Dosimetric comparison of short and full arc in spinal PTV in volumetric-modulated arc therapy-based craniospinal irradiation

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Dosimetric comparison of short and full arc in spinal PTV in volumetric-modulated arc therapy-based craniospinal irradiation Biplab Sarkar, M.S., Dip. R.P., Ph.D. ∗,∗∗, Anusheel Munshi, M.D., D.N.B., M.N.A.M.S. ∗, Tharmarnadar Ganesh, Ph.D., D.A.B.R. ∗, Arjunan Manikandan, M.S., Dip. R.P. †, Bidhu K. Mohanti, M.D. ∗ ∗ †

Radiation Oncology, Manipal Hospitals, New Delhi, Delhi 110075, India Radiation Oncology, Nagarjuna Hospitals, Vijayawada, Andhra Pradesh 520007, India

a r t i c l e

i n f o

Article history: Received 2 November 2018 Revised 8 March 2019 Accepted 22 March 2019 Available online xxx Keywords: VMAT CSI Medulloblastoma PNET Short arc Conformity 3DCRT

a b s t r a c t Since 2011 when it was first described, the volumetric-modulated arc therapy (VMAT) technique for craniospinal irradiation (CSI) has always seen the use of large arc lengths for the spine fields ranging from 200° to 360°. This study was aimed to do a dosimetric comparison between the large and shorter spinal arc for CSI. For a cohort of 10 patients, 2 VMAT CSI plans were created for each patient, one using the conventional full 360° arc (VMAT_FA) for the spine and the other using 100° posterior arc (VMAT_PA) for 23.4 Gy and 35 Gy prescriptions. In both the plans, 360° arc fields were employed for treating cranial volume. Spillage dose (DBody-PTV ) to Body-PTV (DBody-PTV : dose to body excluding planning target volume) was compared with VMAT_FA and VMAT_PA plans. In addition to these VMAT plans, a 3-dimensional conformal radiotherapy plan was also created for all these patients to compare the DBody-PTV and target volume related dose constraints. Mean D95% difference between the two VMAT plans did not exceed 1.3% for cranial and spinal targets for both prescription levels. The conformity index (CI) was averaged over both prescription doses. Average CI shows a similar value for VMAT_FA (0.84 ± 0.04) and VMAT_PA (0.82 ± 0.05) plans. D95%, V110% and CI did not exhibit a statistically significant difference between partial and full-arc VMAT plans. However, the VMAT_PA plan exhibited a lower DBody-PTV compared to VMAT_FA plans (0.007 ≤ p < 0.05) in the 1 to 5 Gy range. Nevertheless, partial arc plans could not offer a statistically significant dose reduction for delineated organs compared to full arc plans, except for bilateral kidneys. © 2019 American Association of Medical Dosimetrists. Published by Elsevier Inc. All rights reserved.

Introduction Medulloblastoma and primitive neuroectodermal tumors are common pediatric tumors treated by craniospinal irradiation (CSI). Radiotherapy techniques include 2D technique, 3-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiotherapy, and volumetric-modulated arc therapy (VMAT).1-4 Conventional techniques requires multiple junction shifts to neutralize hot/cold spots at the junction.2 Several authors have described intensitymodulated radiotherapy-based CSI technique.1 In recent times, VMAT-based CSI technique is gaining popularity and wide acceptance.3-14 Field junctions pose formidable challenges in the conventional planning of CSI. The advantage of the VMAT technique

is that it does not require any junction shifts since it employs overlapped fluence pattern between brain-spine and spine-spine fields to take care of dose matching in the junctions.6 , 10-11 , 13 At our tertiary cancer care center, we are practicing a VMAT-based CSI technique since October 2013. In contrast to the techniques described in the literature, we use a shorter posterior arc span of 100° for spinal isocenter(s).11-13 The underlying rationale is the reduction in spillage of dose to normal tissue (Body-PTV) and dose to the specified organs at risk (OARs). The aim of this study is to compare partial and full-arc VMAT plans in terms of target volume coverage, OAR doses, and spillage of dose to normal tissues for supine CSI. As a corollary, a dosimetric comparison with 3DCRT plans has also been done in the study. Materials and Methods

∗∗ Reprint requests to Biplab Sarkar, M.S., Dip. R.P., Ph.D., Radiation Oncology, Manipal Hospitals, New Delhi, Delhi 110075, India. E-mail address: [email protected] (B. Sarkar).

