Tumor type and vascularity: important variables in infusional brachytherapy with colloidal 32P

This study investigated the role of histologic tumor characteristics, in comparison with a normal tissue, and of tumor vascularization on the uptake and retention of colloidal 32P used in infusional brachytherapy of solid cancers. The cytotoxicity of colloidal 32P was also evaluated for two tumors of different radiosensitivity, a melanoma, and a squamous cell carcinoma.

An in vitro analysis of colloidal 32P uptake was carried out on a human melanoma cell line, HBL, a human squamous cell carcinoma, SCC1, and normal fibroblasts, F-NBB. Tumor retention of colloidal 32P was studied in vivo for the HBL and the SCC1 tumors implanted subcutaneously in nude mice. Tumor vascular density was determined by microscopic study of Masson's trichrome slides of HBL and SCC1 tumors of about 1 cm diameter.

In vitro studies showed that the time required for maximal cell uptake of colloidal 32P was only 10-20 min for the SCC1 and HBL tumors, while it took at least 60 min for the fibroblasts. After intratumoral injection of macroaggregated albumin (MAA), followed by 50 microCi of colloidal 32P, Bremsstrahlung imaging performed at 6 and 24 h showed that the activity remained in the HBL tumor while some of the radiocolloids leaked from the SCC1 tumor and was trapped in the reticuloendothelial system of the liver. Organ activity counting confirmed this finding: 32P activity was three to four times higher in the HBL than in the SCC1 tumor, whereas the activity in the liver, insignificant in the HBL mice (less than 0.1 microCi/g), was as high as 24 microCi/g in the SCC1 mice. This phenomenon may be explained by the difference in tumor vascular density, estimated for the HBL to be about four times less than that of the SCC1 tumor (5.7 vs. 21.4 blood vessels per mm2 for the HBL and the SCC1 tumors, respectively).

Intratumoral infusion of colloidal 32P may be a useful complement of radiation therapy in the treatment of nonresectable but accessible solid tumors. Tumor vascularization must be taken into account for a successful vascular blockade by MAA prior to the infusion of colloidal 32P.