Radioactive holmium (166Ho) therapy for oral squamous cell carcinoma in cats & dogs

Introduction: Inoperable oral squamous cell carcinomas (SCC) in dogs and cats carry a poor prognosis. Radiation therapy has not been able to improve outcome and its dose-rate is limited by side-effects in surrounding tissues. We have developed an interstitial microbrachytherapy technique in which inoperable tumors are injected with radioactive 166-holmium (166Ho) microspheres (Ø 10-20 µm). 166Ho emits β-radiation (Eβ,max=1.84 MeV, t1/2=26.8 hrs) with a maximum tissue penetration of 8.7 mm. This short penetration depth enables application of a high, tumor-ablative radiation dose (200-800 Gy) in a single treatment, without causing extensive collateral damage in surrounding tissues. Objectives: To evaluate 166Ho-microbrachytherapy in dogs and cats with unresectable oral SCC. Methods: Fourteen patients (11 cats, 3 dogs) with oral SCC without evidence of metastasis (except 2 dogs with a localized metastatic lesion of tonsilar carcinoma) were treated by intratumoral injections of 166Ho-microspheres. Tumors were debulked using laser surgery if possible. Cats routinely received an esophageal feeding tube. A standardized treatment and follow-up protocol was used. Results: Complete response rate, being complete remission or sufficient (>70%) tumor volume reduction for subsequent surgical removal, was 43%. Median survival time was 78 days overall, and 382 days for animals with complete response. Side effects were minimal. Conclusions: 166Ho microbrachytherapie has great potential for targeted radio-ablation of unresectable tumors with minimal morbidity. The high rate of incomplete responders may be due to suboptimal spatial distribution of microspheres inside the tumor after injection. Experiments are ongoing to improve treatment technique using CT/MRI/SPECT visualization of 166Ho-microsphere distribution.