S.M. Jafari

Glass beads and Ge-doped optical fibres as thermoluminescence dosimeters for small field photon dosimetry.

An investigation has been made of glass beads and optical fibres as novel dosimeters for small-field photon radiation therapy dosimetry. Commercially available glass beads of largest dimension 1.5 mm and GeO2-doped SiO2 optical fibres of 5 mm length and 120 µm diameter were characterized as thermoluminescence dosimeters. Results were compared against Monte-Carlo simulations with BEAMnrc/DOSXYZnrc, EBT3 Gafchromic film, and a high-resolution 2D-array of liquid-filled ionization chambers. Measurements included relative output factors and dose profiles for square-field sizes of 1, 2, 3, 4, and 10 cm. A customized Solid-Water® phantom was employed, and the beads and fibres were placed at defined positions along the longitudinal axis to allow accurate beam profile measurement. Output factors and the beam profile parameters were compared against those calculated by BEAMnrc/DOSXYZnrc. The output factors and field width measurements were found to be in agreement with reference measurements to within better than 3.5% for all field sizes down to 2 cm2 for both dosimetric systems, with the beads showing a discrepancy of no more than 2.8% for all field sizes. The results confirm the potential of the beads and fibres as thermoluminescent dosimeters for use in small photon radiation field sizes.

A national dosimetry audit for stereotactic ablative radiotherapy in lung

BACKGROUND AND PURPOSE A UK national dosimetry audit was carried out to assess the accuracy of Stereotactic Ablative Body Radiotherapy (SABR) lung treatment delivery. METHODS AND MATERIALS This mail-based audit used an anthropomorphic thorax phantom containing nine alanine pellets positioned in the lung region for dosimetry, as well as EBT3 film in the axial plane for isodose comparison. Centres used their local planning protocol/technique, creating 27 SABR plans. A range of delivery techniques including conformal, volumetric modulated arc therapy (VMAT) and Cyberknife (CK) were used with six different calculation algorithms (collapsed cone, superposition, pencil-beam (PB), AAA, Acuros and Monte Carlo). RESULTS The mean difference between measured and calculated dose (excluding PB results) was 0.4±1.4% for alanine and 1.4±3.4% for film. PB differences were -6.1% and -12.9% respectively. The median of the absolute maximum isodose-to-isodose distances was 3mm (-6mm to 7mm) and 5mm (-10mm to +19mm) for the 100% and 50% isodose lines respectively. CONCLUSIONS Alanine and film is an effective combination for verifying dosimetric and geometric accuracy. There were some differences across dose algorithms, and geometric accuracy was better for VMAT and CK compared with conformal techniques. The alanine dosimetry results showed that planned and delivered doses were within ±3.0% for 25/27 SABR plans.

Feasibility of using glass-bead thermoluminescent dosimeters for radiotherapy treatment plan verification

OBJECTIVE To investigate the feasibility of using glass beads as novel thermoluminescent dosemeters (TLDs) for radiotherapy treatment plan verification. METHODS Commercially available glass beads with a size of 1-mm thickness and 2-mm diameter were characterized as TLDs. Five clinical treatment plans including a conventional larynx, a conformal prostate, an intensity-modulated radiotherapy (IMRT) prostate and two stereotactic body radiation therapy (SBRT) lung plans were transferred onto a CT scan of a water-equivalent phantom (Solid Water(®), Gammex, Middleton, WI) and the dose distribution recalculated. The number of monitor units was maintained from the clinical plan and delivered accordingly. The doses determined by the glass beads were compared with those measured by a graphite-walled ionization chamber, and the respective expected doses were determined by the treatment-planning system (TPS) calculation. RESULTS The mean percentage difference between measured dose with the glass beads and TPS was found to be 0.3%, -0.1%, 0.4%, 1.8% and 1.7% for the conventional larynx, conformal prostate, IMRT prostate and each of the SBRT delivery techniques, respectively. The percentage difference between measured dose with the ionization chamber and glass bead was found to be -1.2%, -1.4%, -0.1%, -0.9% and 2.4% for the above-mentioned plans, respectively. The results of measured doses with the glass beads and ionization chamber in comparison with expected doses from the TPS were analysed using a two-sided paired t-test, and there was no significant difference at p < 0.05. CONCLUSION It is feasible to use glass-bead TLDs as dosemeters in a range of clinical plan verifications. ADVANCES IN KNOWLEDGE Commercial glass beads are utilized as low-cost novel TLDs for treatment-plan verification.

Latest developments in silica-based thermoluminescence spectrometry and dosimetry.

Using irradiated doped-silica preforms from which fibres for thermoluminescence dosimetry applications can be fabricated we have carried out a range of luminescence studies, the TL yield of the fibre systems offering many advantages over conventional passive dosimetry types. In this paper we investigate such media, showing emission spectra for irradiated preforms and the TL response of glass beads following irradiation to an 241Am-Be neutron source located in a tank of water, the glass fibres and beads offering the advantage of being able to be placed directly into liquid. The outcomes from these and other lines of research are intended to inform development of doped silica radiation dosimeters of versatile utility, extending from environmental evaluations through to clinical and industrial applications.

