: In craniospinal irradiation (CSI), prone position has been commonly used but, in some patients, especially pediatric cases where anesthesia is needed prone position may not help. In planning CSI by supine technique, beam geometry and field matching have to be considered, and immobilization is essential to ensure reproducibility of treatment. Materials and Methods: Data of four patients (3 with medulloblastoma and 1 with lymphoma) treated between March 2012 and October 2013 were included in this retrospective dosimetric study and analyzed. Patients were evaluated for dose coverage, organs at risks (OARs) sparing, number of monitor units, and daily table position shifts. Results: All patients underwent CSI by volumetric-modulated arc therapy (VMAT) technique in supine position. All four cases developed Grade 1 skin changes, and only one case developed a Grade 2 change at the end of radiotherapy; also both pediatric cases developed Grades 2 and 3 anemia and leukopenia toward their 4th week of treatment onward. The Rapid Arc®-CSI plans were able to generate dose distributions with high planning target volume (PTV) conformity and homogeneity and with sparing of OAR. The cumulative conformity index for all patients was 0.986, and homogeneity index was 0.1007. The mean PTV doses were within 108% with V110% of <12%, and V107% was 20%. On the evaluation of patient setup errors, a maximal shift of 3 mm in longitudinal direction was noted. At 3 years, all medulloblastoma cases except 1 were in remission. One adult patient with medulloblastoma had multiple spinal metastases at 1 year. Conclusions: Treatment of patients in supine position is reproducible and easily maintained with minimal acute reactions. VMAT technique helps avoid junctions, field-matching, and benefits with image guidance for precise dose delivery, conformity, and OAR sparing.
Keywords: Central nervous system lymphomas, craniospinal irradiation, medulloblastoma, supine craniospinal irradiation, volumetric modulated arc therapy craniospinal irradiation
|How to cite this article:|
Neelakandan V, Christy S S, Mukherji A, Reddy K S. Craniospinal irradiation by rapid Arc® technique in supine position: A dosimetric and clinical analysis. J Curr Oncol 2018;1:16-22
| Introduction|| |
Craniospinal irradiation (CSI) is indicated in tumors with high risk of having craniospinal axis involvement., The CSI fields consist of isocentric parallel-opposed lateral cranial fields and one or two direct posterior spine fields with matched junctions. For CSI, the prone position is the standard method of patient positioning. The prone position has been commonly used by radiation oncologists to confirm craniospinal junction and between the superior and inferior spine fields by direct visualization. Thus, overlapping between the matching fields can be easily avoided. However, prone position is not a good option in some patients, especially pediatric cases where there is need for anesthesia for immobilization and then maintenance of airway during general anesthesia (GA). In addition, the prone position is more difficult to reproduce daily and to ensure patient's cooperation during setup and treatment.
In planning CSI by supine technique, beam geometry and field matching have to be carefully considered. Careful positioning of the patient and optimal placement of the junction is important to avoid over or under dosage, and immobilization is essential to ensure reproducibility of treatment during fractions. In conventional CSI, beams were matched either by gap junctions or feathering of beams at the junctions. In many centers, a moving junction (feathering) between the brain and spine field is used to reduce the chances of under- or over-dose in the cervical spinal cord., Parker et al. have recently reported the feasibility of conventional linear accelerator (LA)-based intensity-modulated radiotherapy (IMRT) for CSI in small children. However, even with this advanced technique, matching of cranial and spinal fields is a difficult process in LA-based IMRT for CSI; especially in older children where a separate field may have to be added for lower spine.
Rapid Arc ® (RA) volumetric-modulated arc therapy (VMAT) is a form of radiotherapy that uses dynamic rotational therapy to deliver a highly conformal dose to the planned target volume. During treatment delivery, the multileaf collimator (MLC) positions, gantry speed, and dose rate continually change which allow for faster treatment times and will also deliver a dose distribution similar to other helical techniques. Lee et al. conducted a study to compare VMAT-CSI with traditional 3-dimensional conformal radiotherapy (3DCRT)-CSI. It was found that VMAT CSI plans created greater conformity and homogeneity, with a more uniform dose distribution specifically around the vertebral column and hence cause reduced incidence of uneven bone growth in children receiving CSI. In this study, we analyzed the radiation dose distribution and plan fidelity of CSI by RA ® technique in supine position on the central nervous system axis and surrounding normal organs.
