Abstract ID: 65 Dosimetry for treatment of retinoblastoma with external photon beams

Abstract

Retinoblastoma is the most common intraocular malignancy in the early childhood. Patients treated with external beam radiotherapy respond very well to the treatment. The University Hospital of Essen has successfully treated these patients on a daily basis during nearly 30 years using a dedicated “D”-shaped collimator inserted in the head of a Varian Clinac 2100 C/D operating at 6 MV. The collimator conforms a “D”-shaped off-axis field whose irradiated area can be either 5.2 or 3.1 cm2. In this work, a dosimetric analysis of the technique has been carried out by using the Monte Carlo (MC) code PENELOPE. Experimental depth dose distributions and lateral profiles were compared with those obtained with Monte Carlo and with the analytical anisotropic algorithm (AAA). PENELOPE simulations agree well with the experimental data with differences in the dose profiles less than 3 mm/3%. Discrepancies between AAA and the experiment reach 3 mm/6%. [1] Absolute dosimetry has been estimated for the collimator with two MC approaches in which the backscattered dose in the monitor chamber is determined [2] . Results are compared to experiment and to those provided by AAA. The MC results for the absolute doses differ from the experimental ones by 2.6% and 1.7%, depending on the tracking parameter set used for the electron transport in the target of the linac head. For the studied radiation fields, the two MC approaches produce absolute doses that are statistically compatible. AAA underestimates the experimental absolute doses with discrepancies larger than those found for MC results. A new “D”-shaped optimized collimator has been proposed [3] . It permits to reduce the risk of radio-induced secondary malignancies and may be easily built up. The new collimator delivers a dose distribution that is 0.3 cm narrower than that obtained with the current collimator, maintaining the other relevant dosimetric characteristics (depth doses at clinically relevant positions, penumbrae width, and shape of the lateral profiles). This allows using the clinical experience gained in the past.