PO-0790: In-vivo dosimetry for kV radiotherapy: clinical use of micro-silica bead TLD & Gafchromic EBT3 film

Purpose or Objective kV radiotherapy continues to be an important modality in modern radiotherapy, but has received less research attention in recent years. There remains a challenge to accurately calculate and verify treatment dose distributions for clinical sites with significant surface irregularity or where the treated region contains inhomogeneities, e.g. nose and ear. The accuracy of current treatment calculations has a significant level of uncertainty [1, 2]. The objective of this work was to characterise two novel detectors, micro-silica bead TLDs and Gafchromic EBT3 film, for in-vivo measurements for kV treatments, and to compare measured doses with conventional treatment calculations. [1. Currie (2007) Australas Phys Eng Sci Med, 2. Chow (2012) Rep Pract Oncol Radiother.] Material and Methods Micro-silica bead TLDs (1 mm diam.) and Gafchromic EBT3 film were calibrated against an NPL traceably calibrated ionisation chamber using an Xstrahl D3300 kV radiotherapy treatment unit. Energy response was evaluated over 70 to 250 kV and compared to 6 MV, useable dose range was evaluated from 0 to 25 Gy, and uncertainty budgets determined. Silica beads were cleaned, annealed, and TL response individually calibrated. EBT3 film was used with triple-channel dosimetry via FilmQAPro® with procedures to reduce uncertainties. Commissioning tests were undertaken in standard conditions using Solid Water blocks and in simulated clinical treatment condition using a custom made ‘wax face with nose’ phantom. Pilot in vivo measurements were made for a consecutive series of eight clinical patient treatments, including cheek, ear, nose and rib sites, over 70 to 250 kV, and 4 to 18 Gy. Results for the two dosimetry systems were compared to conventional treatment planning calculations. Results Energy response varied by 460% for beads and 9% for film, from 70 kV to 6 MV, necessitating energy-specific calibration. Both dosimeters were useable up to 25 Gy. Standard uncertainty was 3.1% for beads, 2.1% for film. The figure shows typical film and bead positions within the lead cut-out of a kV treatment to the cheek. The table provides calculated and measured doses. Average deviation over 6 patients was -1.3% for beads, -0.9% for film. 3 patients had larger deviations; See table note 1: tumour sitting over the maxillary sinus may reduce dose. Note 2: beads placed along surface of tumour into ear, most distal bead received dose -17.5% from prescription, doctor made compensation. Note 3: Increased uncertainty due to curved surface, film required offset to corner as patient sensitive to contact. Note 4: Uncertainty increased due to large respiratory motion at treatment site. Conclusion Both micro-silica bead TLDs and EBT3 film were characterised as suitable for in vivo dosimetry in kV radiotherapy, providing assurance of delivered doses. Film is simpler to prepare, use and read. A line of beads allows conformation to irregular anatomy across the field. A clinical service is now available to verify dose delivery in complex clinical sites.