Room: Exhibit Hall | Forum 7
Purpose: To develop a step-and-shoot method to assess in real-time and with sub-mm resolution the accuracy of the treatment planning system (TPS) calculated dose in the case of static small photon radiation fields collimated under a jaws/MLC combination and delivered with a medical linear accelerator (linac).
Methods: The TPS dose calculation accuracy was evaluated by comparing the measured and calculated output factors (OFs) and 2D dose distributions in terms of FWHM and penumbra width for 1 cm square fields. Investigated fields were collimated either with jaws (MLC retracted), MLC (jaws at 10 cm square field), or MLC/jaws (jaws at 1.2 cm square field). Fields centred on the machine central axis as well as off-axis, using 0Â° or 90Â° collimator rotation, were studied.All TPS plans were generated using Eclipse 13.6 (Varian Medical Systems) for a Varian TrueBeam STx linac and calculated with the analytic anisotropic algorithm (AAA) using a single isocentre. The minimum available calculation grid spacing of 1.0 mm was applied.Measurements were performed with the Octa, a 2D monolithic silicon array detector with 0.3 mm resolution and real-time read-out, at 90 cm SSD, 10 cm depth in solid water. The dosimeter had been previously validated for small-field dosimetry and had been shown to be â€˜correction freeâ€™ for OF measurements.
Results: TPS calculated MLC defined dose profiles were accurate in terms of FHWM and penumbra width within 0.3 mm. Accuracy was reduced under a combined jaws/MLC collimation, especially for the in-plane at 0Â° and cross-plane at 90Â° profiles.
Conclusion: The proposed step-and-shoot method proved to be quick, simple and efficient in assessing the accuracy of the TPS calculated dose in the case of small-field dosimetry, where special attention needs to be paid to the modelling of secondary and tertiary collimators, as well as their interplay.
Funding Support, Disclosures, and Conflict of Interest: We acknowledge the Gross Foundation for financial support.