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.