Session Item

Peripheral dose is an issue of growing interest in today’s radiotherapy, as it contributes to the generation of deterministic late-effects on organs near the field edge as well as secondary cancer. The impact of out-of-field doses is particularly relevant for IMRT treatments, which are typically characterized by large irradiated volumes to low doses. Although this dose is not negligible, it is usually ignored, and then the cancer risk associated with the treatment. Knowing the dose to any organ within the patient would allow generating more personalized therapies. This work presents a new model that calculates peripheral photon dose (PPD) in 3D outside the 5% isodose. The model is easy to use, fast, and only requires two treatment parameters.

Considering the physical sources of PPD and a Monte Carlo (MC) simulation of a reference treatment (eight coplanar isocentric 10x10 cm² square fields equally distributed at mid-abdomen on the ICRP110 phantom), an analytical expression for PPD, as a function of the point´s position within the patient, was proposed. The model was fitted to the MC data using MATLAB (for doses outside the 5% isodose). Then, it was tested with a) experimental (TLD) data from a VMAT prostate treatment, and b) MC simulation of three 5x5 cm² fields lung treatment on the ICRP110 phantom. A graphical user interface (GUI) was created to calculate the PPD 3D distribution on any CT.

Maximum differences between our PPD model prediction and the data from the MC lung plan and measured data from VMAT plan, were 29 and 9 and mGy/Gy, respectively. The PPD distribution on the abdomen (isodoses on a coronal plane) is depicted in figure 1. A screenshot of the GUI for the lung plan showing the PPD on a transversal view is shown in figure 2. The GUI allows for fast visualization of the 3D dose distribution in a loaded CT and the dosimetric information (DVH, maximum, minimum, and average organ dose) of outlined organs (in this example, heart)

Figure 1 Coronal view of PPD dose distribution on the ICRP110 of the eight-field abdomen plan. The central region is not shown as it belongs to the volume inside the 5% isodose, which is not considered by the model.

Figure 2 Upper left shows PPD at a transversal plane on the ICRP110 in a lung treatment (dose normalized to the slice). The upper right window shows the PPD on the organ (heart in this case). The cumulative DVH for the organ is depicted.

A relatively simple model (Periphocal 3D) was generated. It allows for a fast 3D calculation of PPD in isocentric treatments with coplanar beams. This information is essential for a more accurate estimation of secondary cancer risk at the organ. In addition to that, and as a first step to translating the research to the clinic, a GUI was created in MATLAB for 3D visualization of PPD calculated on any whole-body CT (see abstract E22-1144). The GUI allows visualization of PPD distributions, as well as the DVH of out-of-field organs. 

Acknowledgments: Fondecyt N1181133