Evaluation of a 32P patch for therapy of squamous cell carcinoma in cats

Full text: Brachytherapy, that is, the placement of radioactive sources into or near the tumour, has been a big challenge in nuclear medicine for the therapy of cancer. Pirocarbotrat, is a gelatin-protected charcoal suspension labeled with chromic [32P] pyrophosphate that behaves very much like a sealed B-radiation source for the treatment of solid tumours. The aim of this work was to make use of these properties in order to design a silicon patch coated with Pirocarbotrat for topical application in skin cancer lesions. Materials and methods: we selected four adult cats with nasal, state II squamous cell carcinoma (SCC). Measurements of the lesions sizes were taken and the patches were specially designed for its application on the lesion surface with minimal contact with the normal surrounding tissue. Animals were then anesthetized to get immobilized to both facilitate patches application and to prevent their remotion. Dosimetric calculations were done in each case taking into account the time of exposure and the activity contained in the patch. Total surface of the patches was 4.5 cm2 and the activity per surface varied between 30.7 - 32.2 MBq (871.3 - 830.1 uCi / cm2). The patches were applied on the surface of the nose SCC lesions for 3.5 - 4.0 h for a total radiation dose of 28 - 33 Gy (2800 - 3300 rads). During the time of exposition, the animals were isolated in cages specially conditioned for this purpose. When exposition time finished, the patches were removed and the animals were returned to their owners. Clinical evaluation after fifteen days of the treatment showed that in one case tumour disappeared and an erythema with alopecia and hypopigmentation developed in the treated site. In the other three cases lesion reductions were about 50% of their original size with concomitant development of peritumoural fibrosis as well as central necrosis in the treated site. The shared feature in the four cases was the great local inflammatory response after brachytherapy at the site where patches were applied. All these responses are in accordance with those expected after radiation therapy. Furthermore, as total radiation dose (28-33 Gy) was delivered in only one session of 3.5-4.0 h, tissue damage such as fibrosis, necrosis and erythema of the treated site is also indicative of the effectiveness of the treatment. However, the histopathological results of the follow-up biopsies, showed that total remission was not achieved in none of the four cats. Conclusions: Surface applicators are used for superficial tumours as the maximum dose is at the surface and falls off rapidly with depth. On the other hand, dose fractionation is related to tissue repair which depends on the cell turnover rate of the tissue nature (normal or neoplasic). Therefore, the treatment planning should take into account both lesion depth and tissue rate of cell turnover in order to achieve the local control of the tumour for a longer period and to prolong survival. Although total remission was not achieved, invasion and dissemination of the tumours were prevented after brachytherapy. Therefore, this clinical experience allow us to confirm treatment efficacy of the 32P patch for skin cancer but signalling the importance of the planning dose scheme since until now, only partial remission was achieved. Future directions will lie on finding optimal dose fractionation or extending exposure time with patches of minor activity per surface in order to achieve total remission. (author)