A total of 10 patients (mean ± SD age 14.4 ± 8.5 years) who were treated for medulloblastoma/primitive neuroectodermal tumors in our clinic using shorter spinal arcs (VMAT_PA) were incorporated in this study. For all patients, 3-mm slice

https://doi.org/10.1016/j.meddos.2019.03.003 0958-3947/© 2019 American Association of Medical Dosimetrists. Published by Elsevier Inc. All rights reserved.

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Fig. 1. (A). Immobilization for brain-chest area using 5 clamp thermoplastic for the stable patients with less movement in the thorax-abdomen region. (B). Brain to thorax and abdomen-pelvis immobilization for the unstable patients. (Color version of figure is available online.)

thickness simulation CT was acquired with appropriate immobilization in head first supine position; partial arc, full arc, and 3DCRT plans were carried out in the same image set. Two types of immobilization were used; if patients could keep abdomenpelvis immobilized without any external inhibitor then only upper part of the body (brain to thorax) were immobilized with a 5-clamp thermoplastic (Fig. 1A). For unstable and anaesthetized patient’s abdomen-pelvis were also immobilized along with the brain (Fig. 1B). Contouring consists of brain clinical target volume which included complete brain and the upper portion of the spinal cord ending at the C5-C6 vertebra level. Spinal clinical target volume extended from the junction of C5-C6 level up to the end of the thecal sac. A 7-mm margin was applied to generate the planning target volume (PTV). Contoured OARs included optic apparatus, eyes, oral cavity, parotid, thyroid, larynx, oesophagus, heart, bilateral lung, bowel bag, liver, bilateral kidneys, rectum, bladder, and genitals.12-13 A special structure, PTV subtracted from the body, was created to evaluate the dose to nontarget tissue (DBody-PTV ). If combined craniocaudal length of brain and spine PTV exceeded 60 cm, two spine isocenters were employed, otherwise one isocenter was sufficient. Prescription doses for average and high-risk group patients were 23.4 Gy in 13 fractions (RX D_Low) and 35 Gy in 21 fractions (RX D_High), respectively. Our therapeutically delivered VMAT_PA plans used a 360° brain field along with one or two spine field(s) as per the cranio-caudal PTV length. Spine field was 100° symmetric about 180° position. Optimization for both PTVs was done simultaneously in the synchronous setting, creating a field-overlapping of more than 10 cm.10 , 12-13 Additionally, for this study, we created a VMAT plan with 360° arc spine field called VMAT_FA (full arc) and a 3DCRT plan for comparison with VMAT_PA. All plans, VMAT_PA, VMAT_FA, and 3DCRT plans were created using 6 MV flattened photon beam with a dose calculation bin of 3 mm × 3 mm × 3 mm. Figure. 2 presents the beam arrangement and dose distribution VAMT_PA (panel-A) and VMAT_FA (panelB) plans. The plan acceptance criteria for all plans (VMAT and 3DCRT) was at least 95% of PTV received 95% of prescription dose (D95% ≥ 95% of prescription dose). Both VMAT plans were optimized using 2 independent sets of parameters to avoid any a priori bias towards our clinical practice. The optimization was carried out until the OAR dose could be reduced without compromising on target volume coverage and the increase of excesses dose (≥ 107%); detail can be found elsewhere.11-14 In the 3DCRT plan, we used bi-lateral fields for treating brain and 1 or 2 posterior field(s) depending on the craniocaudal length of the spine PTV was used. If total craniocaudal length of the brain and spine PTV was less than 60 cm, 1 single spine field was used. If the same exceeded 60 cm, another additional field was added inferiorly. Couch, gantry, and collimator angles were adjusted for all fields to avoid field overlapping due to divergence.12-13 Treatment plans were done in Monaco (CMS Monaco, Sunnyvale, CA) (V 5.0.11) planning system. Clinical acceptability of all plans was evaluated by an experienced radiation oncologist. The plans were evaluated in terms of PTV dose coverage D95% (% dose received by 95% of the PTV volume), V110% (% PTV volume receiving excess of 110% dose), V2

5% Paddick conformity index (PI = TV RX ), heterogeneity index (HI= DD95% ), DBody-PTV , and ∗VRI OAR doses where TV is the volume of PTV, VRX is the volume of PTV covered by prescription isodose line, and VRI is the volume of tissue receiving the prescription dose. D5% and D95% are the doses received by 5% and 95% of the target volume, respectively15 . Paired sample t-test was carried out for calculating the statistical significance at 95% confidence interval to compare different techniques.

