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Bladder and rectal doses from external-beam boosts after gynecologic brachytherapy
1Department of Radiation Oncology, University of Miami School of Medicine, Fla., USA.
Radiology
|December 9, 1998
Summary
Pelvic midline shields deliver higher radiation doses to the bladder and rectum than previously assumed. Actual doses can reach 15% of the unshielded dose, potentially exceeding tolerance levels.
Area of Science:
- Radiation oncology
- Medical physics
- Gynecologic oncology
Background:
- Pelvic midline shields are used in radiation therapy to protect the bladder and rectum.
- Accurate dose assessment under these shields is crucial for treatment planning and toxicity reduction.
Purpose of the Study:
- To determine typical radiation doses delivered to the bladder and rectum when using pelvic midline shields.
- To compare measured and computed doses under shielding conditions.
Main Methods:
- Dose computation using standard irregular field routines in 25 patients with advanced cervical cancer.
- In vivo dosimetry using a solid-state probe during external beam boosts.
- Ionization chamber measurements in a solid phantom for comparison.
Main Results:
- Bladder and rectal doses under 5- or 6-half-value layer (HVL) shields were consistently higher than the commonly assumed 5% of the unshielded dose.
- Actual doses could reach up to 15% of the unshielded dose, frequently exceeding planned dose limits.
- Doses were similar for both 5- and 6-HVL shields.
Conclusions:
- Radiation doses to the bladder and rectum under pelvic midline shields are significantly higher than previously understood.
- Parametrial boosts can contribute substantially to the total bladder and rectal dose, potentially up to 3.0 Gy.
- Current shielding practices may not adequately protect these organs at risk, necessitating re-evaluation of dose constraints.