Use of Cystourethroscopy to Define the Gross Tumour Volume in Radiation Treatment Planning for Canine Genitourinary Carcinomas.

Radiotherapy (RT) is increasingly utilised for definitive-intent treatment of canine genitourinary carcinomas (CGUC). At our institution, the standard approach is to irradiate tomographically abnormal tissues gross tumour volume (GTV) plus a clinical target volume (CTV) expansion of 2 cm. Cystourethroscopy is often incorporated into the treatment planning workflow, though an optimal approach has yet to be defined. This observational study evaluated cystourethroscopy as a tool for identifying gross lesions that can be targeted with RT. We hypothesised that in most cases, addition of cystourethroscopy would result in a larger GTV than would be drawn with computed tomography (CT) alone. Medical records from 54 dogs diagnosed with CGUC between 2013 and 2023 were reviewed; each had been evaluated before RT using CT and cystourethroscopy. The GTV was initially defined as the tomographically evident disease on a post-contrast sagittal plane CT scan, and then lesions visualised with cystourethroscopy (suspected or confirmed to be tumour) were added. Beyond what was visible on CT, cystourethroscopy extended the GTV by a median of 6.5 cm distally into the urethra (range: 1.5-31.8 cm) and therefore resulted in GTV enlargement in 26 of 54 (48%) cases. Addition of our standard 2 cm CTV expansion to a CT-defined GTV (without use of data from cystourethroscopy) would have underestimated the extent of grossly abnormal tissue in 35% (19/54) of cases. These results suggest that incorporating cystourethroscopy into treatment planning workflows may improve local tumour control by reducing the risk of a geographic miss.