BLADDER AND RECTAL DOSES FROM EXTERNAL-BEAM BOOSTS AFTER GYNECOLOGIC BRACHYTHERAPY

PURPOSE:To establish a typical value for radiation doses under pelvic midline Shields. MATERIALS AND METHODS: Three methods were used to det ermine bladder and rectal doses under 5- or 6-half-value layer (HVL) s hields for 10- and 24-MV external beams. First, dose was computed with a standard irregular field routine in 25 consecutive patients (aged 3 5-70 years) with stage IIB or IIIB disease treated with cesium-137 bra chytherapy followed by a parametrial external-beam boost. Second, in v ivo measurements with a solid-state probe were recorded during the fir st boost after completion of brachytherapy in each patient. Third, mea surements obtained with an ionization chamber in a solid phantom (wate r-equivalent material) were compared with computed and in vivo results . RESULTS: All three dosimetric methods yielded bladder and rectal dos es higher than the commonly assumed 5% of the unshielded primary beam dose. Doses within the shielded volume may be as high as 15% of the un shielded dose. Doses are similar under 5- and 6-HVL midline shields. O ften, the actual bladder and rectal doses exceeded the planned dose li mits and their corresponding maximum radiation dose tolerance levels. CONCLUSION: Bladder and rectal doses are higher than previously unders tood. Parametrial boosts may contribute as much as 3.0 Gy to the bladd er and rectal doses.