The influence of an immobilization mask on the absorbed dose in the build-up region of photon beams
Abstract
The purpose of this work is to study how a mask, used for immobilization, alters the depth dose in the build up region of photon beams as compared to the situation in the absence of the mask. Depth dose curves in the build up region of photon beams (@Co,6MV) and with field sizes 5 x 5 cm2 and 15 x 15 cm2, were measured by 3-4 batches of LiF TLDs (more than 50 LiF TLDs, 0.13 mm thin) alternatively by a plane parallel ionisation chamber (NACP 02). The experiments were carried out in the presence and absence of the mask. To ensure the linearity of the dosemeters, the dose was limited to 1 Gy. The data show that the mask can significantly increase the dose at the surface from 11% to 76% for 5 5 x 5 cm2 and from 25% to 83% for 15 x 15 cm2 in the &Co. In the 6 MV beam it alters the surface dose from 7% to 48% for 5 x x 5 cmp and from 18% to 56% in 15 x 15 cm? field. The mask will actually change the dose in the whole build up region but the dose increase will be less with increasing depth up to nearly the maximum dose where no more influence of the mask is shown.The data show that the relative influence of the mask in term of increased absorbed dose in the build up region is higher in small fields compared to large fields but in term of the absolute absorbed dose, in large fields is of course higher than in small fields. The effect of the mask on the surface dose is higher in Co as compared to 6 MV. The results show that the mask will substancially increase the absorbed dose in the build up region. This is particularly important at high doses where skin reactions can be a problem. This dose increase should be verified experimentally for each radiation beam and type of mask in order to obtain for considering to decrease early and late radiation induced effects in normal tissues.
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