59. Verification of the 3-D dose calculation algorithm during total skin electron irradiation with the rotary-dual field technique

Abstract

Total skin electron irradiation is the commonly used procedure in the treatment of mycosis fungoides. The aim of this paper was to verify the elaborated algorithm for dose calculation during total skin electron irradiation with rotary-dual fields technique (TSEI-RD).

Material and method

: Authors modified the 2-D algorithm published by Podgorsak taking account of dose distribution along the body midline and doses in the body on a larger depth than in the skin. Depth-dose function, beam profile were measured in TSEI-RD conditions (spoiler, source-skin distance SSD=350cm, field size: 36×36cm at 100cm). Cylindrical vax phantom was used to calculate and then to measure the doses in a depth of 0.4cm during exposure to the electron beam of 6 MeV (at the output of Clinac-2300CD accelerator). Phantom was rotating with the pre-calculated speed during constant exposure to two fields executed one by one in each fraction. Thermoluminescent detectors (TLD) were used for in-phantom dose measurements and Marcus ionization chamber was used for calibration of TLD. Dose homogeneity on the phantom surface was checked for three phantoms with different diameters of 20, 30 and 40cm. Phantoms were irradiated at different rotating speeds.

Initial results

: Doses measured by TLD were on average by 4% lower than doses pre-calculated using the modified algorithm. Mean doses normalized to those pre-calculated and their standard deviations (both in per cents) were respectively: 96.1% and 0.4% for phantom with diameter of 20; 96.5% and 0.7% for diameter of 30cm and 96.0% and 0.8% for diameter of 40cm.

Conclusions

: Measured doses proved correctness of elaborated algorithm. Very low standard deviations are resulting from regular cylindrical shape of the phantoms.