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 3 mm (�6 mm to 7 mm) and 5 mm (�10 mm to +19 mm) 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

Development and Implementation of an End-To-End Test for Absolute Dose Verification of Small Animal Preclinical Irradiation Research Platforms

Purpose Lack of standardization and inaccurate dosimetry assessment in preclinical research is hampering translational opportunities for new radiation therapy interventions. The aim of this work was to develop and implement an end-to-end dosimetry test for small animal radiation research platforms to monitor and help improve accuracy of dose delivery and standardization across institutions. Methods and Materials The test is based on a bespoke zoomorphic heterogeneous mouse and WT1 Petri dish phantoms with alanine as a reference detector. Alanine measurements within the mouse phantom were validated with Monte Carlo simulations at 0.5 mm Cu x-ray reference beam. Energy dependence of alanine in medium x-ray beam qualities was taken into consideration. For the end-to-end test, treatment plans considering tissue heterogeneities were created in Muriplan treatment planning systems (TPS) and delivered to the phantoms at 5 institutions using Xstrahl's small animal irradiation platforms. Mean calculated dose to the pellets were compared with alanine measured dose. Results Monte Carlo simulations and in phantom alanine measurements in NPL's reference beam were in excellent agreement, validating the experimental approach. At 1 institute, initial measurements showed a larger than 12% difference between calculated and measured dose caused by incorrect input data. The physics data used by the calculation engine were corrected, and the TPS was recommissioned. Subsequent end-to-end test measurements showed differences <5%. With an anterior field, 4 of the participating institutes delivered dose within 5% to both phantoms. Conclusions An end-to-end dosimetry test was developed and implemented for dose evaluation in preclinical irradiation with small animal irradiation research platforms. The test was capable of detecting treatment planning commissioning errors and highlighted critical elements in dose calculation. Absolute dosimetry with alanine in relevant preclinical irradiation conditions showed reasonable levels of accuracy compared with TPS calculations. This work provides an independent and traceable dosimetric validation in preclinical research involving small animal irradiation

Μulti-institutional dosimetric delivery assessment of intracranial stereotactic radiosurgery on different treatment platforms

BACKGROUND AND PURPOSE: Assessment of dosimetric accuracy of radiosurgery on different treatment platforms. MATERIAL AND METHODS: Thirty-three single fraction treatment plans were assessed at thirty centres using an anthropomorphic head phantom with target and brainstem structures. The target being a single irregular shaped target, ∼8cc, 10 mm from the brainstem. The phantom was "immobilised", scanned, planned and treated following the local protocols. EBT-XD films and alanine pellets were used to measure absolute dose, inside both the target and the brainstem, and compared with TPS predicted dose distributions. RESULTS: PTV alanine measurements from gantry-based linacs showed a median percentage difference to the TPS of 0.65%. Cyberknife (CK) had the highest median difference of 2.3% in comparison to the other platforms. GammaKnife (GK) showed the smallest median of 0.3%. Similar trends were observed in the OAR with alanine measurements showing median percentage differences of1.1%, 2.0% and 0.4%, for gantry-based linacs, CK and GK respectively. All platforms showed comparable gamma passing rates between axial and sagittal films. CONCLUSIONS: This comparison has highlighted the dosimetric variation between measured and TPS calculated dose for each delivery platforms.. The results suggest that clinically acceptable agreement with the predicted dose distributions is achievable by all treatment delivery systems. Radiosurgery, Dosimetry, End-to-end, Audit, Anthropomorphic Phantom, Alanine, Radiochromic film

Results of an independent dosimetry audit for scanned proton beam therapy facilities

Purpose: An independent dosimetry audit based on end-to-end testing of the entire chain of radiation therapy delivery is highly recommended to ensure consistent treatments among proton therapy centers. This study presents an auditing methodology developed by the MedAustron Ion Beam Therapy Center (Austria) in collaboration with the National Physical Laboratory (UK) and audit results for five scanned proton beam therapy facilities in Europe.Methods: The audit procedure used a homogeneous and an anthropomorphic head phantom. The phantoms were loaded either with an ionization chamber or with alanine pellets and radiochromic films. Homogeneously planned doses of 10 Gy were delivered to a box-like target volume in the homogeneous phantom and to two clinical scenarios with increasing complexity in the head phantom.Results: For all tests the mean of the local differences of the absolute dose to water determined with the alanine pellets compared to the predicted dose from the treatment planning system installed at the audited institution was determined. The mean value taken over all tests performed was -0.1 +/- 1.0%. The measurements carried out with the ionization chamber were consistent with the dose determined by the alanine pellets with a mean deviation of -0.5 +/- 0.6%.Conclusion: The developed dosimetry audit method was successfully applied at five proton centers including various "turn-key" Cyclotron solutions by IBA, Varian and Mevion. This independent audit with extension to other tumour sites and use of the correspondent anthropomorphic phantoms may be proposed as part of a credentialing procedure for future clinical trials in proton beam therapy

Detector to detector corrections: a comprehensive experimental study of detector specific correction factors for beam output measurements for small radiotherapy beams

Purpose: The aim of the present study is to provide a comprehensive set of detector specific correction factors for beam output measurements for small beams, for a wide range of real time and passive detectors. The detector specific correction factors determined in this study may be potentially useful as a reference data set for small beam dosimetry measurements.Methods: Dose response of passive and real time detectors was investigated for small field sizes shaped with a micromultileaf collimator ranging from 0.6 × 0.6 cm2 to 4.2 × 4.2 cm2 and the measurements were extended to larger fields of up to 10 × 10 cm2. Measurements were performed at 5 cm depth, in a 6 MV photon beam. Detectors used included alanine, thermoluminescent dosimeters (TLDs), stereotactic diode, electron diode, photon diode, radiophotoluminescent dosimeters (RPLDs), radioluminescence detector based on carbon-doped aluminium oxide (Al2O3:C), organic plastic scintillators, diamond detectors, liquid filled ion chamber, and a range of small volume air filled ionization chambers (volumes ranging from 0.002 cm3 to 0.3 cm3). All detector measurements were corrected for volume averaging effect and compared with dose ratios determined from alanine to derive a detector correction factors that account for beam perturbation related to nonwater equivalence of the detector materials.Results: For the detectors used in this study, volume averaging corrections ranged from unity for the smallest detectors such as the diodes, 1.148 for the 0.14 cm3 air filled ionization chamber and were as high as 1.924 for the 0.3 cm3 ionization chamber. After applying volume averaging corrections, the detector readings were consistent among themselves and with alanine measurements for several small detectors but they differed for larger detectors, in particular for some small ionization chambers with volumes larger than 0.1 cm3