Technical Report: Evaluation of peripheral dose for flattening filter free photon beams.

PURPOSE: To develop a comprehensive peripheral dose (PD) dataset for the two unflattened beams of nominal energy 6 and 10 MV for use in clinical care. METHODS: Measurements were made in a 40 × 120 × 20 cm(3) (width × length × depth) stack of solid water using an ionization chamber at varying depths (dmax, 5, and 10 cm), field sizes (3 × 3 to 30 × 30 cm(2)), and distances from the field edge (5-40 cm). The effects of the multileaf collimator (MLC) and collimator rotation were also evaluated for a 10 × 10 cm(2) field. Using the same phantom geometry, the accuracy of the analytic anisotropic algorithm (AAA) and Acuros dose calculation algorithm was assessed and compared to the measured values. RESULTS: The PDs for both the 6 flattening filter free (FFF) and 10 FFF photon beams were found to decrease with increasing distance from the radiation field edge and the decreasing field size. The measured PD was observed to be higher for the 6 FFF than for the 10 FFF for all field sizes and depths. The impact of collimator rotation was not found to be clinically significant when used in conjunction with MLCs. AAA and Acuros algorithms both underestimated the PD with average errors of -13.6% and -7.8%, respectively, for all field sizes and depths at distances of 5 and 10 cm from the field edge, but the average error was found to increase to nearly -69% at greater distances. CONCLUSIONS: Given the known inaccuracies of peripheral dose calculations, this comprehensive dataset can be used to estimate the out-of-field dose to regions of interest such as organs at risk, electronic implantable devices, and a fetus. While the impact of collimator rotation was not found to significantly decrease PD when used in conjunction with MLCs, results are expected to be machine model and beam energy dependent. It is not recommended to use a treatment planning system to estimate PD due to the underestimation of the out-of-field dose and the inability to calculate dose at extended distances due to the limits of the dose calculation matrix.

Utilizing an electronic portal imaging device to monitor light and radiation field congruence.

A method to investigate light and radiation field congruence utilizing a commercially available amorphous silicon electronic portal imaging device (EPID) was developed. This method employed an EPID, the associated EPI software, and a diamond-shaped template. The template was constructed from a block tray in which Sn/Pb wires, 1 mm in diameter, were embedded into a diamond shaped groove milled into the tray. The collimator jaws of the linac were aligned such that the light field fell directly on the corners of the diamond. A radiation detection algorithm within the EPI software determined the extent of the radiation field. The light and radiation field congruence was evaluated by comparing the vertexes of the diamond reference structure to the detected radiation field. In addition, the digital jaw settings were recorded and later compared to the light field detected on the films and EPIs. Three linear accelerators were tracked for a period ranging from 2-8 months. Light radiation field congruence tests with films and EPIs were comparable, yielding a difference of less than 0.6 mm, well within the allowed 2-mm tolerance. A disparity was observed in the magnitude of the detected light field. The X and Y dimensions of the light field measured with film differed by less than or equal to 1.4 mm from the digital collimator settings, whereas the values extracted from the EPIs differed by up to 2.5 mm. Based on these findings, EPIs were found to be a quick and reliable alternative to film for qualitative and relative analyses.