Table 1 Relative dose and its difference for brain and spine PTV yielded by VMAT_FA, VMAT_PA and 3DCRT techniques

PTV PTV brain_FA PTV brain_PA PTV brain_3D % difference (FA-PA) % difference (PA_3D) % difference (FA-3D) PTV spine_FA PTV spine_PA PTV spine-3D % difference (FA-PA) % difference (PA_3D) % difference (FA-3D) PTV brain_FA PTV brain_PA PTV brain_3D % difference (FA-PA) % difference (PA_3D) % difference (FA-3D) PTV spine_FA PTV spine_PA PTV spine-3D % difference (FA-PA) % difference (PA_3D) % difference (FA-3D)

35 Gy prescription V95% (Mean ± SD)

23.4 Gy prescription

101.1 ± 2.2 100.8 ± 1.7 97.1 ± 1.9 0.3 ± 2.5 3.7 ± 2.5 4.0 ± 2.9 101.5 ± 3 100.1 ± 3.6 96.2 ± 2.2 1.4 ± 4.7 3.9 ± 4.2 5.3 ± 3.7 V110% (Mean ± SD) 1.3 ± 1.7 1.2 ± 1.6 0.04 ± 0 0.1 ± 2.3 1.2 ± 1.6 1.3 ± 1.7 1.2 ± 0.9 1.5 ± 1.3 18.9 ± 5.9 −0.3 ± 1.6 −17.4 ± 6.0 −17.7 ± 6.0

99.5 ± 1.2 100.2 ± 1.5 98.8 ± 4 −0.7 ± 1.9 1.4 ± 4.3 0.7 ± 4.2 101.1 ± 0.05 100.1 ± 1.9 96.4 ± 2 1.0 ± 1.9 3.7 ± 2.8 4.7 ± 2.0 1.3 ± 1.8 3.0 ± 3.7 0.1 ± 0.0 −1.7 ± 4.1 2.9 ± 3.7 1.2 ± 1.8 2.2 ± 3.1 6.0 ± 6.1 19.3 ± 6.8 −3.8 ± 6.8 −13.3 ± 9.1 −17.1 ± 7.5

Results The dose and dose difference for PTV attributed to VMAT and 3DCRT techniques presented in Table 1. For D95% (brain PTV), the variation in the 3 planning techniques was within ± 2%. Brain PTV average D95% difference between (FA-PA), (PA3DCRT), and (FA-3DCRT) plans were 0.3 ± 2.5%, 3.7 ± 2.5%, and 4.0 ± 2.9%, respectively for RX D_High. For RX D_Low, differences in the same sequence were −0.7 ± 1.9%, 1.4 ± 4.3%, and 0.7 ± 4.2%, respectively. For spine PTV, the D95% was lowest for the 3DCRT technique, 96.2 ± 2.2% and 96.4 ± 2.0% for RX D_High and RX D_Low, respectively. All the volumetric techniques yielded similar D95% for PTV spine for both prescription doses; and value ranged between 101.1 ± 0.05% to 100.1 ± 3.6%. The mean difference of D95% for PTV spine between VMAT_FA and VMAT_PA plans were 1.4 ± 4.7% and 1.0 ± 1.9% for high and low prescriptions, respectively. For V110% in PTV brain, the average percentage difference between FA and PA plan was 0.1 ± 2.3% and 1.7 ± 4.1% for RX D_High and RX D_Low, respectively.

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Fig. 2. Relative position of the isocenter for a 2 isocentric VMAT-based CSI plan. Spine full arc plan (A) and spine partial arc plan (B). (Color version of figure is available online.)

