Intratumoral injection of radioactive holmium (166Ho) microspheres for treatment of soft tissue sarcomas in dogs and cats.

Title • Intratumoral injection of radioactive holmium (166Ho) microspheres for treatment of soft tissue sarcomas in dogs and cats Introduction • A “microbrachytherapy” was developed as treatment option for inoperable tumors by direct intratumoral injection of radioactive holmium (166Ho) microspheres. 166Ho emits β-radiation which enables a high, ablative, radioactive-absorbed dose on the tumor while sparing surrounding tissues. Materials and methods • Twelve client-owned patients (8 dogs, 4 cats) with soft tissue sarcomas of several histologic types for which complete surgical excision was no option received 166Ho-microbrachytherapy. Standard disease staging protocol included CT evaluation of the tumor, thorax, and abdomen. Tumor response and side effects were evaluated using a standard follow-up protocol. Results • Treatment was well tolerated. Tumor-absorbed dose varied from 36 to >1000 Gy. A tumor volume reduction of >50% was observed in 6 animals; 20-50% in 4 animals; and <20% in 2 animals. Subsequent surgical excision was performed in 9/12 patients resulting in complete local remission. Local recurrence occurred in 5 animals after 121 – 1919 days. One patient received a second 166Ho-microbrachytherapy. Median survival time for all patients was 755 days (range 93–1826 days). Four animals were alive at the time of writing, seven died and one was lost to follow-up. Three deaths were disease-related. Side effects were minimal and not associated with radiation dose. Possible side effects consisted of one case of chronic skin ulceration at the injection site, 2 cases of delayed wound healing after surgery, and one case of chronic wound infection after surgery. Conclusion • 166Ho-microbrachytherapy is a safe and effective treatment for downstaging of soft tissue sarcomas allowing subsequent surgical excision.