Comparison of Volumetric Modulated Arc Therapy and Fixed‐Field Intensity‐Modulated Radiotherapy for Canine Prostatic Carcinoma Using a 4.3 Gy × 10‐Fraction Protocol: A Dosimetric Planning Study

Advancements in radiotherapy, including intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), have reduced radiation-induced toxicities, enabling hypofractionated protocols to be feasible for treating intrapelvic tumours in dogs. This study aimed to determine the optimal planning technique for canine prostatic carcinoma using a radiotherapy protocol delivering 43 Gy in 10 fractions. Four techniques were compared-two full arcs (2FA), two partial arcs (2PA), 9-field IMRT (9F) and 7-field IMRT (7F). Computed tomography datasets from 22 dogs were analysed. While the conformity index did not differ significantly among techniques, IMRT demonstrated significantly lower homogeneity index than VMAT. IMRT yielded the lowest rectal normal tissue complication probabilities (NTCPs), maximum dose (Dmax) and V45, whereas 2PA achieved the lowest mean dose (Dmean) and V10, V15, V20, V25 and V30. Additionally, IMRT showed significantly lower NTCPs and V20, V25 and V30 for the colon; NTCPs and V20, V25, V30 and V35 for the small intestine; and Dmax, V20, V25 and V45 for the bladder. For the urethra, IMRT showed the lowest Dmax and V45, whereas 2PA demonstrated the lowest Dmean and V10, V15 and V20. The spinal cord (Dmean, V10, V15) and cauda equina (Dmean, Dmax, V10, V15 and V20) were best spared with 2PA. However, 2PA resulted in the highest femoral head NTCPs, Dmean, Dmax and V20, V25 and V30. IMRT showed significantly greater monitor units and beam-on time than VMAT, especially with 9F. In conclusion, under the predefined and unaltered planning objective and constraints used in this study, IMRT showed more favourable OAR sparing and improved dose homogeneity than VMAT. Among the IMRT techniques, 7F may be preferred over 9F due to reduced beam-on time and monitor units. However, these findings reflect the specific equipment, planning setting and optimisation approach applied in this study and different planner, systems or optimisation strategies may yield different results. Therefore, treatment planning should be individualised according to patient-specific dosimetric characteristics.