Bladder and rectal doses from external-beam boosts after gynecologic brachytherapy

To establish a typical value for radiation doses under pelvic midline shields.

Three methods were used to determine bladder and rectal doses under 5- or 6-half-value layer (HVL) shields for 10- and 24-MV external beams. First, dose was computed with a standard irregular field routine in 25 consecutive patients (aged 35-70 years) with stage IIB or IIIB disease treated with cesium-137 brachytherapy followed by a parametrial external-beam boost. Second, in vivo measurements with a solid-state probe were recorded during the first boost after completion of brachytherapy in each patient. Third, measurements obtained with an ionization chamber in a solid phantom (water-equivalent material) were compared with computed and in vivo results.

All three dosimetric methods yielded bladder and rectal doses 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 unshielded dose. Doses are similar under 5- and 6-HVL midline shields. Often, the actual bladder and rectal doses exceeded the planned dose limits and their corresponding maximum radiation dose tolerance levels.

Bladder and rectal doses are higher than previously understood. Parametrial boosts may contribute as much as 3.0 Gy to the bladder and rectal doses.