Reducing margins for abdominopelvic tumours in dogs: Impact on dose-coverage and normal tissue complication probability

Radiation toxicity grading after chemoradiotherapy of canine urinary tract carcinomas: Comparing VRTOG to VRTOG_v2.0

Radiation toxicities may be underestimated after treatment of transitional cell carcinoma in dogs' lower urinary tract. Assessing acute and late toxicities and differentiating them from progressive disease (PD) impacts further therapeutic approach. We retrospectively assessed dogs treated with definitive-intent chemoradiotherapy (12 × 3.8 Gy, various first-line chemotherapeutics). Local tumour control, radiation toxicities and survival were evaluated. We classified radiation toxicities according to the previously published radiation toxicity scheme "VRTOG" as well as the updated version, "VRTOG_v2.0". Fourteen dogs with transitional cell carcinoma of bladder ± urethra (n = 8), +prostate (n = 3) or solely urethra (n = 3), were included. Median follow-up was 298 days (range 185-1798 days), median overall survival 305 days (95%CI = 209;402) and 28.6% deaths were tumour-progression-related. Acute radiation toxicity was mild and self-limiting with both classification systems: In VRTOG, 5 dogs showed grade 1, and 1 dog grade 2 toxicity. In VRTOG_v2.0, 2 dogs showed grade 1, 3 dogs grade 2, and 3 dogs grade 3 toxicity. Late toxicity was noted in 14.2% of dogs (2/14) with the VRTOG, both with grade 3 toxicity. With VRTOG_v2.0, a larger proportion of 42.9% of dogs (6/14) showed late toxicities: Four dogs grade 3 (persistent incontinence), 2 dogs grade 5 (urethral obstructions without PD resulting in euthanasia). At time of death, 5 dogs underwent further workup and only 3 were confirmed to have PD. With the updated VRTOG_v2.0 classification system, more dogs with probable late toxicity are registered, but it is ultimately difficult to distinguish these from disease progression as restaging remains to be the most robust determinant.

Retrospective assessment of radiation toxicity from a definitive-intent, moderately hypofractionated image-guided intensity-modulated protocol for anal sac adenocarcinoma in dogs

A recent calculation study predicted acceptable toxicity in pelvic organs at risk for a new definitive-intent, moderately hypofractionated radiation therapy (RT) protocol (12 x 3.8 Gy), when used with image-guided intensity-modulated radiation therapy (IG-IMRT). We hypothesized this protocol to result in clinically acceptable radiation toxicities. Dogs diagnosed with and irradiated for anal sac adenocarcinoma (ASAC) were retrospectively assessed. Eleven dogs were included, six had prior surgery. Before any therapy, staging according to Polton et al. resulted in the following distribution: stage 1 (n = 1), stage 2 (n = 1), stage 3a (n = 6), stage 3b (n = 3). We scored radiation toxicities at the end of therapy, at weeks 1, 3 and every 3 months after RT according to Veterinary Radiation Therapy Oncology Group radiation toxicity criteria. Clinical follow-up was maintained on regular intervals combined with computed tomography (n = 3). Median follow-up time for dogs still alive was 594 days (range: 224-972 days). Within 1 week post treatment, eight dogs (73%) developed grade 2 and four dogs (36%) grade 1 acute toxicity in the perianal region. All acute toxicities resolved or improved to grade 1 within 3 weeks after treatment. Late toxicity, for example, chronic colitis/diarrhoea, ulcerations, strictures or myelopathies was not observed in any patient. Five dogs were euthanized 105, 196, 401, 508 and 908 days after RT and six dogs were still alive, one in spite of progressive disease. The median progression-free survival was 908 days (95%CI: 215; 1602). The previous theoretically described definitive-intent, moderately hypofractionated protocol using IG-IMRT for the treatment of advanced ASAC showed clinically acceptable acute and late toxicities.

Reducing margins for abdominopelvic tumours in dogs: Impact on dose-coverage and normal tissue complication probability

Image‐guided, intensity modulated radiation therapy (IG‐IMRT) reduces dose to pelvic organs at risk without losing dose coverage to the planning target volume (PTV) and might permit margin reductions potentially resulting in lower toxicity. Appropriate PTV margins have not been established for IG‐IMRT in abdominopelvic tumours in dogs, and herein we explore if our usual PTV 5 mm margin can be reduced further. Datasets from dogs that underwent IG‐IMRT for non‐genitourinary abdominopelvic neoplasia with 5 mm‐PTV expansion were included in this retrospective virtual study. The clinical target volumes and organs at risk (OAR) colon, rectum, spinal cord were adapted to each co‐registered cone‐beam computed tomography (CBCT) used for positioning. New treatment plans were generated and smaller PTV margins of 3 mm and 4 mm evaluated with respect to adequate dose coverage and normal tissue complication probability (NTCP) of OAR. Ten dogs with a total of 70 CBCTs were included. Doses to the OAR of each CBCT deviated mildly from the originally planned doses. In some plans, insufficient build‐up of the high dose‐area at the body surface was found due to inadequate or missing bolus placement. Overall, the margin reduction to 4 mm or 3 mm did not impair dose coverage and led to significantly lower NTCP in all OAR except for spinal cord delayed myelopathy. However, overall NTCP for spinal cord was very low (<4%). PTV‐margins depend on patient immobilization and treatment technique and accuracy. IG‐IMRT allows treatment with very small margins in the abdominopelvic region, ensuring appropriate target dose coverage, while minimizing NTCP.