Peripheral neutron and photon doses in radiotherapy
Miércoles 26 de abril, 15:30, auditorio JK
Irazola L 1,2, Terrón JA2,1, Sánchez-Nieto3 B and Sánchez-Doblado F1,2.
1Departamento de Fisiología Médica y Biofísica, Universidad de Sevilla, Spain
2Servicio de Radiofísica, Hospital Universitario Virgen Macarena, Sevilla, Spain
3Instituto de Física, Pontificia Universidad Católica de Chile, Santiago, Chile
The study of Secondary Malignant Neoplasms (SMNs) as a consequence of peripheral doses in radiotherapy is becoming a topic of interest nowadays. This is due to the higher healing rates and life expectancy accomplished with current diagnose procedures and treatment modalities . In the case of modern techniques, there is a tendency to prefer low (i.e. 6 MV) energies to high ones (e.g. 15 or 18 MV), sometimes to the detriment of treatment conformity as the latter are known to increase equivalent dose to patients due to neutron production [2,3].
Our group developed a simple and universal methodology for the estimation of peripheral neutron doses, based on the correlation between thermal neutron fluence at a reference location (far from the patient) and those inside an adult anthropomorphic phantom . This correspondence model directly links detector readings (referring to thermal neutron fluence in the room) to equivalent neutron doses at specific organs in the patient. Two different model locations (i.e. H&N and abdomen) were considered for this methodology, as they demonstrated to be general enough to cover any specific high-energy treatment .
The acquisition of a new prototype of a miniaturized active thermal neutron detector (TNRD), initially designed for nuclear purposes, opened the possibility of improving this methodology to be used at any facility with any thermal neutron detector, following a simple characterization procedure . Despite the good performance of these new devices, critical conditions during some specific treatments, made necessary the development of special procedures to improve their behavior in clinical routine. For that, several studies to ensure the good performance of TNRD devices, in terms of stability and photon rejection, were performed [7-10].
We considered that patient age and anatomy is an aspect of special importance for specific group of patients such as children, whose life expectancy and radiosensibility are greater. Thus, we aimed to include it as an additional parameter in the model. This methodology was applied to 510 patients, starting the generation of a database that would allow a more patient specific analysis of second cancer risk, as a consequence of neutron contamination .
Besides, the analytical peripheral photon model simultaneously developed in collaboration with other institutions, has allowed the generation of a piece of software for the estimation of both peripheral photon and neutron doses . This would enable an easy assessment of photon and neutron peripheral doses in clinical routine from readily available parameters. Estimations of these doses were finally evaluated for some of the most common tumor locations, comparing conventional techniques and fractionations (3D-CRT, IMRT, VMAT) to newer ones (SBRT, FFF), regarding the three main linac vendors and energies (6, 10 and 15 MV). As a general pattern, hypofractionated modality, 10 MV photon energy and FFF irradiation mode have shown as the best alternatives in terms of peripheral dose reduction [13,14]. Thus, a combined use of these options would imply a decrease of second cancer probability.
Additionally, second cancer risk estimations could be easily performed from the here presented procedures by the direct use of the existing risk models, established by the international organisms (i.e. ICRP or BEIR). The universal methodology presented aims to provide an objective additional criterion to be used in combination with the previously existing radiobiological parameters as Tumor Control Probability (TCP) and Normal Tissue Complication Probability (NTCP), for the choice of the best radiotherapy strategy, thanks to its easy implementation in Treatment Planning Systems .
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