Evaluation of Dosimetric Uncertainties in Diminutive MLC Fields by “Fixed Virtual Cone Technique” using Measurements and Monte Carlo Simulations

Quantification of dosimetric uncertainties due to variable field sizes in virtual cone technique during stereotactic radiosurgery (SRS) can be challenging. We propose a virtual cone technique defined by a fixed field size to mimic a physical cone. It is characterized by 0.5cm x 0.5cm field, defined by two central leaves of high-definition MLC with a fixed opening of jaws at 100cm SAD. To generate spherical dose distributions equivalent to physical cones and ensure an optimized dose coverage, approximately 12–14 non-coplanar partial arcs to be used. Retrospective measurements (SRS MapCHECK) of 30 SRS cases treated on Varian EDGE system were performed using our virtual cone to quantify dosimetric uncertainties due to gravity or low maintenance of MLC, when the gantry at 90°/270° and the collimator at 90°. Field output factor and dose/MU were compared between measurements (SRS MapCHECK) and Eclipse TPS (Acuros XB) for 6FFF and 10FFF photon beams at two clinical settings (95/5 cm & 90/10 cm). MC study on the EDGE linac verified our method. Preliminary results showed a notable dosimetric variation due to a minute (± 0.5–1.0)mm leaf shift. The 10FFF beam yielded more accurate (≤4%) dosimetric results than the 6FFF beam (≤7%). MC simulations exhibited similar dosimetric results but more consistent than the TPS ones. Overall, the 10FFF beam delivered more accurate and reliable dosimetric results than the 6FFF at different clinical settings, whereas the 90/10cm setting was more effective than the 95/5cm.