| Materials and Methods|| |
The computed data of four cases of which three patients had diagnosis of medulloblastoma and one case was a spinal lymphoma treated between March 2012 and October 2013 were included into this retrospective dosimetric study and analyzed postradiotherapy. Patients were placed in the supine position and fixed by a VacLoc body immobilizer and thermoplastic head mask [Figure 1]. Computed tomography (CT) simulation was done using a two-slice CT-simulator. Treatment planning was based on a series of consecutive CT slices. All patients were planned on Eclipse Treatment Planning System (Varian Medical Systems, Palo Alto, CA, US) using Millennium-120 MLCs for beam shaping. RA ® plans were generated for 6 MV photons with progressive resolution optimizer Version III and volume dose calculation by the anisotropic analytic algorithm with a resolution of 2.5 mm × 2.5 mm. Treatment was delivered on Clinac iX LA (M/s Varian Medical Systems, Palo Alto, USA) using dynamic MLCs. Radiotherapy was delivered using standard fractionation (5 fractions per week) with a daily dose of 180 cGy for craniospinal axis.
|Figure 1: Patient immobilized in treatment position with thermoplastic mask and VacLoc|
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Plans were analyzed for planning target volume (PTV) dose coverage, organs at risks (OARs) dose sparing, overall maximum dose, and total MUs required. Cumulative dose-volume histograms (DVHs) for PTV and OARs were computed for analysis. In addition, daily table position shifts done were noted, and the data were analyzed. The treated patients were assessed for skin and hematological toxicities, response, and treatment breaks.
| Observations and Results|| |
Three patients had histologically proven medulloblastomas, while one case was that of a spinal lymphoma with craniospinal fluid involvement postchemotherapy with complete remission. In two medulloblastoma patients, the site involved was the 4th ventricle; while in the third patient, the site involved was the vermis. Two patients were young adults while the other two patients were children but above 5 years' age. Three patients presented with symptoms of headache and visual loss. Two cases of medulloblastoma were classified as having standard risk while one case of medulloblastoma (a young adult) presented to us more than a month after her surgery and a fresh magnetic resonance imaging (MRI) showed tumor regrowth. A neurosurgery opinion was taken regarding reexcision, but was deemed not operable. The patient was therefore staged as “High risk” and treatment planned accordingly. All three medulloblastoma patients had undergone ventriculoperitoneal shunt followed by surgery. Procedures involved craniotomy and gross total excision in two cases and near total excision in one case. There was no residual disease immediate postsurgery in any case. The lymphoma case was a young male presenting with paresthesias and diagnosed as having spinal lymphoma. He had completed chemotherapy and was in complete remission and was planned for CSI as consolidation therapy. The earliest patient, an 11-year-old boy, started treatment in March 2012; while the last studied patient, a 23-year-old young man completed treatment in October 2013.
Plans for RA were optimized by selecting those plans that gave acceptable PTV coverage with minimum OAR volume doses. For all RA plans, collimator was set to 181° with a limited opening, which allowed finer modulations for each arc. Two arcs with two isocenters were sufficient in three patients, but one patient required four arcs [Figure 2]. Description of each patient setup is given in [Table 1]. The PTV for the cranial CSI field included the whole brain and the meningeal reflections till the level of the foramen magnum. The PTV for the spinal part of the CSI fields included the spinal cord and the thecal sac. Details of isocenter used in each patient and their positions are given in [Table 1]. Two coplanar full arcs, with opposite directions of rotation (clockwise, CW, and counterclockwise, CCW), were used to cover the superior portion of the PTV (brain and upper portion of the spinal cord). A single partial arc was used to cover the inferior portion of the PTV (remaining part of spinal cord). Care was taken to prevent as far as possible beams from entering the PTV through the eyes. A conformity structure was created as a shell around PTV (2 cm volumetric expansion beginning from the PTV) to improve dose conformity and control dose gradient outside PTV. Both arcs were optimized together at same time, and plans were optimized by dose-volume constraints to PTV, OARs, and control structures.
|Figure 2: Collimator and beam setup for volumetric-modulated arc therapy-craniospinal irradiation|
Click here to view
Treatment given and complications
All patients underwent adjuvant CSI by VMAT technique in supine position, with concurrent weekly Vincristine in two cases of medulloblastoma. Two of the treated patients received a dose of 3600 cGy to the whole craniospinal axis with a boost to the posterior fossa to a total dose of 4500 cGy. They then received a further boost of 1300 cGy for a total dose of 5800 cGy to the tumour bed with sparing of brainstem beyond 4500 cGy. Another patient was irradiated to a dose of 2340 cGy to whole craniospinal axis followed by posterior fossa boost up to 4500 cGy and then tumor bed boost up to 5580 cGy. The patient with lymphoma received a craniospinal axis dose 3060 cGy and no boost.
