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Energy Correction of TLD Dosimetry at Out-Of-Field Region for Arc Therapy

J Yoon

J Yoon1*, R Zhang1,2 , (1) Louisiana State University, Baton Rouge, LA, (2) Mary Bird Perkins Cancer Center, Baton Rouge, LA


SU-I-GPD-T-305 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Thermoluminescence dosimeters (TLDs) are widely used as in-vivo dosimeters for plan evaluation. Since the response of TLDs is sensitive to lower energy photons and electrons at out-of-field regions, the accuracy of measured dose for arc therapy is limited. This study proposed a method to improve the accuracy of in-vivo dosimetry using TLDs for volumetric modulated arc therapy (VMAT).

Methods: Two stacks of solid water phantom (gross dimension 30x60x20 cm³) were scanned by a computed tomography scanner and a VMAT plan was generated using Pinnacle 9.8 with the maximum field size of 30x30 cm². Due to the characteristic of arc therapy, out-of-field region (lower than 5% of prescription dose) was separated into two regions depending on whether a point of interest is included in the beam path (in-beam out-of-field; IBOF) or not (out-of-beam out-of-field; OBOF). In this study, ion chamber (IC) measurement was used as the ground truth since IC’s over-response at out-of-field region is negligible. TLDs and IC measurements were calibrated on the beam axis. For each measurement position, TLDs and IC measurements were performed at multiple depths. The VMAT plan was delivered by Elekta Versa HD using 6, 10 and 15 MV photons.

Results: The TLD measured doses were higher than IC at all regions except 6MV IBOF. Energy correction factors from TLD doses to IC measurement were 0.90, 0.85 and 0.80 for 6, 10 and 15 MV photons at OBOF region, respectively; the correction factors were 1.00, 0.95 and 0.90 for 6, 10 and 15 MV photons at IBOF region, respectively.

Conclusion: When TLD was used for arc therapy, using appropriate energy correction factors for different points of interest and beam energies is suggested.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by National Cancer Institute Award 1 K22 CA204464-01, Louisiana State University (LSU) Faculty Research Grant and LSU Economic Development Award.

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