ESTRO 2025 - Abstract Book

S2812

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

2345

Digital Poster The effect of patient size on OAR doses and secondary cancer risk in craniospinal radiotherapy using VMAT and 3D-CRT Antti Vanhanen 1,2 , Annele Heikkilä 1,2 , Maija Rossi 1,2 , Sirpa-Liisa Lahtela 3 1 Department of Oncology, Tampere University Hospital, Tampere, Finland. 2 Department of Medical Physics, Tampere University Hospital, Tampere, Finland. 3 Department of Oncology, Helsinki Comprehensive Cancer Center, Helsinki, Finland Purpose/Objective: In craniospinal radiotherapy, VMAT produces more conformal dose distribution than 3D-CRT and spares certain OARs better. However, as the patient size affects the dose distribution due to varying location of the organs relative to the target, the benefit of VMAT over 3D-CRT is not always clear. The aim of this study was to evaluate the effect of patient length on OAR doses and secondary cancer risk using three different VMAT beam arrangements in comparison with 3D-CRT. Material/Methods: Fourteen patients (age 4-54 years) with PTV length from 47 to 85 cm were retrospectively included in the study. Three VMAT plans and a 3D-CRT plan were created for each patient. Dose prescription was 36.0/1.8 Gy. VMAT plans differed in spinal arc arrangements: VMATfull consisted of two 360° arcs, VMATp of 140° posterior arcs and VMATap of 100° anterior and 140° posterior arcs. 3D-CRT consisted of lateral cranial fields and posterior spinal fields. Investigated OARs included lenses, oral cavity, parotid and submandibular glands, thyroid, heart, lungs, kidneys, liver, pancreas, esophagus, stomach, small and large bowel. Volume weighted average of the mean OAR doses was calculated for each plan and compared as a function of PTV length. Secondary cancer risk for stomach, colon, liver and lungs was evaluated using OED concept for carcinoma induction [1]. Total risk was calculated as weighted mean OED, using radiation-related secondary cancer incidence of each organ [2] as the weighting factor, and compared between different techniques as a function of PTV length. Results: The weighted mean OAR dose (fig. 1) and mean OED (fig. 2) of the VMAT plans increased relative to 3D-CRT as the PTV length increased. VMATp produced the lowest OAR mean doses. Except for one patient, the secondary cancer risk was higher for all VMAT plans when compared to 3D-CRT and the relative risk increased as the PTV length increased. The correlation of the risk ratio and PTV length was weakened due to obesity of one patient, for whom the risk was similar regardless of the treatment technique.