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  4. Small Static Fields: Real-Time Assessment of the Accuracy of a Treatment Planning System (TPS) Calculated Dose with a Step-And-Shoot Method

Small Static Fields: Real-Time Assessment of the Accuracy of a Treatment Planning System (TPS) Calculated Dose with a Step-And-Shoot Method

G Biasi1*, N Hardcastle12 , M Petasecca1 , S Guatelli1 , A Rozenfeld1 , T Kron123 , (1) University of Wollongong, Wollongong, NSW, Australia, (2) Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia, (3) Sir Peter MacCallum Cancer Institute, University of Melbourne, Victoria, Australia

Presentations

(Wednesday, 8/1/2018) 10:30 AM - 11:00 AM

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 0° or 90° 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 ‘correction free’ 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 0° and cross-plane at 90° 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.

Keywords

Small Fields, Quality Assurance, Semiconductor Detectors

Taxonomy

TH- Radiation dose measurement devices: diodes/solid state

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