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

Radiology. 1998 Dec;209(3):825-30. doi: 10.1148/radiology.209.3.9844682.

Abstract

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

Materials and methods: 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.

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.

Conclusion: 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.

MeSH terms

  • Adult
  • Aged
  • Brachytherapy*
  • Female
  • Humans
  • Middle Aged
  • Radiotherapy Dosage*
  • Rectum*
  • Retrospective Studies
  • Urinary Bladder*
  • Uterine Neoplasms / radiotherapy*