OC-0155: UK SABR Consortium Lung Dosimetry Audit; absolute dosimetry results

Purpose/Objective: The UK SABR Lung Consortium dose audit was designed to assess the positional and dosimetric accuracy of SABR lung treatment delivery. The audit has been carried out in 21 radiotherapy centres between October 2013 and July 2014 in order to provide an independent check of safe implementation and to identify problems in the modelling and delivery of SABR lung treatment. Materials and Methods: A mail based audit using EBT3 GafChromic film and alanine dosimeters was designed. A CIRS Model 002LFC anthropomorphic thorax phantom which contained 9 adjacent alanine pellets in the tip of a Farmer chamber shaped insert was scanned, structure sets for the ITV and alanine pellets were pre-delineated, and was sent to radiotherapy centres to be loaded into their treatment planning system. Each centre used this CT scan set to create a SABR plan using their current planning protocol (including dose, fractionation and coverage) and technique. The centres used their own margin to create the PTV. A range of delivery techniques were used including conformal, VMAT and Cyberknife and calculated using local algorithms (AAA, Collapsed Cone, Monte Carlo and Pencil beam). The doses determined by the alanine dosimeters were compared to expected doses determined by treatment plan system (TPS) calculation, film and local ionisation chamber measurements. Results: The mean % difference between the alanine measured doses, the TPS calculated doses, and the local chamber measurements found to be within 2% (1 SD) as given in table 1. As shown, alanine findings were supported by the film results.

OC-0154: UK SABR Consortium Lung Dosimetry Audit; relative dosimetry results

Purpose/Objective: The UK SABR Consortium QA group conducted a postal dosimetry audit of SABR lung plans at 21 UK centres. The purpose of this was to verify the accuracy of calculated dose distributions, improve confidence of centres in the early stages of implementing lung SABR and to establish a benchmark QA method. Here the results of the GafChromic film relative dosimetry arm of the audit are given. Materials and Methods: Individual centres were asked to plan a treatment to a pre-defined PTV in the CIRS Thorax phantom, using their clinical method and prescription dose. EBT3 GafChromic film was used to measure an axial plane of dose. Pins in the phantom facilitated alignment of the film and calculated dose planes. Gantry linac and Cyberknife centres were audited, using a variety of TPS with pencil beam, AAA, CCC, Acuros and Monte Carlo algorithms. Scanned films were compared to dose distributions calculated by the individual centres, using single red-channel dosimetry and a purpose-built Matlab application. Centres were also asked to irradiate additional calibration films to provide output-normalised optical density to dose calibration. Measured and calculated isodoses corresponding to 120, 100, 70 and 50% of prescription dose were compared (figure 1), and conformity and maximum distance to agreement were measured. For the areas bound by the 100, 50 and 30% calculated isodoses, local gamma analysis, mean gamma and gamma pass rate (at 3%, 2mm) and a mean dose comparison was performed. The latter was compared to the alanine dosimetry results. Results: The dosimetry of the calibration films was reproducible to ±0.9% (1.S.D), for doses ranging from 4.3 to 26.9 Gy. The audit relative dosimetry results are reported in table 1. Mean dose differences within the 100% calculated isodose line agreed well with alanine dosimetry; -0.1 ± 2.0 % (1.S.D). Gamma pass rates (%) and mean gamma results varied with some outlying measurements, mostly caused by small dose deviations within the PTV or at low doses. Isodose line agreement (figure 1) was generally much closer at the 70 and 100% dose levels, indicated by the lower S.D. (table 1, column 5). The exception was the centre using a pencil beam algorithm, where the measured prescription dose covered a significantly smaller area than that calculated, consistent with the algorithm’s known limitations calculating dose in low density lung surrounding tumour. Conclusions: Of the 21 UK centres audited, 74% of measurements were within ±3% agreement compared to calculated doses. Where appropriate, outlying centres have been offered support from the QA Group to bring their results into line. The EBT3 GafChromic film was found to be highly suited to a postal audit, reliably giving detailed information about the geometric and dosimetric accuracy of treatment.

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.

Evaluation and clinical implementation of in vivo dosimetry for kV radiotherapy using radiochromic film and micro-silica bead thermoluminescent detectors

PURPOSE kV radiotherapy treatment calculations are based on flat, homogenous, full-scatter reference conditions. However, clinical treatments often include surface irregularities and inhomogeneities, causing uncertainty. Therefore, confirmation of actual delivered doses in vivo is valuable. The current study evaluates, and implements, radiochromic film and micro silica bead TLD for in vivo kV dosimetry. METHODS The kV energy and dose response of EBT3 film and silica bead TLD was established and uncertainty budgets determined. In vivo dosimetry measurements were made for a consecutive series of 30 patients using the two dosimetry systems. RESULTS Energy dependent calibration factors were required for both dosimetry systems. The standard uncertainty estimate for in vivo measurement with film was 1.7% and for beads was 1.5%. The mean measured dose was -2.1% for film and -2.6% for beads compared to prescription. Deviations up to -9% were found in cases of large surface irregularity, or with underlying air cavities or bone. Dose shielding by beads could be clinically relevant at low kV energies and superficial depths. CONCLUSIONS Both film and beads may be used to provide in vivo verification of delivered doses in kV radiotherapy, particularly for complex situations that are not well represented by standard reference condition calculations.

PO-0796: Dosimetric characterisation of glass bead TLDs in proton beams

Purpose/Objective: To investigate the feasibility of using glass beads as a novel thermoluminescence dosimeter (TLD) in clinical proton radiotherapy. The glass beads have several physical characteristics which suggest their suitability for use as TLDs in this area: a spherical physical shape with a hole in the middle that facilitate their use in 2D and 3D arrangements; chemically inert nature; small size of 1.5 mm diameter and 1 mm thickness; inexpensive and readily available; reusable; and importantly their TL light transparency with negligible self-attenuation which is very important for high LET beams. Proton beams have high LET and therefore can deposit dose nonuniformly across a detector. Readout of TL detectors can be influenced if any opacity is present causing self-attenuation of TL light [1], [2]. The transparency of glass beads to TL light means the beads have the potential to avoid such issues.