Entrance and exit dose regions for a Clinac-2100C

While significant advances have taken place in confirming dose in homogeneous media and accounting for changes due to distant heterogeneities, interface dosimetry is still a dose assessment problem. The entrance and exit surfaces of the patient are a prime example where dose assessment is questionable, but important, in many clinical situations.

Data were taken to examine the effect of such parameters as field size, source-to-skin distance, blocking tray, compensation materials (lead, aluminum and brass) and various patient support materials (Mylar, graphite, thermal plastic, and foam) on the surface doses from a dual energy (6 MV, 18 MV) linear accelerator. Measurements were conducted with a thin window parallel-plate chamber.

Relative surface dose was found to have a proportional dependence on field size for both energies with surface doses ranging from 6% to 53%. A surface depth dose of 22.6% for a 15 cm field was measured for the low energy beam while a surface dose of 22.3% was observed for the 18 MV beam. The surface dose increased significantly for short source to skin distances and with the presence of a blocking tray. Compensating filter materials had little influence on surface dose. However, patient support devices such as Alpha Cradle and the graphite of the table can increase the surface dose to as much as 92%.

We found there was a loss in dose at the exit surface (in comparison with percent depth dose tables, which assume infinite depth) on the order of 15% and 11% for the 6 MV and 18 MV beams, respectively. However, this loss is quickly compensated for with the introduction of most inherent backscattering media, [which in fact can increase the dose] for example, graphite in the patient support assembly tabletop.