The dose difference between the VMAT and 3DCRT plans for PTV Brain ranged between 1.2 ± 1.6% and 2.9 ± 3.7% for both prescription doses. For PTV spine, average V110% was higher for VMAT_PA (6.0 ± 6.1%) compared to VMAT_FA (2.2 ± 3.1%) for RX D_Low; however, for RX D_High, no significant difference was observed (1.5 ± 1.3% vs 1.2 ± 0.9%). 3DCRT resulted in a significantly high dose to PTV spine (V110% = 18.9 ± 5.9% and 19.3 ± 6.8% for RX D_High and RX D_Low, respectively) which was statistically higher than that for VMAT plans (0.01 ≤ p ≤ 0.05). For PTV Spine, volume receiving in excess of 100% dose was −17.4 ± 6.0% and −17.7 ± 6.0% in VMAT_PA-3DCRT and VAMT_FA-3DCRT plans, respectively for RX D_High. For RX D_Low, the corresponding values were −13.3 ± 9.1% and −17.1 ± 7.5%. Table 2 shows the OAR doses in competing VMAT plans for the 2 different dose prescriptions. In VAMT_PA, reduced doses were observed for the midline structures like larynx, liver, heart, bladder, and for lateral organs like bilateral kidneys and bilateral lungs at both prescription doses. However, in the case of thyroid, while the VMAT_FA yielded a smaller dose to the organ as compared to VMAT_PA (1147.2 ± 51.8 cGy vs 1148.8 ± 126.2) at higher dose prescription, VMAT_PA resulted in lower dose to the organ (925.5 ± 558 cGy vs 845.1 ± 457.9 cGy) at smaller dose prescription. Bladder is the only structure which showed an elevated dose for partial arc plans by 114 cGy. The dose difference between VMAT_FA and VMAT_PA for all organs, varied over a wide range of −1.6 ± 136.4 cGy to 520.9 ± 188.2 cGy. The minimum dose difference between FA and PA plan was obtained for thyroid (−1.6 ± 136.4 cGy) at RX D_High level and bowel bag (4.6 ± 74.5 cGy) at RX D_Low. The highest dose differences between FA and PA plans were observed for bilateral kidneys at RX D_Low. Left and right kidney showed a difference of 520.9 ± 188.2 cGy and 499.9 ± 158.5 cGy, respectively between these 2 plans. At RX D_High, these dose differences between FA and PA plan for left and right kidneys were less pronounced at 310.5 ± 233.6 cGy and 266.5 ± 175.2 cGy, respectively. At RX D_High left (p = 0.01) and right kidney (0.03) shows a statistically significant dose difference between VMAT_PA and VMAT_FA plans. No other OAR exhibited statistically significant dose differences between partial and full arc plans, with organs like bowel, larynx, liver, heart, and bilateral lung showing only 50 cGy to 275 cGy average dose reduction in VMAT_PA compared to VMAT_FA. The PI and HI values averaged for both prescription doses are presented in Table 3. PI and HI did not exhibit any statistically significant difference between VMAT_PA and VMAT_FA plans. Figures. 3A and 3B show the variation of mean DBody-PTV between the 3 competing techniques for RX D_Low and RX D_High, respectively. The 3DCRT plan gave lower body doses for low dose regions, but after 7 to 10 Gy, it was found to be spilling high doses to large volume of the body. When compared to VMAT_PA plan, VMAT_FA plan resulted in a substantially higher spillage dose to body below 10 Gy for both prescription doses. The mean difference in the percentage volume of the body receiving dose in 1-5 Gy range from VMAT_FA and VMAT_PA plans were 13.2 ± 2.9% and 15.4 ± 2.4%, respectively for RX D_Low and RX D_High. Statistical analysis revealed p for DBody-PTV was significant (0.007 ≤ p < 0.05) in the 1 to 5 Gy range for both prescription levels. While

Table 2 Absolute dose and its difference for different organ at risk for full and partial arc VMAT technique

Organ at risk

35Gy prescription Dose cGy (Mean ± SD)

23.4Gy prescription Dose cGy (Mean ± SD)

