ESTRO 2022 - Abstract Book

S383

Abstract book

ESTRO 2022

resulting change in mean dose to high uptake areas V0 and V2 ( Δ D V0,V2 ) was evaluated with respect to the cause of change. A Chi-square test was used to test if a low overlap OF 02 (<0.7) was correlated with a large decrease in dose Δ D V2-V0 (>3Gy). Results The geometric stability of the high uptake volumes was high, with median (min;max) values of OF 01 : 0.83 (0.42;1.00) and OF 02 : 0.82 (0.38;1.00). OF 01 and OF 02 for all patients are shown in fig1.

In 56 (82%) and 55 (80%) of the patients, the OF was above 0.7 at PET/CT1 and PET/CT2, respectively. Both increase (negative Δ D V0,V2 ) and decrease (positive Δ D V0,V2 ) in dose to V2 compared to V0 were observed. Increase was in general due to tumour shrinkage as illustrated in fig2a. There was a significant correlation between patients with OF 02 <70% and patients with mean dose decrease >3Gy (p<0.001). Patients with >3Gy decrease are highlighted in fig1. For patients with OF 02 above 0.7, the median change in dose, Δ D V0,V2 , was 0.2Gy(-4.4;5.5), and in only 5% of these patients (3/55) the dose decrease >3Gy. For the 13 patients where the OF 02 was below 0.7, the median change in dose, Δ D V0,V2 , was 2.1Gy(-0.9;17.0), and in 38% of these patients (5/13) the dose decreased >3Gy. The primary cause for OF 02 <0.7 was the decline of SUVpeak to ~2 times background activity, as this resulted in large changes of the region included in V2 for 54% of patients (7/13), see example in Fig 2c. Additionally, OF 02 <0.7 was observed for patients with larger anatomical changes, such as necrotic regions partly dissolved and changing atelectasis, where plan adaptation was warranted.