Program Information

Analytical Model for Photon Peripheral Dose in Radiotherapy Treatments

B Sanchez Nieto

B Sanchez Nieto¹*, R El far ¹*, M Romero-Exposito² , J Lagares³ , JC Mateo⁴ ,JA Terron⁵ , L Irazola^5,6 , F Sanchez-Doblado^5,6 (1) Instituto de Fisica, Pontificia Universidad Catolica de Chile, Santiago, Santiago De Chile, Chile (2) Universitat Autonoma de Barcelona, Barcelona, Spain (3) Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, Madrid, Spain (4) Hospital Duques del Infantado, Sevilla, Spain (5) Servicio de Radiofisica, Hospital Universitario Virgen Macarena, Sevilla, Spain (6) Departamento de Fisiologia Medica y Biofisica, Universidad de Sevilla, Sevilla, Spain

Presentations

SU-E-T-43 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
The higher survival rate of radiotherapy patients entails a growing concern on second cancers associated to peripheral doses. Currently, dosimetry of out-of field doses is still under development. Our group has developed a methodology to estimate neutron equivalent dose in organs (1,2). We aimed to propose a model to estimate out-of-field photon doses in isocentric treatments from basic clinical data.

Methods:
The proposed function models the dose as the sum of leakage and scatter terms. The latter is modeled as a virtual source at the collimator, which suffers from attenuation in air and tissue, corrected by the inverse-square-law.

The model was parameterized using experimental measurements with TLD700 chips placed inside an anthropomorphic phantom (6-18MV) irradiated with conformal and modulated techniques in Elekta, Siemens and Varian linacs. This model provides photon dose at a point as a function of clinical parameters as prescription dose/UM, PTV volume, distance to the field edge, height of the MLC leaves and distance from the the MLC to the isocenter. Model was tested against independent measurements (TLD100) for a VMAT treatment on a Elekta. Dose to organs is modeled from dose to points along the head-to-feet axis of the organ of a “standard man” escalated by patient height.

Results:
Our semi-empirical model depends on 3 given parameters (leakage parameter can be individualized). A novelty of our model, over other models (e.g., PERIDOSE), arises from its applicability to any technique (independently of the number of MU needed to deliver a dose). Differences between predictions and measurements were < 0.005mSv/UM.

Conclusion:
We have proposed a unique model which successfully account for photon peripheral organ dose. This model can be applied in the day-to-day clinic as it only needs a few basic parameters which are readily accessible.

1. Radiother. Oncol. 107:234-243, 2013.
2. Phys. Med. Biol. 57:6167-6191, 2012.


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