Bowel bag_FA Bowel bag_PA Difference (FA-PA) Bladder_FA Bladder_PA Difference (FA-PA) Larynx_FA Larynx_PA Difference (FA-PA) Left kidney_FA Left kidney_PA Difference (FA-PA) Right kidney_FA Right kidney_PA Difference (FA-PA) Liver_FA Liver_PA Difference (FA-PA) Heart_FA Heart_PA Difference (FA-PA) Thyroid_FA Thyroid_PA Difference (FA-PA) Right lung_FA Right lung_PA Difference (FA-PA) Left lung_FA Left lung_PA Difference (FA-PA)

890.2 ± 76.1 844.4 ± 28.4 45.8 ± 81.2 678.7 ± 70.1 791.8 ± 46.8 −113.1 ± 84.3 1367.8 ± 176.6 1305.9 ± 67.5 61.9 ± 189.1 655.4 ± 180.2 344.9 ± 148.6 310.5 ± 233.6 558.8 ± 163.3 322.3 ± 63.6 266.5 ± 175.2 615.8 ± 6.7 498.1 ± 2.5 117.7 ± 7.1 786.0 ± 117.4 728.6 ± 24.8 57.4 ± 120 1147.2 ± 51.8 1148.8 ± 126.2 −1.6 ± 136.4 760.6 ± 42.7 670.6 ± 58.1 90.0 ± 72.2 1092.3 ± 177.7 818.0 ± 192 274.3 ± 261.6

666.9 ± 58.1 662.3 ± 46.7 4.6 ± 74.5 257.2 ± 299.6 372.4 ± 430.4 −115 ± 524.4 1159.9 ± 180.6 949.7 ± 162.6 210.2 ± 243.0 826.6 ± 157.8 305.7 ± 102.7 520.9 ± 188.2 811.8 ± 156.4 311.9 ± 25.5 499.9 ± 158.5 514.4 ± 54.9 396.4 ± 41.2 145.0 ± 68.6 767.5 ± 69.3 679.6 ± 131 87.9 ± 148.2 925.5 ± 558 845.1 ± 457.9 80.4 ± 721.8 574.7 ± 26 493.5 ± 123.7 81.2 ± 126.4 549.7 ± 17.4 499.9 ± 24.2 49.8 ± 29.8

comparing between 3DCRT and VMAT plans, p<3DCRT:VMAT_FA> and p<3DCRT:VMAT_PA> , DBody-PTV were significantly different in the dose ranges 1 to 5 Gy and 1 to 3 Gy, respectively. MUs for the 2 different prescription doses as a function of planning technique are given in Table 3. The mean reduction in MU between PA and FA plan was 178 ± 129.3 MU for 23.4 Gy and 206.7 ± 160.9 MU for 35 Gy prescription, respectively.

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Table 3 Paddick conformity index and heterogeneity index for 3 different planning strategies, averaged over both prescription level (as they are not a function of dose fractionation). MU for each prescription levels (as it is a function of dose fractionation) for 3 different planning strategies Difference

PI (averaged over both prescription levels) HI (averaged over both prescription levels) MU (23.4Gy/13 Fractions) MU (35.0Gy/21 Fractions)

VMAT_PA

VMAT_FA

3DCRT

< VAMT_PA- VMAT_FA>

< VMAT_FA-3DCRT>

< VAMT_PA-3DCRT>

0.82 ± 0.05 1.1 ± 0.03 1104.9 ± 98.7 984 ± 123.4

0.84 ± 0.04 1.09 ± 0.02 1283.5 ± 83.5 1190.7 ± 103.2

0.55 ± 0.08 1.12 ± 0.01 523.1 ± 61.2 474.2 ± 78.2

−0.02 ± 0.06 0.01 ± 0.04 −178.6 ± 129.3 −206.7 ± 160.9

0.29 ± 0.09 −0.03 ± 0.02 760.4 ± 103.5 716.5 ± 129.5

0.27 ± 0.1 −0.02 ± 0.03 581.8 ± 116.1 509.8 ± 146.1

Fig. 3. (A). Mean dose (±SD) to Body-PTV (DBody-PTV : nontumor integral dose) for 3DCRT, partial arc VMAT, and full arc VMAT for prescription dose of 23.4 Gy. (B). Mean dose (±SD) to Body-PTV (DBody-PTV : nontumor integral dose) for 3DCRT, partial arc VMAT, and full arc VMAT for prescription dose of 35 Gy. Mean differences in MU between VMAT_PA and 3DCRT MU were 581.8 ± 116.1 MU and 760.4 ± 103.5 MU for 23.4 and 35 Gy prescriptions, respectively. For same order of prescription doses, differences between VMAT_FA MU and 3DCRT MU were 509.8 ± 146.1 MU and 716.5 ± 129.5 MU, respectively.

