No abstract

No abstract

Purpose: To determine experimentally the intrinsic energy response, k bq , of EBT3 GafChromic â radiochromic film with kilovoltage x rays, 137 Cs, and 60 Co in therapeutic and diagnostic dose ranges through direct measurement with an accompanying mathematical approach to describe the physical processes involved. Methods: The EBT3 film was irradiated with known doses using 60 Co, 137 Cs, and 13 NIST-matched kilovoltage x-ray beams. Seven dose levels, ranging from 57 to 7002 mGy, were chosen for this work. Monte Carlo methods were used to convert air-kerma rates to dose rates to the film active layer for each energy. A total of 738 film dosimeters, each measuring (1.2 9 1.2) cm 2 , were cut from three film sheets out of the same lot of the latest version of EBT3 film, to allow for multiple dosimeters to be irradiated by each target dose and beam quality as well as unirradiated dosimeters to be used as controls. Net change in optical density in excess of the unirradiated controls was measured using the UWMRRC Laser Densitometry System (LDS). The dosimeter intrinsic energy response, k bq , for each dose level was determined relative to 60 Co, as the ratio of dosimeter response to each beam quality relative to the absorbed dose to the film active volume at the same dose level. A simplified, single-hit mathematical model was used to derive a single-free-parameter, b, which is a proportionality constant that is dependent on beam quality and describes the microdosimetric interactions within the active layer of film. The response of b for each beam quality relative to 60 Co was also determined. Results: k bq was determined for a wide range of doses and energies. The results show a unique variation of k bq as a function of energy, and agree well with results from other investigations. There was no measurable dose dependence for k bq within the 500-7002 mGy range outside of the expanded measurement uncertainty of 3.65% (k = 2). For doses less than 500 mGy, the signal-to-noise ratio was too low to determine k bq accurately. The single-free-parameter, b, fit calculations derived from the single-hit model show a correlation with k bq that suggests that b, at least in part, characterizes the microdosimetric interactions that determine k bq . Conclusions: For the beam qualities investigated, a single energy-dependent k bq correction can be used for doses between 500 and 7002 mGy. Using the single-hit model with the single-free-parameter fit to solve for b shows promise in the determination of the intrinsic energy response of film, with b being the mathematical analog of the measured k bq .

Purpose An interlaboratory comparison of radiation dosimetry was conducted to determine the accuracy of doses being used experimentally for animal exposures within a large multi-institutional research project. The background and approach to this effort are described and discussed in terms of basic findings, problems and solutions. Methods Dosimetry tests were carried out utilizing optically stimulated luminescence (OSL) dosimeters embedded midline into mouse carcasses and thermal luminescence dosimeters (TLD) embedded midline into acrylic phantoms. Results The effort demonstrated that the majority (4/7) of the laboratories was able to deliver sufficiently accurate exposures having maximum dosing errors of ≤ 5%. Comparable rates of ‘dosimetric compliance’ were noted between OSL- and TLD-based tests. Data analysis showed a highly linear relationship between ‘measured’ and ‘target’ doses, with errors falling largely between 0–20%. Outliers were most notable for OSL-based tests, while multiple tests by ‘non-compliant’ laboratories using orthovoltage x-rays contributed heavily to the wide variation in dosing errors. Conclusions For the dosimetrically non-compliant laboratories, the relatively high rates of dosing errors were problematic, potentially compromising the quality of ongoing radiobiological research. This dosimetry effort proved to be instructive in establishing rigorous reviews of basic dosimetry protocols ensuring that dosing errors were minimized.

Purpose To establish a method of accurate dosimetry required to quantify the expected linear energy transfer (LET) quenching effect of EBT3 film used to benchmark the dose distribution for a given treatment field and specified measurement depth. In order to facilitate this technique, a full analysis of film calibration which considers LET variability at the plane of measurement and as a function of proton beam quality is demonstrated. Additionally, the corresponding uncertainty from the process was quantified for several measurement scenarios. Materials and methods The net change in optical density (OD) from a single version of GafchromicTM EBT3 film was measured using an Epson flatbed scanner and NIST‐traceable OD filters. Film OD response was characterized with respect to the known dose to water at the point of measurement for both a NIST‐traceable 60Co beam at the UWADCL and several clinical single‐energy and spread‐out Bragg peak (SOBP) proton beam qualities at the Northwestern Medicine Chicago Proton Center. Increasing proton LET environments were acquired by placing film at increasing depths of Gammex HE Solid Water® whose water‐equivalent thickness was characterized prior to measurement. Results A strong LET dependence was observed near the Bragg peak (BP) consistent with previous studies performed with earlier versions of EBT3 film. The influence of range straggling on the film's LET response appears to have a uniform effect toward the BP regardless of the nominal beam energy. Proximal to this depth, the film's response decreased with decreasing energy at the same dose‐average LET. The opposite trend was observed for depths past the BP. Changes in the SOBP energy modulation showed a linear relationship between the film's relative response and dose‐averaged LET. Relative effectiveness factors (RE) were observed to range between 2%–7% depending on the width of the SOBP and depth of the film. Using the field‐specific calibration technique, a total k = 1 uncertainty in the absorbed dose to water was estimated to range from 4.68%–5.21%. Conclusion While EBT3 film's strong LET dependence is a common problem in proton beam dosimetry, this work has shown that the LET dependence can be taken into account by carefully considering the depth and energy modulation across the film using field‐specific corrections. RE factors were determined with a combined k = 1 uncertainty of 3.57% for SOBP environments and between 3.17%–4.69% for uniform, monoenergetic fields proximal to the distal 80% of the BP.