Combined interstitial 125I and external X-irradiation on a rat rhabdomyosarcoma

The effect of graded external radiation doses and a dose administered by an implanted 125I seed on growth delay and cure rate of a rat rhabdomyosarcoma was investigated. One 125I seed (0.40 or 0.50 mCi) was implanted per tumour when the tumours had grown to a predetermined size. The irradiation by the 125I seed did not cause significant tumour growth delay. When implantation of an 125I seed was combined with graded external radiation doses, the growth delay observed after these combined regimens was significantly greater than that observed after treatment with external doses alone. The same was observed for tumour cure rates. The effectiveness of the 125I seed could be assessed as being equivalent to a single dose of external radiation of about 6-20 Gy, depending on the external dose applied. The application of 125I seeds combined with external irradiation to obtain local control might be recommended in those cases where the total dose of external radiation alone is limited by normal tissue reactions.