Dosimetric Benefits with a Lower Energy Photon Beam in Stereotactic Radiosurgery

Abstract

<h3>Purpose/Objective(s)</h3> Stereotactic radiosurgery (SRS) relies on very small field sizes on the order of a few millimeters to ablate lesions and spare normal structures. Currently a 6 MV photon beam is most often utilized beam energy although there has been an interest in developing lower energy photon beams for stereotactic radiosurgery due to their sharper dose fall off. This study aims to find if there is a dosimetric benefit of using a lower energy photon beam over 6 MV photons for SRS. <h3>Materials/Methods</h3> A 2.5 MV is developed for imaging purpose and is available from a linear accelerator. Monte Carlo simulations were used to simulate both 2.5 MV and 6 MV beams and calculate dose distribution. The details of the incident beams including cone accessory were simulated, with calculated doses benchmarked against measurements. The dose comparison between using 6 MV and 2.5 MV beams were based on a realistic treatment plan for thalamotomy treatment. <h3>Results</h3> For the same target dose coverage, the dose fall off is significantly faster with 2.5 MV beams. Based on calculated 3D dose distributions for a realistic patient treatment plan using 21 arcs with 4 mm cone beams delivered 15000 cGy to the target, the DVH analysis showed the maximum doses to brainstem, chiasm, and optic nerves were 691 cGy (435 cGy), 266 cGy (135 cGy) and 103 cGy (38 cGy) for 6 MV (2.5 MV) beams respectively. The mean organ doses are 136 cGy (64 cGy), 155 cGy (66 cGy) and 51 cGy (19 cGy) to brain stem, Chiasm and optic nerves for 6 MV (2.5 MV) beams respectively. <h3>Conclusion</h3> The dose reductions to organs at risk (OARs) with 2.5 MV beams are very significant ranging from 37% maximum dose to brain stem to 50% mean dose to chiasm, optical nerves and eyes. The sharper dose fall-off with 2.5 MV beam reduced unintended doses to OARs while providing the same target dose coverage. A dosimetric advantage of using 2.5 MV has been observed for delivery of an extremely high dose with very small circular field in SRS.