The normal tissue sparing potential of an adaptive plan selection strategy for re-irradiation of recurrent rectal cancer

Background and purpose: Radiotherapy (RT) of rectal cancer is challenged by potentially large inter-fractional anatomy changes. The risk of radiation-induced morbidity is a particular concern in patients receiving re-irradiation for recurrent disease. We propose an adaptive RT plan selection strategy for these patients and report on its clinical feasibility and normal tissue sparing potential. Material and methods: Eight patients with pelvic recurrence were re-irradiated according to a hyper-fractionation protocol (ReRAD-I; 40.8 Gy) using margins around the clinical target volume (CTV) of 15 mm trimmed to anatomical barriers (Plan L). Two new library plans (S and M) were created for each patient, with the target volumes covering the CTV with isotropic margins of 5 and 10 mm. Pre-treatment cone beam CTs were assessed to determine which plan would cover the CTV following soft-tissue match. The selected plans were compared to the clinically delivered plan in terms of normal tissue volume receiving 95% of the dose (V95%) and the volume of bone receiving 30 Gy (V30 Gy). Results: Plan selections could be performed on all CBCTs for all patients. Plan S was chosen in 213 fractions (79%), plan M in 53 (20%) and plan L in 2 fractions. Normal tissue V95% was reduced by 67% (median; range 30–79%) while bone V30 Gy was reduced by 66% (median; range 40–100%). Conclusion: The CTV and/or surrogate structures were visible on all CBCTs. Margins smaller than those used clinically would have accounted for 99% of the observed target deformations, translating into a considerable normal tissue sparing potential. Keywords: Adaptive radiotherapy, Rectal recurrence, Plan selectio

Treatment plan comparison of proton vs photon radiotherapy for lower-grade gliomas

Background and purpose: Patients with lower-grade gliomas are long-term survivors after radiotherapy and may benefit from the reduced dose to normal tissue achievable with proton therapy. Here, we aimed to quantify differences in dose to the uninvolved brain and contralateral hippocampus and compare the risk of radiation-induced secondary cancer for photon and proton plans for lower-grade glioma patients. Materials and methods: Twenty-three patients were included in this in-silico planning comparative study and had photon and proton plans calculated (50.4 Gy(RBE = 1.1), 28 Fx) applying similar dose constraints to the target and organs at risk. Automatically calculated photon plans were generated with a 3 mm margin from clinical target volume (CTV) to planning target volume. Manual proton plans were generated using robust optimisation on the CTV. Dose metrics of organs at risk were compared using population mean dose-volume histograms and Wilcoxon signed-rank test. Secondary cancer risk per 10,000 persons per year (PPY) was estimated using dose-volume data and a risk model for secondary cancer induction. Results: CTV coverage (V95%>98%) was similar for the two treatment modalities. Mean dose (D-mean) to the uninvolved brain was significantly reduced from 21.5 Gy (median, IQR 17.1-24.4 Gy) with photons compared to 10.3 Gy(RBE) (8.1-13.9 Gy(RBE)) with protons. D-mean to the contralateral hippocampus was significantly reduced from 6.5 Gy (5.4-11.7 Gy) with photons to 1.5 Gy(RBE) (0.4-6.8 Gy(RBE)) with protons. The estimated secondary cancer risk was reduced from 6.7 PPY (median, range 3.3-10.4 PPY) with photons to 3.0 PPY (1.3-7.5 PPY) with protons. Conclusion: A significant reduction in mean dose to uninvolved brain and contralateral hippocampus was found with proton planning. The estimated secondary cancer risk was reduced with proton therapy