Accuracy of the convolution/superposition dose calculation algorithm at the condition of electron disequilibrium.

Using Monte Carlo simulation and the convolution/superposition algorithm, this work examines percent depth dose curves of the central axis in an acrylic phantom (20 x 20 x 20 cm(3)) with variously sized air cavities (20 x 20 x 1.0, 20 x 20 x 2.0, 20 x 20 x 3.0, 20 x 20 x 4.0 and 20 x 20 x 4.95 cm(3) for study of longitudinal electron disequilibrium (ED) and 3.6 x3.6 x 4.95, 4.5 x 4.5 x 4.95, 5.4 x 5.4 x 4.95 and 20 x 20 x 4.95 cm(3) for study of lateral ED). Radiochromic film samples are also measured to verify the Monte Carlo results. The Monte Carlo simulation is performed using OMEGA/BEAM and DOSXYZ codes, and the convolution/superposition calculation relies on an ADAC commercial treatment planning system. Underestimating the dose kernel expansion leads to overestimating the dose of what was found in the air cavity of ED using the convolution/superposition algorithm. Consequently, the dose in the rebuild-up region is influenced. The influenced region is on the acrylic phantom surface to a depth of about 0.5 cm. The density scaling method of the convolution/superposition algorithm, applied to heterogeneous media, should be enhanced to account for the over-expansion of the dose kernel in the cavity of ED.