The Use of a Milled NIST Traceable Tld for End-to-End Validation in Mlc and Cone SRS Programs. πŸ“

Author: Ross Bland, Zachary A. Christ, Sophy Mangana, Feba Mathew, Stephanie K. Phoenix, Joseph Salloum, Jason D. Stanford, William Zollinger πŸ‘¨β€πŸ”¬

Affiliation: Forrest General Cancer Center, Northeast Louisiana Cancer Institute, Florida Atlantic University 🌍

Abstract:

Purpose: The objective of this paper is twofold: 1) to explore the efficacy of using referenced beam models supplied by the vendor (Brainlab) and 2) the efficacy of NIST traceable TLD for end-to-end validation in extremely small MLC and cone SRS program.

Methods: Varian Edge and Truebeam STx were evaluated, and where necessary corrected to meet the mechanical and imaging specification for SRS/SBRT delivery per TG-142.

Beam data (PDD, profiles and output factors) were collected and processed using vendor and published guidelines. The β€œdaisy chain” method was used to determine a correction factor for the SNC diode at 0.5 cm x 0.5 cm field. Milled TLDs in the shape of Exradin A16 ion chamber were ordered from university of Wisconsin. Each probe has a single TLD microcube inside with dimensions of 1mm3. Before sending out to the clinical end user, each probe is imaged to ensure their position relative to the included insert block is within 0.6 mm of the center cross hair (most are within 0.3mm). MLC plans were created using Brainlab Pencil beam and Monte Carlo algorithm. Cone plans were created with Cone Dose Calculation (CDC) Algorithm. These plans were created to validate the TPS calculated dose.

Results: 1. The dose for SRS cones TLD are within 4% agreement. 2. MLC delivery plans for pencil beam and Monte Carlo agrees within 5% down to 1x1 cm. Below 1 cm, pencil beam algorithm deviation exceeds 20%

Conclusion: Vendor supplied referenced beam data is appropriate for SRS; however field size limitation below 1x1 cm exist and must be incorporated in the clinical decision for each case. NIST traceable TLD with 1mm3 are suitable for validating the dose for cones and small MLC fields. This is especially important for small cancer centers with a single physicist commissioning a program.

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