Investigation of a 4.3 Gy × 10-Fraction Volumetric Modulated Arc Therapy Protocol for Canine Prostatic Carcinoma: A Normal Tissue Complication Probability Study

The fraction size in canine pelvic tumours has traditionally been limited to minimisze radiation toxicity. However, advancements in precision radiotherapy techniques have enabled the use of larger fraction sizes, thereby facilitating a reduction in the overall treatment course. This study assessed the radiation toxicity risks of a 10-fraction radiation protocol for canine prostatic carcinoma by calculating normal tissue complication probabilities (NTCPs). Computed tomography data from 22 dogs with prostatic carcinoma were analysed. The new protocol was designed to deliver 43 Gy in 10 fractions over 2 weeks (Monday-Friday), with a biologically effective dose similar to that of standard protocols. Compared to the standard 20-fraction protocol, the 10-fraction protocol demonstrated comparable toxicity risks in most organs except for some rectal endpoints and the urethra, while also offering advantages in treatment time and patient convenience. Nevertheless, under the 10-fraction protocol, the relatively high NTCPs for late rectal toxicities and the identification of patients at high risk of toxicity support the recognition of the rectum as a primary organ at risk in hypofractionated radiotherapy. Rectal toxicity risks were higher in patients with trigonal invasion, dorsal rectal displacement or rectal narrowing. Higher tumour length/L6 height and maximal tumour diameter/L6 height ratios were associated with increased rectal NTCPs. Relative tumour size indices effectively predicted patients at high risk for rectal toxicity. Cut-off values were identified for acute rectal toxicity (Grade ≥ 2; tumour height/pelvic inlet ratio: 0.62), rectal bleeding (Grade 2; tumour height/L6 height: 4.03) and proctitis (Grade 2; maximal tumour diameter/pelvic inlet ratio: 0.82). These findings highlight the importance of relative tumour size indices as predictive markers for rectal toxicity risk in the 10-fraction protocol. The results suggest that the 10-fraction, 43 Gy protocol may be safely applied when tumour size remains below specific thresholds.