The planned radiotherapy course was for 44 days in each case of medulloblastoma and 23 days for lymphoma case; among the medulloblastoma cases, one patient completed treatment in 55 days, another in 53 days, and the last in 48 days. The patient with lymphoma completed therapy in 28 days. We found that the pediatric cases took longer time due to treatment breaks caused by anemia and leukopenia due to extensive field and chemotherapy. An assessment was made of acute sequelae encountered in these cases during their radiotherapy course. We found that all cases developed Grade 1 skin changes and only one case (the adult medulloblastoma case) developed a Grade 2 change toward the end of her radiotherapy course. These reactions developed typically after 3rd week of radiotherapy and were mostly self-limiting and resolved within 2 weeks of completion of radiotherapy [Figure 3].
In view of extensive treatment volumes, all patients developed acute hematological sequelae during their radiotherapy course, the grade of which was more severe in the pediatric cases. Both the pediatric cases developed Grades 2 and 3 anemia and leukopenia, especially towards their 4th week of treatment onward. The adult cases developed anemia and leukopenia from the 3rd week of treatment, but it was only Grade 1. Grade 3 anemia and leukopenia were seen in the 11-year-old boy at 3rd week of treatment and resulted in a week long break in treatment. The lymphoma patient at end of 4th week radiotherapy developed Grade 1 oral mucositis which was transient and resolved in next 3 days.
Plans were normalized so that at least 95% of PTV was covered by 95%–98% of prescription dose. Evaluation of plans was performed by DVH analysis of the target volume and relevant OARs. For the PTV, the dosimetric parameters analyzed included mean dose, D1cc, D2cc, D2%, D5%, D95%, D98%, V95%, V107%, and V110%, wherein D1cc was defined as maximum dose to 1 cm 3 of PTV, and Vy % was defined as the percentage of PTV receiving at least y% of the prescribed dose [Table 2]. Conformity of the prescription dose to the PTV was expressed by the conformity index (CI), which represented the volume of the PTV receiving more than 95% of the prescribed dose divided by the volume of the PTV i.e., V95/VPTV. The homogeneity index (HI) of the PTV was defined as the ratio of the difference of D2% and D98% to D50% i.e., (D2–D98)/D50 for the PTV, where D2%, D50%, and D98% corresponded to the dose delivered to 2%, 50%, and 98% of the PTV, respectively. The delivered monitor units ranged from 511 to 694 in all cases. For the relevant OARs, the mean and maximum doses for each organ were reported for dosimetric comparison [Table 3]. As we can see from this table, the dose constraints for all critical OAR have been maintained.
The RA-CSI plans were able to generate dose distributions with high PTV conformity and homogeneity [Figure 4]. The cumulative CI for all patients was 0.986 (standard deviation [SD] ± 0.005), and the HI was 0.1007 (SD ± 0.052). The mean PTV doses were within 108% with V110% of <12%, and V107% at 20%. The PTV coverage was found to be excellent with mean D98 being 98.47% (SD ± 6.06), mean D95 was 100.6% (SD ± 4.69) and D2 or Dmax being 112.3% and SD ± 7.82. The point maximal dose was on average found to be 115% ± 11.65% [Table 2]. On evaluation of patient setup errors, a maximum shift of 3 mm in longitudinal direction was noted. The vertebral column was covered within 50% isodose of PTV and spinal canal in 80% which is consistent with reviewed literature. Mean doses to both eyes were restricted to planned values of 16 Gy or below; at follow-up of more than 1 year no visual loss or sequelae have been noted [Table 3]. Similarly, planned mean doses to heart, lung, thyroid, and other organs have been achieved.
Response and follow-up
All cases at the end of radiotherapy showed complete response. MRI scans taken at 3 and 6 months postradiotherapy showed complete resolution and no fresh recurrence in all cases. Two cases; the 6-year-old girl who received 2340 cGy CSI and the adult medulloblastoma patient with recurrent disease received further maintenance chemotherapy with lomustine, cisplatin, and vincristine. Both patients have completed six chemotherapy cycles and the child is still in remission. The lymphoma patient is symptom free and is on follow-up. The adult medulloblastoma patient with recurrent gross disease who was classified as “High-Risk” was in remission for 1 year when she presented with sudden onset loss of motor power. An MRI of the craniospinal axis done revealed multiple spinal deposits intramedullarly. Patient has been planned for further chemotherapy. Therefore, at 3 years follow-up, two out of the three medulloblastoma cases have been in remission since (while the lymphoma patient has completed 2 years and 8 months in remission).
