FETAL DOSE EVALUATION DURING BREAST-CANCER RADIOTHERAPY

Purpose: The aim of the work was to estimate the radiation dose delive red to the fetus in a pregnant patient irradiated for breast cancer. M ethods and Materials: A 45-year woman was treated for left breast canc er using a 6 MV photon beam with two isocentric opposing tangential un wedged fields. Daily dose was 2.3 Gy at 95% isodose line given by two fields/day, 5 days/week. A total dose of 46 Gy was given in 20 fractio ns over a 4-week period. Pregnancy confirmed during the second therape utic week. Treatment lasted between the second and sixth gestation wee k. Radiation dose to fetus was estimated from in vivo and phantom meas urements using thermoluminescence dosimeters and an ionization chamber . In vivo measurements were performed by inserting either a catheter w ith TL dosimeters or ionization chamber into the patient's rectum. Pha ntom measurements were performed by simulating the treatment condition s on an anthropomorphic phantom. Results: TLD measurements (in vivo an d phantom) revealed fetal dose to be 0.085% of the tumor dose, corresp onding to a cumulative fetal dose of 3.9 cGy for the entire treatment of 46 Gy. Chamber measurements (in vivo and phantom) revealed a fetal dose less than the TLD result: 0.079 and 0.083% of the tumor dose corr esponding to cumulative fetal dose of 3.6 cGy and 3.8 cGy for in vivo and phantom measurement, respectively. Conclusions: It was concluded t hat the cumulative dose delivered to the unshielded fetus was 3.9 cGy for a 46 Gy total tumor dose. The estimated fetal dose is low compared to the total tumor dose given due to the early stage of pregnancy, th e large distance between fundus-radiation field, and the fact that no wedges and/or lead blocks were used. No deterministic biological effec ts of radiation on the live-born embryo are expected. The lifetime ris k for radiation-induced fatal cancer is higher than the normal inciden ce, but is considered as inconsequential. (C) 1998 Elsevier Science In c.