Discussion VMAT radiation treatment techniques are gaining popularity due to their simplicity and faster treatment delivery time. VMATbased CSI is increasingly being accepted as the choice of treatment technique over conventional techniques in clinics since it does not require any junction-shifts and it results in more conformal dose distribution.6,10,13 While all VMAT-based CSI studies invariably have used full-arcs for brain field, substantial interuser variations are observed in the arc lengths used for spine fields (Table 4).3-10 Mean arc lengths for the upper and lower spine fields are 315.7° ± 62.0° (0° to 360° cyclic) and 314.6° ± 60.9° (0° to 360° cyclic), respectively. Nevertheless, the most common arc angle used in spine fields is a full arc (8 out of 12 institutions). Our study established spine VAMT_PA could reduce the dose to kidneys, lung, larynx, and body/Body-PTV even though dose reduction in liver, thyroid, heart, and bowel were not significant. The only statistically different OAR doses were observed for bilateral kidneys for RX D_High. As reported by earlier investigators, the variations in the OAR doses are wide and inconsistent (Table 4).3-10 For 36 Gy prescription, kidney doses were in the range of 4.4 to 13.2 ± 12 Gy. In our study for 35 Gy dose, dose to kidney(s) were 3.2 to 3.4 Gy for VMAT_PA plans and 5.5 to 6.5 Gy for VMAT_FA plans.3-4 , 7 For 35 Gy prescription, present study yielded a mean lung dose of 6.7 to 8.2 Gy for VMAT_PA plan whereas the earlier reported doses were in the range 5.4 to 11 Gy. Similarly, for other OARs also, we have obtained the doses consistently lower than the earlier reported values. As described in our study, VMAT_PA plans could reduce the low doses spillage to the body compared to the VMAT_FA plans, however, FA plans could neither increase PI nor reduce HI. Reduction in DBody-PTV by PA plans was statistically significant below 10 Gy.

The other salient finding of our study is that VMAT_PA plans reduce the volume of 7 Gy spillage dose in comparison to the 3DCRT plan for RX D_Low. With full arc plans, Lee et al. demonstrated a reduction of only 10 Gy spillage dose.4 The DBody-PTV crossover point between 3DCRT-VMAT_PA plan did not vary between 2 different prescription levels. 3DCRT-VMAT_FA crossover point increased by only 2 Gy for an increase in the prescription dose from 23.4 Gy to 35 Gy. These observations indicate perhaps the spillage dose for VMAT plan either saturated or varies slowly as a function of delivered dose. Paired sample t-test was chosen for this study as it aims to compare mean of 2 sets of data originated from similar radiotherapy planning technique(s). Primary aim of the statistical analysis in this study was to compare the mean of different dose volume parameters between 2 competing VMAT technique. Additionally, a statistical analysis between VMAT and 3DCRT technique was also provided for PTV and dose spillage related parameters. The volume of normal tissue receiving a low dose of radiotherapy has become an important clinical parameter. Low doses, especially in the paediatric population, have been correlated to a higher incidence of side effects including secondary malignancies which makes the findings of our study even more important.16 A recent AAPM report (AAPM TG-158) summarizes the contribution of the out-of-field doses in external radiotherapy which indicated an increased susceptibility of the second malignancy for the childhood cancer patients and the cardiac toxicity.17 Roughly 10% of the cancer survivors develop second cancer; out of which 2% is attributed to early radiotherapy.17-18 The dose risk (second cancer) relationship is linear in our dose range of interest.17-18 Therefore, lower spillage dose offered by partial arc may yield a decreased risk of radiotherapy-induced second malignancy. We have described the rationale of choosing shorter arc length in appendix section. Target volume conformity obtained in 3DCRT is inferior to VMAT plans. However full arc VMAT plans do not attribute any gain in the conformity over partial arc plans. Heterogeneity in 3DCRT, VMAT_PA, and VMAT_FA was comparable.

