Potential Benefit of a Library of Plans Strategy for Pre-Operative Gastric Cancer Radiotherapy
M Bleeker1*, M Hulshof1, K Goudschaal1, A Bel1, J-J Sonke1,2, A van der Horst1, (1) Department of Radiation Oncology, Amsterdam UMC - University of Amsterdam, Amsterdam, NL, (2) Department of Radiation Oncology, Netherlands Cancer Institute, Amsterdam, NL
Presentations
PO-GePV-M-105 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: As the stomach experiences large interfractional changes in shape and size, a library-of-plans (LoP) strategy may be beneficial (coverage and organs-at-risk sparing) in pre-operative gastric cancer radiotherapy. Here, we assess differences in PTV between LoP and single-plan (SP) strategy.
Methods: Twelve patients who received pre-operative gastric cancer radiotherapy (CRITICS-II study) and additional imaging (a pre-treatment CT (pCT) and 2-3 repeat CTs (rCT; total=34)) were included. For this study, clinical target volume (CTV) consisted of stomach +10 mm for surrounding lymph nodes minus organs-at-risk (=liver/spleen/lungs). The pCT and a healthy volunteer-based gastric deformation model were used to create 5 equidistant stomachs with fixed volumes (1=150;2=254;3=373;4=540;5=750mL). The stomach-CTV expansion on pCT was propagated to the LoP stomachs. For each rCT, plan selection (coarse selection based on best-fitting CTV, followed by optimization to minimize PTV margin) was performed after a translations-only-match.Local CTV-to-PTV margins were calculated for this dataset (non-linear van Herk formula) and include uncertainties due to delineation (3mm), intrafractional motion (2.4mm), respiratory motion (0.358*A, A:SI=8;AP=3;LR=2mm) and interfractional shape variation. This shape variation was assessed by determining distances from CTV (SP: CTV=CTV on pCT; LoP: CTV=selected CTV on rCT) to CTV on rCT following a translations-only-match, resulting in different PTV margin distributions between LoP and SP. Irradiated volume (PTV) difference was evaluated for each rCT.
Results: Observed stomach (CTV) volumes ranged between 112−597mL (323−1062mL). Selected plans (34 rCTs) were 1:(N=3), 2:(N=11), 3:(N=16), 4:(N=3), 5:(N=1). PTV margins were significantly smaller for LoP than SP (median[5th−95th percentile], 14.0[10.4−18.7]mm vs 17.4[13.9−22.6]mm, Mann–Whitney–Wilcoxon: p<0.0001). For 25 rCTs (74%), LoP resulted in a smaller PTV than SP. Median PTV difference over all rCTs [5th−95th percentile]: -151[-921−472]mL.
Conclusion: LoP shows potential for benefit, mainly expressed in smaller PTV margins and reduced irradiated volume; to assess dosimetric benefit, dose calculation/accumulation with plan selection for all fractions is needed.
Keywords
Shape Deformation, Radiation Therapy
Taxonomy
TH- RT Interfraction Motion Management: Development (new technology and techniques)
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