ESTRO 2021 Abstract Book

S891

ESTRO 2021

Purpose or Objective To study whether differences in planning techniques using proton radiotherapy may play a role in the appearance of non-desired effect due to high linear energy transfer (LET). Materials and Methods We selected 26 patients treated for meningioma with pencil-beam scanning proton radiotherapy that developed abnormal changes in MRI after treatment. MRI images were registered to the planning CT and dose and dose-averaged LET (LETd) distributions were calculated for all the patients. We sampled values of LETd across abnormal areas and the rest of the patient, matching the dose distributions. LETd values of both samples were compared to determine whether abnormal areas presented significantly higher LETd values. Finally, we divided patients with and without higher LETd into two groups and attempted to identify similar characteristics in their planning technique. Results 11 of 26 patients demonstrated significantly higher LETd in tissue volumes with imaging changes relative to other tissue, whereas 15 patients did not. Figure 1 condensates the results and the differences in mean LETd for all 26 patients. No significant differences between both groups were found for age, gender or appearance lapse after end of treatment. However, patients with higher LETd had significantly shallower tumors (5.0 cm vs 6.6 cm) and employed fewer beams (2.1 vs 2.6 on average) and narrower beam arrangements (69 deg vs 117 deg between each pair of beams on average). Figure 2 illustrates representative planning technique and beam arrangement for both group of patients.

Figure 1