Central Nervous System
PV QA 2 - Poster Viewing Q&A 2
Purpose/Objective(s): Radiotherapy technique typically used in the post-operative management of high grade glioma (HGG) is 3-dimensional conformal radiotherapy (3D-CRT). Volumetric modulated arc therapy (VMAT) can provide better coverage of target volumes and sparing of organs at risk (OARs). We aimed to dosimetrically compare 3D-CRT with VMAT plans in the same.
Materials/Methods: Computed tomographic simulation data of 26 patients of HGG fused with post-operative magnetic resonance imaging data and treated with 3D-CRT, were used for dosimetric comparison with subsequently generated VMAT plans. For 3D-CRT, low risk planning target volume (LR-PTV; resection cavity plus edema) was treated to 46 Gray (Gy) in 23 fractions over 5.5 weeks and high risk planning target volume (HR-PTV; resection cavity plus contrast enhancement) was further boosted by 14 Gy in 7 fractions over 1.5 weeks. For VMAT, 46 Gy in 30 fractions was delivered to LR-PTV and simultaneous integrated boost of 60 Gy in 30 fractions to HR-PTV over 6 weeks. Planning objective was to cover ≥ 95% of PTV with 95% of prescription dose. Standard dose constraints were applied to OARs for VMAT planning and tried to be respected in 3D-CRT plan.
Results: Median age was 46.5 years. Left sided and right sided tumors were 46.15% and 53.84%. Tumor location was frontal, parietal, temporal and multi-lobed in 26.92%, 15.38%, 23.0% and 34.6% respectively. Histopathology was glioblastoma multiforme, anaplastic oligodendroglioma and anaplastic astrocytoma in 88.46%, 7.69% and 3.84% patients respectively. Mean PTV D95 (dose received by 95% volume) in 3D-CRT vs. VMAT were 96.66% vs. 98.88% (95% CI -3.23 to -1.20, p<0.001) for LR-PTV and 97.38% vs. 99.01% for HR-PTV (95% CI -2.39 to -0.87, p value<0.001) respectively. Mean conformity index in 3D-CRT vs. VMAT plans for LR-PTV and HR-PTV were 0.96 vs. 0.98 (95% CI -0.032 to -0.011; p<0.001) and 0.97 vs. 0.99 (95% CI -0.025 to -0.009; p<0.001). Mean Dmax (maximum point dose received by the organ) of right optic nerve 31.59 Gy vs. 25.57Gy (95%CI; 0.86 to 11.17;p=0.02), left optic nerve 28.81 vs. 22.14 (95% CI;1.16 to 12.16;p=0.019) optic chiasma 42.24 Gy vs.37.12 Gy (95% CI; -0.11 to 10.37;p=.055),brainstem 50.60 Gy vs. 49.08 Gy (95% CI;-1.50 to 4.53;p =0.31), right lens 5.71 Gy vs. 9.33 Gy (95% CI;-7.94 to 0.80;p=0.10),left lens 5.08 Gy vs. 9.33 Gy (95%CI; -8.32 to 0.19; p=0.040) for 3D-CRT and VMAT respectively. Mean biologically effective dose (BED) for 3D-CRT vs. VMAT were 68.72 Gy vs. 69.20 Gy (95%CI -3.01 to 2.05; p=0.70) for LR-PTV and 72.38 Gy vs. 73.54 Gy (95%CI -1.94 to -0.366; p=0.006) for HR-PTV respectively.
Conclusion: VMAT as compared to 3D-CRT achieved better coverage of the PTV with significant sparing of optic nerve and optic chaisma in post-operative patients of HGG treated with radiotherapy. The sparing of OARs is influenced by the tumor location and laterality.
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