Intratumoral injection of radioactive holmium ( (166) Ho) microspheres for treatment of oral squamous cell carcinoma in cats

Background & Aims A “microbrachytherapy” was developed as treatment option for inoperable tumours by direct intratumoral injection of radioactive holmium-166 ( 166Ho) microspheres (MS). 166Ho emits β-radiation which potentially enables a high, ablative, radioactive-absorbed dose on the tumour tissue while sparing surrounding tissues. Materials & Methods Safety and efficacy of 166Ho microbrachytherapy were evaluated in a prospective cohort study of 13 cats with inoperable oral squamous cell carcinoma without evidence of distant metastasis. Results Local response rate was 55%, including complete response or partial response (downstaging) enabling subsequent marginal resection. Median survival time was 113 days overall, and 296 days for patients with local response. Side effects were minimal. Tumour volume was a significant predictor of response. Discussion Response rate may be further improved by optimizing the intratumoral spatial distribution of 166Ho MS. Conclusion 166Ho microbrachytherapy has potential as a minimally invasive, single procedure radio-ablation treatment of unresectable tumours with minimal morbidity.