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Table 4 Reviewed papers for VMAT-based craniospinal irradiation (first/corresponding author and publication year). Data shows a comparison between arc angles for different spinal isocenters, prescription dose, and organ at risk doses Mean dose in Gy Prescription Dose (Gy)

No of fractions

Heart

Thyroid

Esophagus Left Lung

Antonella Fogliata (2011) Antonella Fogliata(2011) Matthew T. Studenski (2013) Qilin Li (2015)

Antonella Fogliata(2011) Lee YK (2012) Pamela A. Myers (2014) Antonella Fogliata(2011) Antonella Fogliata(2011)

centre a

181°/179° = 358°

181°/179° = 358°

36

20

7.1

14.9

19.2

11

9.7

8.5

18.8

centre b

181°/179° = 358°

181°/179° = 358°

36

17

9

13.6

15.7

5.4

6.2

4.4

13.6

200°

-

36

20

5

11

13

9.8

5.5

(180°−240°) + (300°−60°) + (120°−180°) = 240° 180°/180° = 360°

-

36

20

6.3

12.9

15.1

6.2

5.3

4.8

179°/180° = 360°

23.4

13

5.3

9.9

13.7

8.2

6

7.4

179°/181° = 358° 360°

-

23.4 23.4

13 13

4 13.7

15 19

14

4

6 5.6

(180°/310°) + (50°/179°) = 260° 180°/180° = 360°

(180°/310°) + (50°/179°) = 260° (180°/215°) + (276°/84°) + (142°/180°) = 237° -

30.6

8

Different prescription

12

13

Different prescription

Andrej Strojnik (2016) Anders T. Hansen (2015) Pamela Myers (2013)

30.6

17

Not presented

-

36

20

Not presented

-

23.4

13

Not presented

centre e

centre c centre d

(180°/310°) + (50°/179°) = 260° 182°/179° = 357° 360°

4.1

Rt Lung

5.8

Combined Liver Lung

Rt Kidney

13.2

9 5.3

Lt Kidney

Combined Larynx kidney

10

9.5 4.2

13.6

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Upper spine isocenter (start angle/end angle)

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Conclusion

References

Spine full arcs are redundant, compared to partial arcs, in terms of PTV-related dosimetric characteristics. Doses to certain OARs are low in partial arc plans and dose to body/ DBody-PTV is also reduced yielding an anticipatory gain in the quality of life; thereby giving the partial-arc plans an advantage over the full-arc plans. We recommend avoiding full arc for spinal PTV.

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Conflict of Interest The authors declare no conflicts of interest. Source of Funding No funding was involved in preparation of this manuscript. Appendix The rationale of choosing the optimal arc angle for spine PTV. As per the discussion in this article, it is evident that a large arc angle or full arcs for spine PTV does not result in any dosimetric advantage for partial arc VMAT plan in CSI. However, it is essential to justify what should be the optimal arc angle for spine field(s). We started clinical use of VMAT-based CSI late 2013 with an initial choice of the arc angle as 40 for all age group patients.11 Using this arc angle, we observed an increased GI toxicity especially esophagitis in the patients during the therapy. While correlating the dosimetry with the toxicity, it was realized that short arcs like 40° creating a forward projecting isodose distribution and this consequently increased dose to the mid-line structures like liver, bowel, heart, and especially esophagus. Subsequently, we increased the spine posterior arc angle to 60° so as to reduce the anterior throw off of dose.13 We also realized that 60° arc angle is insufficient on the inferior side of the spine for adult age group (especially where the lateral dimension of the spine PTV exceeds 4.5 cm). For few patients, differential arc lengths of 60° for upper and 100° for lower spine were used.12 Further, a dosimetric optimization between dose volume parameters against arc angle was carried out. We settled for an optimal arc angle of 100° for both upper and lower spine isocenter for adolescent and adult age group patients. We also observed that for early childhood age group (3 to 6 years) it is preferable to reduce the arc length as the maximum lateral dimension of spine PTV reduces (≤ 1.5 cm). The preferable arc length should be 60° to 70° and definitely not less than 40°. However, we accept that it is difficult for us to make a conclusive remark on the arc angle required for posterior spine arc as the number of such patients is very less.