Effects of beam modifiers and immobilization devices on the dose in the build-up region.
OBJECTIVE
To analyze the effect that immobilization devices used in conjunction with beam modifiers may have on the dose to the skin and build-up region.


METHODS
Central axis depth dose measurements were made in a polystyrene phantom in the build-up regions using the 6 and 15 MV photon beams, at two different source-to-phantom distances, and various field sizes. The effects of acrylic blocking trays, lead wedges, and cerrobend blocks were assessed in conjunction with the enhancement of dose in the build-up region due to immobilizing devices using plaster and thermoplastic casting materials of different thicknesses.


RESULTS
For the 6 MV photons, solid (3 mm) thermoplastic casting material was found to have the greatest effect on surface dose: for a 12 x 12 cm field we measured 79% of maximum dose when treating through the material versus 22% of maximum dose when no beam modifiers or immobilization devices are used. Measurements were also made to evaluate the effect of the immobilization of patients receiving three-dimensional conformal treatments using a 15 MV photon beam.


CONCLUSIONS
The relevance of these results to treatments in the pelvis, breast, and head and neck regions is discussed. For 6 MV beams, special consideration should be given if the need arises to treat through the immobilization device, as unacceptable skin reactions may result.
Fontenla
D P
DP
Memorial Sloan-Kettering Cancer Center, New York, NY 10021.
Napoli
J J
JJ
Hunt
M
M
Fass
D
D
McCormick
B
B
Kutcher
G J
GJ
D004307:Dose-Response Relationship, Radiation; D006801:Humans; D007103:Immobilization; D008954:Models, Biological; D011829:Radiation Dosage; D011835:Radiation Protection; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011882:Radiotherapy, High-Energy / Q000379:methods; D012867:Skin / Q000528:radiation effects*