ESTRO 2024 - Abstract Book

S4664

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

node (PPN) were planned on Eclipse with VMAT class solutions and standard OAR dose constraints which did not change during the 2-year period. For the dosimetric analysis, we utilised Aria (Varian©) to extract radiotherapy fractionation schedules, treatment volumes and organ at risk (OAR) dose volume histograms (DVH). All doses were adjusted to 2-Gy equivalent schedules (EQD2) using α/β = 3 with Withers formula. Data was analysed in R for calculation of odds ratios between groups and univariate logistic regression of relative rectal volumes at 10 Gy dose bins. Additionally, to derive dose constraints, we calculated the area under the receiver operating curve (AUC) for each dose bin; if the AUC confidence interval lower limit was >0.5, this was considered statistically significant. To then determine optimal OAR dose constraints, we used the averaged result of Youden and Closest to top Left (CTL) indices, determined across 1000 stratified bootstraps with replacement.

Results:

We included 761 patients; 39 had GI toxicity (5.1%). Baseline characteristics of age, Charlson score and IMD5 were similar between both groups (see table). Patients with high-risk prostate cancer were more likely to develop GI toxicity (OR: 6.36, 95% CI: 2.97-13.6). Patients who had PPN treated were significantly more likely to develop lower GI toxicity, compared to any PO schedules (OR: 3.41, 95% CI: 1.74-6.68).

Lower GI toxicity (n=39)

Non-toxicity (n=722)

All patients (n=761)

Statistics

Demographics

Age (SD)

70.5 (6.4)

71.5 (6.4)

70.4 (6.4)

Not significant

Charlson (SD)

0.57 (0.79)

0.52 (0.79)

0.58 (0.79)

Not significant

IMD5 (SD)

3.03 (1.49)

3.05 (1.50)

3.03 (1.49)

Not significant

Risk group

OR:

6.36,

Low/Intermediate

489 (64.3%)

9 (23.1%)

480 (66.5%)

p<0.005

High/Metastatic

266 (35.0%)

30 (76.9%)

236 (32.7%)

95% CI: 2.97-13.6

Insufficient data

6 (0.08%)

0 (0%)

6 (0.8%)

Radiotherapy

PPN schedule

271 (35.6%)

25 (64.1%)

248 (34.3%)

OR: 3.41, p<0.005

PO schedule

490 (64.4%)

14 (35.9%)

474 (65.7%)

95% CI: 1.74-6.68