ESTRO 2023 - Abstract Book

S1694

Digital Posters

ESTRO 2023

Conclusion In a large cohort of pediatric cancer patients receiving radiotherapy, we found that interfractional motion does not depend on age, but the use of GA in younger patients results in smaller systematic and random errors. Furthermore, our results show that the breathing amplitude increases with age. Differences in systematic and random errors between younger and older children are small, but variations between patients advocate the need for a patient-specific margin approach.

PO-1939 PBS proton therapy for neuroblastoma: dosimetric robustness to bowel gas variation

A. Toltz 1,2 , S.J. Court 3 , V. Rompokos 1 , J. Gains 4 , M. Gaze 4 , Y. Chang 4 , P. Lim 4,2

1 University College London Hospitals, NHS Foundation Trust, Department of Radiotherapy Physics, London, United Kingdom; 2 University College London, Department of Medical Physics and Biomedical Engineering, London, United Kingdom; 3 University College London Hospitals, NHS Foundation Trust, Department of Radiotherapy Physics, London, United Kingdom; 4 University College London Hospitals, NHS Foundation Trust, Department of Oncology, London, United Kingdom Purpose or Objective A challenge in proton therapy is in producing plans that are robust to anticipated changes in the patient anatomy. Small differences in the heterogeneities in the beam path between planning and treatment can produce large discrepancies in both target coverage and tissue sparing. Here we explore whether optimising directly on the overridden bowel structure may yield a plan that is overall more robust to interfractional bowel gas variation. Materials and Methods Three cases previously treated with VMAT and exhibiting complex target volumes were selected: all comprised a midline target abutting both kidneys. Our institutional guidelines for planning objectives (Rx 21 Gy in 14 fractions) and organ at risk constraints (V14Gy contralateral kidney < 10%) were used. Planning in Eclipse (PCS v15.6.05) followed the protocol of our previous work in which we investigated various beam arrangements and optimisation techniques. Each case was planned with single field optimisation and a three-field beam arrangement: an anterior field (included to try to improve kidney sparing) and two posterior oblique fields. The fields were optimised (NUPO v15.6.05) on the original bowel filling from the planning CT as well as two scenarios using manual overrides to simulate full (120 HU) and mean (-293 HU) over the bowel structure. Weekly CBCTs were obtained for each case; three in total. Plans were then recalculated on the bowel filling from each CBCT (using manual HU overrides), and plan sums were created for evaluation.