AAPM WGTG51 Report 374: Guidance for TG‐51 reference dosimetry

Purpose To quantify the potential error in outputs for flattening filter free (FFF) beams associated with use of a lead foil in beam quality determination per the addendum protocol for TG‐51, we examined differences in measurements of the beam quality conversion factor k Q when using or not using lead foil. Methods Two FFF beams, a 6 MV FFF and a 10 MV FFF, were calibrated on eight Varian TrueBeams and two Elekta Versa HD linear accelerators (linacs) according to the TG‐51 addendum protocol by using Farmer ionization chambers [TN 30013 (PTW) and SNC600c (Sun Nuclear)] with traceable absorbed dose‐to‐water calibrations. In determining k Q , the percentage depth‐dose at 10 cm [PDD(10)] was measured with 10×10 cm 2 field size at 100 cm source‐to‐surface distance (SSD). PDD(10) values were measured either with a 1 mm lead foil positioned in the path of the beam [%dd(10) Pb ] or with omission of a lead foil [%dd(10)]. The %dd(10)x values were then calculated and the k Q factors determined by using the empirical fit equation in the TG‐51 addendum for the PTW 30013 chambers. A similar equation was used to calculate k Q for the SNC600c chamber, with the fitting parameters taken from a very recent Monte Carlo study. The differences in k Q factors were compared for with lead foil vs. without lead foil. Results Differences in %dd(10)x with lead foil and with omission of lead foil were 0.9 ± 0.2% for the 6 MV FFF beam and 0.6 ± 0.1% for the 10 MV FFF beam. Differences in k Q values with lead foil and with omission of lead foil were −0.1 ± 0.02% for the 6 MV FFF and −0.1 ± 0.01% for the 10 MV FFF beams. Conclusion With evaluation of the lead foil role in determination of the k Q factor for FFF beams. Our results suggest that the omission of lead foil introduces approximately 0.1% of error for reference dosimetry of FFF beams on both TrueBeam and Versa platforms.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Quantification of the role of lead foil in flattening filter free beam reference dosimetry

Gao

Nelson

Wang

et al. 2023

“…The solid phantom is composed of acrylic block slabs, each approximately (20 × 20 × 2.4) cm 3 . Slabs are purchased at nominal 1 in thickness, for which the manufacturer states an actual machined thickness of approximately 2.4 cm, with manufacturing tolerances resulting in slight variation in slab thicknesses.…”

Section: Methodsmentioning

confidence: 99%

“…The American Association of Physicists in Medicine (AAPM) Task Group 51 formalism (TG-51) 1,2 specifically requires measurements in water. Updated AAPM guidance on TG-51 implementation 3 reiterates the requirement for a water phantom; however, TG-51 notes that the protocol does not preclude the use of solid phantoms for routine measurement of output so long as an appropriate transfer factor has been established via a calibration performed in water at least annually. Seuntjens et al 4 describe the framework of using an absorbed-dose-towater calibration coefficient to convert charge measured in a solid phantom to that in water for routine quality assurance of linear accelerators (linacs).…”

Section: Introductionmentioning

confidence: 99%

“…The standardized quality assurance system described in this paper is based on the above AAPM TG-51 protocol. This system uses stacks of polymethylmethacrylate (PMMA, also known as acrylic) blocks, commercially known as Perspex and Lucite, with a mass density of approximately 1.18 g/cm 3 , an electron density relative to water of approximately 1.147. 7 The following equation is used with the acrylic phantom-based setup:…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Therapeutic radiation beam output and energy variation across clinics, technologies, and time

Zaini¹,

Fagerstrom²,

Marshall³

et al. 2023

Over the past several decades, a medical physics service group covering 35 clinical sites has provided routine monthly output and energy quality assurance for over 75 linear accelerators. Based on the geographical spread of these clinics and the large number of physicists involved in data acquisition, a systematic calibration procedure was established to ensure uniformity. A consistent measurement geometry and data collection technique is used across all machines for every calendar month, using a standardized set of acrylic slabs. Charge readings in acrylic phantoms are linked to AAPM's TG‐51 formalism via a parameter denoted kacrylic, used to convert raw charge readings to machine output values. Statistical analyses of energy ratios and kacrylic values are presented. Employing the kacrylic concept with a uniform measurement geometry of similar acrylic blocks was found to be a reproducible and simple way of referencing a calibration completed in water under reference conditions and comparing to other machines, with the ability to alert physicists of outliers.

Assessment of a self‐inspection method and reporting measured electrometer correction factors for reference‐class electrometers in radiotherapy

Kawashima

Varnava

Ozawa

et al. 2023

Background and purpose The standard dosimetry system of medical accelerators in radiotherapy consists of an ionization chamber, an electrometer, and cables. Guidance for TG‐51 reference dosimetry reported that the electrometer correction factor (Pelec) should be checked every few years. Therefore, continuous Pelec measurements have not been reported. The purpose of this study is to measure the Pelec with a charge generator at our institution and to evaluate variations over time. The measurements are compared with calibration data given by an Accredited Dosimetry Calibration Laboratory (ADCL). Materials and methods We used four reference‐class electrometers: RT521R (RTQM system/EMF Japan), Model 35040 (FLUKE), RAMTEC Duo (Toyo medic), and UNIDOS‐E (PTW). Each electrometer was connected to the charge generator, and the required charge was applied. The measurement points used were the same as those used for calibration by the ADCL. From the measured charges at each point, the Pelec was obtained from the slope of the linear regression function. The measurements were repeated over a 3‐month period to evaluate variations over time for each electrometer. Additionally, error budgets for the Pelec measurements were estimated, and the overall uncertainty was determined. Results The measured Pelec values were 1.0000, 0.9995, 1.0009/0.9999, and 0.9995/0.9998 for RT521R, Model 35040, the low/medium (L/M) ranges of RAMTEC Duo, and the L/M ranges of UNIDOS‐E, respectively. The measured Pelec values agreed within 0.1% with those given by the ADCL. We found a small drift in the measurements for one electrometer. Additionally, the uncertainty considered was 0.26% for k = 2 (k, coverage factor). Conclusion In this study, stable Pelec values were obtained for four electrometers using a charge generator over a three‐month period. The measured Pelec values were within the overall uncertainty stated in the electrometer guidelines. However, performing periodic measurements for the Pelec was able to help in detecting small errors.