BACKGROUND: To quantify the effect of implanted||| low-dose-rate iodine seeds combined with fractionated external beam radiation||| on local control rates in an experimental tumor system. MATERIALS AND METHODS:||| Experiments were done on the rhabdomyosarcoma R1H of the rat transplanted s.c.||| into the back of male WAG/Raj albino rats. Tumors were irradiated with 200 kVp||| X-rays with 2 Gy/fraction 5 times weekly. The total dose of the external beam||| irradiation varied between 60 and 98 Gy for external beam radiotherapy alone||| and 10 Gy to 82 Gy for combined external beam radiotherapy and iodine seeds.||| One to 4 iodine seeds with a median activity of 21.05 MBq were permanently||| implanted 3 days before the start of external radiotherapy or 6 and 7 iodine||| seeds alone were used. The median tumor volume at the start of treatment was||| 0.12 cm3. Local tumor control rates were determined and TCD37% values were||| calculated applying the maximum likelihood method. RESULTS: With increasing||| number of implanted iodine seeds the TCD37% (of external beam irradiation)||| decreased. With external beam radiotherapy alone the TCD37% amounted to 103.2||| Gy (95% CI, 101.3 to 105.1 Gy) decreasing to (externally applied doses) 69.7 Gy||| (63.7 to 74.7 Gy) after 1 implanted iodine seed and further to 31.6 Gy (25.6 to||| 37.6 Gy) after 4 implanted iodine seeds. The effective dose (equivalent to||| external dose) per iodine seed decreased with increasing number of implanted||| iodine seeds. One iodine seed gave an effective dose of 33.5 Gy (28.5 to 39.5||| Gy) decreasing to 17.9 Gy (16.4 to 19.4 Gy) after 4 iodine seeds. CONCLUSIONS:||| The combined treatment of tumors with implanted low-dose-rate iodine seeds and||| external beam irradiation can decrease the total dose of the external beam||| irradiation and, hence, offer the possibility of considerable dose sparing of||| normal tissues without compromising local tumor control rates.
