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

Clarissa C. Freemyer, Tracy L. Gieger, Shelly L. Vaden, Michael W. Nolan

Background
Canine genitourinary carcinomas (CGUC) are common malignancies of the lower urinary tract that pose challenges in treatment planning due to their variable extent and resistance to radiotherapy (RT). Geographic miss during RT, caused by underestimating tumour boundaries, is a significant contributor to treatment failure. This study evaluated cystourethroscopy as a tool for improving gross tumour volume (GTV) delineation during RT planning, hypothesizing that it would identify lesions beyond those visible with computed tomography (CT) alone.

Methods
The study retrospectively analyzed data from 54 dogs diagnosed with CGUC between 2013 and 2023. Each dog underwent pre- and post-contrast CT imaging and cystourethroscopy. The GTV was initially defined using CT, then expanded based on findings from cystourethroscopy. Measurements included the length of urethral involvement and the extent of GTV expansion. Descriptive statistics were used to evaluate the data.

Results
-Cystourethroscopy expanded the GTV in 26 of 54 cases (48%).
-Median GTV extension into the urethra was 6.5 cm (range: 1.5–31.8 cm).
-Expansion beyond the standard 2 cm clinical target volume (CTV) margin was identified in 19 cases (35%), suggesting a risk of geographic miss with CT alone.
-GTV extension was predominantly distal into the urethra (22 cases), with a smaller proportion expanding proximally into the bladder (4 cases).
-Cystourethroscopy identified abnormalities not visible on CT in multiple cases, highlighting its potential to refine treatment planning.

Limitations
The study could not confirm that all cystourethroscopically visible lesions were neoplastic, potentially overestimating tumour extent. The retrospective design and absence of histopathological or necropsy data further limited comprehensive conclusions. Instrumentation size restricted the use of cystourethroscopy in small dogs, and resource demands may challenge its routine implementation.

Conclusions
Cystourethroscopy enhances the precision of GTV delineation in dogs with CGUC and reduces the risk of geographic miss during RT. While incorporating cystourethroscopy into treatment planning shows promise, further research is required to validate its impact on treatment outcomes and to explore cost-effective, non-invasive alternatives for routine clinical use.

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