| Discussion|| |
Conventionally, CSI fields have consisted of isocentric parallel-opposed lateral cranial fields and one or two direct posterior spine fields with matched junctions. The prone position has been the standard method of patient positioning., Since the target volume encompasses the whole cranium and spinal cord, as well as subarachnoid space; the use of multiple fields is generally needed. Careful matching of multiple fields in CSI is essential. Cold spots at the junction area may lead to disease recurrence; while significant hot spots can cause overdose on the spinal cord leading to severe complications.
In a study by Lee et al., it was found that VMAT CSI plans created greater conformity and homogeneity, with a more uniform dose distribution specifically around the vertebral column and hence cause reduced incidence of uneven bone growth in children receiving CSI. Lee reported a median CI of 1.22 (1.09–1.45) which was similar to our study. There was a lower mean dose to vital organs such as heart, thyroid, and esophagus of at least <4 Gy with VMAT. This significant reduction in dose to OAR meant reduced late side effects and overall increased quality of life. Another important advantage of VMAT compared to IMRT is the relative ease of implementing the therapy. The time required to deliver treatment in VMAT is reduced from 10 to 16 min for IMRT or tomotherapy to <5–7 min. This results in less time for potential errors due to respiratory or other physiological motions while the patient is on the treatment couch. This was also seen in our study as well as in another study by Chen et al.; where there was a maximal longitudinal shift of 3 mm in daily table variation.
Although the prone position has long been the standard position in CSI technique, the supine position is needed, especially in young patients who need sedation or GA. The prone position provides a direct visualization of field matching and it may be easier for patient setup. However, increasing use of virtual simulation by CT simulation makes it easier to place the spinal field in the supine position. A few reports about the reliability and convenience of supine position in CSI are now available.,, The supine position, in addition, may be the only workable option in children <6–7 years of age who require CSI with intubation. While treating supine, patients are more comfortable, treatment setup more reproducible, and treatment better tolerated. The use of CT simulation significantly decreases both simulation and daily treatment times. CT images provide a better definition of critical organs and help improve field placement and shielding accuracy.,, Parker and Freeman state that with supine patient positioning, the time for CT simulation can be reduced from up to 2 h (with conventional simulation) to about 30 min. The supine technique is much better tolerated and more stable than the prone technique and results in a reduced daily treatment time by up to 15 min. Our study provided a satisfactory result with supine position CSI. In fact, patients were more comfortable, and treatment setup was more reproducible with fewer errors, reducing the treatment delivery time.
Improvements in long-term survival, particularly in children with average-risk medulloblastoma, have led to growing concern for treatment-related long-term side effects and risk of second malignancies. The majority of these late effects are dose and volumes related and have resulted in guidelines for reduced dose CSI for average-risk disease in conjunction with chemotherapy. Two major concerns often raised over the use of IMRT, especially in children, are the increase in whole-body dose and the larger volumes of normal tissues irradiated at relatively lower radiation doses when compared with conventional radiotherapy. These two factors can potentially increase the risk of radiation-induced carcinogenesis, particularly in children and long-term survivors., Very recently, Sharma et al. proposed the use of age- and gender-specific risk coefficients for estimating the risk of radiation-induced carcinogenesis. VMAT technique is designed to use MUs more efficiently, and delivered MUs are similar to that of 3DCRT and much less than conventional IMRT. Therefore, VMAT is thought to have less risk of secondary cancers for patients with expected longer survival when compared with conventional IMRT as was shown in the study by Lee et al. Our study as well as those by Chen et al. and Tongwan et al. showed that RA can achieve highly conformal dose distributions comparable to tomotherapy or IMRT, with much fewer MUs than IMRT.
| Conclusions|| |
Irradiation of the craniospinal axis of patients in the supine position is reproducible and easily maintained. Acute skin, hematological, and gastrointestinal reactions were comparable with declared historical literature. Treatment in supine position would be comfortable for adult patients and would minimize anesthesia-related risks.
RA ® technique is suited to plan complex-shaped target volumes, avoiding junction, field-matching, and abutment dosimetry. It also provides benefit of image guidance for precise dose delivery and is favorable regarding target volume coverage, dose homogeneity, conformity, OAR sparing, treatment time, and reduction of IDs to nontarget tissues.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3]
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