An evaluation of surface-dose increase caused by the thermoplastic shell in head and neck radiation therapy

Some kinds of the thermoplastic shells have been developed to improve the reproducibility of patient immobilization in head and neck radiotherapy treatment. Due to a build-up layer of high energy photons, the consideration of the surface-dose under the shell is important in radiotherapy treatment planning. This paper presents the precise evaluation of the surface-dose affected by the shell. The therapy machine used was 2.8 MV linac X-ray. The shell used was mesh sheet type thermoplastic shell. The TPR (tissue peak ratio) in the build-up layer was measured in various irradiation fields under the conditions with or without a wedge filter. The coefficients m' and K's of the power function of the build-up region were obtained from the relation between TPR and the irradiation fields. As a result, the coefficients were approximately intermediate values between cobalt-60 γ-ray and 4.3 MV X-ray. When a wedge filter was used, the coefficients shifted toward those of higher energy. Before obtaining the dose distribution under the thermoplastic shell, film response was tested under the conditions of various irradiation fields and use of a wedge filter. The results showed that the film response was constant enough for dosimetry in the build-up region. Digital surface-absorbed dose distribution images under the shell were obtained using the microphotometer-microcomputer system. The digital image demonstrated a meshy dose pattern under the shell. In the area of a higher absorbed dose, the dose increases were in the range of 40-50%. The differences depended on not only the irradiation field and presence of a wedge-filter, but especially on the extent of expansion of the thermoplastic material in making the shell mask. (author)