Intratumoral injection of holmium-166 microspheres as neoadjuvant therapy of soft tissue sarcomas in dogs.

Introduction: Minimally invasive microbrachytherapy is in development to treat solid tumors by intratumoral injection of (radioactive) holmium-166 ( ¹⁶⁶ Ho) microspheres (MS). A high local dose can be administered with minimal damage to surrounding tissue because of the short soft tissue penetration depth of ¹⁶⁶ Ho beta radiation. We aimed to prospectively evaluate the safety and efficacy of ¹⁶⁶ Ho microbrachytherapy in client-owned canine patients with soft tissue sarcomas (STS).
Methods: We included seven dogs with STS not suitable for local excision due to tumor size and/or location. ¹⁶⁶ HoMS were suspended in a carrier fluid and multiple needle-injections were performed in predetermined tumor segments to maximize tumor coverage. Tumor response was evaluated using 3D caliper and CT measurements. Follow-up further included monitoring for potential side effects and registration of subsequent treatments and survival, until at least two years after treatment.
Results: Delivered radioactive doses ranged from 70 to 969 Gy resulting in a mean tumor volume reduction of 49.0 ± 21.3% after 33 ± 25 days. Treatment-related side effects consisted of local necrosis ( n = 1) and ulceration of the skin covering the tumor ( n = 1), which resolved with basic wound care, and surgical excision of residual tumor, respectively. Residual tumor was surgically resected in six patients after 22-93 days. After a mean follow-up of 1,005 days, four patients were alive, two patients were euthanized because of unrelated causes, and one patient was euthanized because of disease progression after the owner(s) declined subsequent surgical treatment.
Conclusion: ¹⁶⁶ Ho microbrachytherapy was a safe and effective neoadjuvant treatment option for canine patients with STS.
Competing Interests: Author JN is co-founder and part-time scientific advisor of Quirem Medical which has been acquired by Terumo Europe NV in July 2020. He is entitled to certain milestone payments from Terumo which are related to Quirem's financial, operational, and regulatory performance in the future. Furthermore, he is inventor on the patents related to radioactive microspheres that are assigned to University Medical Center Utrecht Holding BV, Quirem Medical or BASF Corp. The activities of author JN within Quirem Medical are approved and supported by the Board of Directors of the Radboudumc. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
(Copyright © 2022 Morsink, Nijsen, Grinwis, Hesselink, Kirpensteijn and van Nimwegen.)