Development and experimental validation of TPS software to determine the dose outside the radiation beam

Igor Bessières (1) , jean-marc bordy (2) , Bénédicte Poumarède (1)

1 LM2S (CEA, LIST) - Laboratoire Modélisation et Simulation de Systèmes (CEA, LIST)
2 LNHB (CEA, LIST) - Laboratoire National Henri Becquerel (CEA, LIST)

Purpose: Even if the IMRT modality allows a more accurate definition of the target volume in radiotherapy, low doses are still delivered around the tumour to organs at risk. Epidemiological studies demonstrated the relationship between peripheral doses and second cancers or heart diseases. Our aim is to develop a Monte Carlo tool to compute the dose at the target volume and the organs at risk in order to enable a decrease of the peripheral dose by adapting the treatment’s parameters. This tool will be implemented in a TPS (Treatment Planning System). In this paper we expose the first validation step of the out-of-field Monte Carlo calculations, using a comparison with measurements in a specific water tank. Materials: The PENELOPE MC (Monte Carlo) code has been parallelized to save computation time by running the calculations on a cluster of 84 processors.To validate the simulations results, OSL (Optically Stimulated Luminescence) dosimeters made of Al2O3:C were used. The OSL Nanodots are manufactured by LCIE Landauer. The OSL measurements are compared with NE2571 IC (ionization chamber) measurements. Irradiation has been done at LNHB (French Primary Standard Laboratory) on a GE Saturne 43 accelerator at the 6, 12 and 20 MV qualities. The only difference with the irradiation conditions proposed in the AIEA protocol 398 is the use of a large water phantom specially machined for this experiment; it is a 60x30x30 cm3 tank. Results: Three main corrections are done on the OSL measurements: particular sensitivity for each Nanodot, air calibration factor Dwater/Kair and energy dependence correction. The picture shows out-of-field dose profiles at reference depth (10 cm) for MC calculations, ionization chamber and OSL measurements. The doses are normalized to the maximum of dose. In this figure, the energy dependence is not corrected for the OSL. Considering that the ionization chamber dose is the reference value, one can observe that OSLs always over-estimate the dose. The energy response of the OSL dosimeter has been investigated to correct this over response. The MC data are in good agreement with the IC measurements up to 200 mm from the centre of the beam. For further positions, the calculations underestimate the dose in comparison with the IC measurements. In the beam, the data are in good agreement (error < 3 %). Conclusions: After correcting the energy dependence, the OSL dosimeter will give satisfying results so that it can be used to validate the calculations of the new MC calculation tool in a real IMRT configuration with an anthropomorphic phantom. Further research is necessary to explain the discrepancy between measured and calculated dose for large distances. Besides, the implementation of reduction variance techniques in the MC tool should be helpful by allowing a better accuracy of the calculation out of the beam. Part of the work has been done within the frame work of EURADOS WG9

Mots clés: metrology; Particle transport; ionization chamber; dose measurement; radiotherapy; ionizing radiation; dosimetry; simulation; modelling; Treatment Planning System; Monte Carlo; PENELOPE; Optically Stimulated Luminescence; OSL dosimeter; instrumentation; LINAC; linear accelerator; Saturne; nuclear instrumentation; irradiation; phantom; out-of-field dose

Domaines: Physique Médicale [physics.med-ph]; Modélisation et simulation; Méthodologie [stat.ME]; Cancer

Igor Bessières, jean-marc bordy, Bénédicte Poumarède. Development and experimental validation of TPS software to determine the dose outside the radiation beam. 11th Biennial Conference on Physics and Radiation Technology for Clinical Radiotherapy (ESTRO - 2011), May 2011, London, United Kingdom. Radiotherapy and oncology, Volume 99, Supplement 1, May 2011, Page S176, 99 (supplement 1), S176, poster 436, 2001, ⟨10.1016/S0167-8140(11)70558-8⟩. ⟨cea-02558172⟩