ESTRO 2024 - Abstract Book

S3494

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

ESTRO 2024

A total of 40 patients were treated with primary RT (87.5%) or with postoperative RT (12.5%) after incomplete resection. Tumour staging was T1 (≤ 20 mm) for 65% of the patients while the remaining 35% was T2 (>20 mm, but ≤ 50 mm). Dose and fractionation were prescribed in six different fractionation schemes (Table 1). The overall median energy was 8 (4-18) MeV corresponding to a median depth of 27 (14-58) mm and the median circular aperture diameter was 60 (40-90) mm. Median tumour size was 18.5 (5-30) mm.

The median cylindrical volume irradiated to at least 85% of the prescribed dose was 19.1 (1.1-164.0) cm 3 while the lower and the upper quartile were 6.6 cm 3 and 41.3 cm 3 , respectively. A bolus of 3 mm thickness was used in the majority of all patients. At a median follow-up of 72 (9-135) months, seven (17.5%) patients were diagnosed with local failure (LF) (Figure 1). Five (71%) LF were diagnosed within the first 60 months of follow-up, while the remaining two (29%) LF were diagnosed after 64 months and 113 months, respectively (Figure 1).

Significantly more patients with T2 tumors (5/14 (36%)) experienced a LF compared with T1 tumours (2/26 (8%)) ( Χ 2 (1, N =40)=4.9493, p =.026101). There was no significant relation between LF and the prescribed dose as all but one LF occurred in the group of patients receiving an EQD 2Gy in the range of 60.0-61.0 Gy (Table 1). Late grade 2 and 3 toxicity was found in eight (20%) and four (10%) patients, respectively. In four cases (10.0%) potential geometrical miss was possible as the prescribed aperture diameter to tumour size showed an inadequate margin at treatment with an aperture 1-10 mm too small. None of these patients have had LF. Extraction of the data from the clinical records was insufficient in the determination of potential geometrical miss due to a prescription of inadequate electron energy.