Monte Carlo study for designing a dedicated `D'-shaped collimator used in the external beam radiotherapy of retinoblastoma patients.

Purpose: The purpose of the present article is to propose a modified version of the D-shaped collimator used at the Univ. Hosp. of Essen for retinoblastoma treatment that reduces even further the irradiation field with the scope to reduce the risk of radio-induced secondary malignancies. The new dedicated D-shaped collimator is easier to build and produces dose distributions that only differ on the field size with respect to the dose distributions obtained by the current collimator in use. Methods: The Monte Carlo code PENELOPE was used to study the effect that the structural elements of the collimator have on the absorbed dose distribution. The radiation transport through a Varian Clinac 2100 C/D operating at 6MV was simulated in order to tally phase-space files which were used as radiation sources to simulate the considered collimators and the subsequent dose distributions. Results: The proposed collimator delivers a dose distribution which is 2.4cm wide along the inferior-superior direction of the eyeball. This width is 0.3cm narrower than that of the dose distribution obtained with the collimator currently in use. The other relevant characteristics of the dose distribution obtained with the new collimator, namely, depth doses, penumbrae width and shape of the lateral profiles, are statistically compatible with the results obtained for the collimator in use. Conclusions: The smaller field size delivered by the proposed collimator still fully covers the planning target volume with at least 95% of the maximum dose at a depth of 2cm and provides a safety margin of 0.2cm, so ensuring an adequate treatment while reducing the dose absorbed by surrounding structures of the eye.