Ponente
Descripción
Objective: The quantitative relationship between dose delivered outside the treated volume and stochastic effects is not well understood, although epidemiological evidence supports the hypothesis that low to moderate doses of ionizing radiation are associated with measurable excess risk of several types of cancer. Current studies of second primary cancer after radiotherapy demand the possibility of computing whole-body dose distributions on patients treated with modern photon and proton external beam radiotherapy (EBRT) techniques.
Methods: In the framework of the Horizon Euratom HARMONIC project a database of pediatric modern radiotherapy patients, treated with photon or proton EBRT, was created. This database stores for each patient DICOM-RT computed tomography images and treatment plans. The whole-body dose computation requires a patient geometry in which to compute the corresponding dose distribution. A software developed within HARMONIC was employed for that purpose using anthropomorphic phantoms. For the computation of the whole-body absorbed dose distribution in photon treatments, the Monte Carlo system PRIMO was used, while proton treatments were computed with the Monte Carlo code TOPAS/Geant4 . The imaging whole-body dose associated with diagnostic and positioning procedures during therapy was also computed with the Monte Carlo code PENELOPE. The developed codes and geometries were experimentally validated by means of an anthropomorphic pediatric material phantom (ATOM) irradiated with the considered therapeutic and imaging procedures.
Results: Out-of-field organ equivalent dose of proton therapy is up to two orders of magnitude lower when compared to modern photon therapy. As an example, for a brain glioma treatment, with a total target dose of 50.4Gy(RBE), it was found that the organ doses from the proton treatment ranged between 0.6 mSv (testes) and 120 mSv (thyroid), while for photon therapy (IMRT) 43 mSv (testes) and 575 mSv (thyroid). Dose delivered by planning CT ranged between 0.01 mSv (testes) and 72 mSv (scapula), while for the 28 fractions, the imaging procedure for positioning (CBCT) yielded doses ranging between 56 uSv (testes) and 36 mSv (thyroid).
Conclusions: The developed and validated code systems for dose computation have shown to be suitable for the computation of whole-body dose distributions of patients stored in the HARMONIC database. The lower out-of-field dose of proton treatments respect to equivalent photon irradiations results in the fact that the imaging dose from diagnostic and positioning procedures becomes significant in proton therapy. This research is the result of the work conducted at the West German Proton Therapy Centre Essen, the Belgian Nuclear Research Centre, the French Alternative Energies and Atomic Energy Commission, the University Hospital Essen, the University Hospital of Zürich and the Institut Gustave Roussy.
