Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non‐equilibrium conditions

Abstract

Abbreviations: %dd(10) x , The photon component of the percent depth dose at 10 cm depth in water for a 10 cm 2 × 10 cm 2 field; L∕ w air , Restricted mass collision stopping power ratio of water to air; en ∕ , Spectrum-averaged mass energy-absorption coefficient; AAPM, American Association of Physicists in Medicine; CPE, Charged Particle Equilibrium; DLG, Dosimetric leaf gap; D f msr w,Q msr , Absorbed dose to water at the reference depth z ref in water in the absence of the detector in a field specified by f msr and beam quality Q msr .; f clin , Clinical (clin) non-reference radiation field; f msr , Machine-specific reference (msr) field; f ref , Reference field (ref) specified in dosimetry protocols for which the calibration coefficient of an ionization chamber in terms of absorbed dose to water is provided by a standards laboratory; FWHM, Full-width at half-maximum; GUM, Guide to the expression of Uncertainty in Measurements.; IAEA, International Atomic Energy Agency; ICRU, International Commission on Radiation Units and Measurements; IMRT, Intensity-modulated radiation therapy; k Q,Q 0 , The beam-quality correction factor, which corrects for the differences between the response of an ionization chamber in the reference beam of quality Q o used for calibrating the chamber and the beam of quality Q (defined as k Q . in TG-51 4 ); k f ref Q,Q 0 , Correction factor that accounts for the differences between the response of a detector in field f ref in a beam of quality Q and reference beam quality Q 0 as defined in TRS-483. 1 and Palmans et al 2 ; k fclin,fmsr Qclin,Q msr , The detector-specific correction factor that accounts for the difference between the responses of the detector in fields f clin in a beam of quality Q clin and in fields f msr in beam of quality Q msr as defined by Alfonso et al. 3 ; LCPE, Lateral charged particle equilibrium; M fmsr Qmsr , Detector reading in field f msr and beam quality Q msr corrected for influence of changes in pressure and temperature, incomplete charge collection, polarity effect and electrometer correction factor (TRS-483 1 ); MU, Monitor unit; N D,w,Q 0 , This is N D,w in TG-51, 4 and defined as the calibration coefficient in terms of absorbed dose to water for an ionization chamber at a reference beam of quality,Q 0 and field size f ref ; N f ref D,w,Q 0

Selection Strategy of Jaw Tracking in VMAT Planning for Lung SBRT

“…A high level of delivery accuracy is crucial for the implementation of SBRT. Due to the extensive use of small-field dose delivery in SBRT and the difficulties in measurements of small field, a number of reviews, guidelines, and recommendations have been published on the subject of small-field dosimetry, such as the IPEM report 103 ( 12 ), the IAEA-AAPM TRS-483 code of practice ( 13 ), and the more recent AAPM TG-155 report ( 14 ). However, due to limited resources or lack of qualified staff, many treatment centers may not be able to perform reliable small-field measurements, and the avoidance of uncertainties would be practical.…”

“…Small-field uncertainty is one of the main contributors to the discrepancy between calculated and delivered doses. In accordance with the corresponding guidelines and recommendations ( 12 – 14 , 28 ), the delivered dose for several FJ and JT plans was measured in the present study. However, due to the large uncertainties in the small-field data acquisition, TPS commissioning, and dose verification, obtaining reliable results to demonstrate the differences in the delivery accuracy between FJ and JT plans was difficult.…”

“…With the extensive implementation of SBRT, interest in the use of small photon fields has been rapidly growing (9)(10)(11). However, due to lack of lateral charged-particle equilibrium, collimator effects, and limited choices of radiation detectors, small-field dosimetry has always been challenging, and large uncertainties are generally expected in small-field measurements (12)(13)(14). Therefore, small-field dosimetry is one of the main factors related to the delivery accuracy of SBRT.…”

Potential dose variability for small‐field plans delivered with Elekta Agility collimators

“…While the results obtained from the current study support the notion that additional investigations should analyze the effects of small fields, highly modulated fields, and the relationship between planned and measured dose grids on PSQA, they do not necessarily indicate that discrepancies in these measurements lie within the Agility TM MLC itself. To fully understand the implications of differences in calculated and measured dose distributions, additional studies are required in the context of smallfield dosimetry and beam modeling, which TG-155 8 has addressed. While we respectfully suggest that the authors of the cited publication 1 may have used unnecessarily alarming language that could otherwise undermine the confidence of the radiotherapy community, we are looking forward to the authors elaborating on their findings in future publications.…”

A multi-detector comparison to determine convergence of measured relative output factors for small field dosimetryAuthor InformationDo Duc Chi1, Tran Ngoc Toan2 and Robin Hill3,4,5

“…There are published codes of practice (CoP) on the dosimetry of small fields from the International Atomic Energy Agency (IAEA) TRS-483 (2017) and the American Association of Physicists in Medicine (AAPM) and TG-155 (2021), which generally consider field size equal or less than 3 × 3 cm 2 as small field for most common photon beam energies [6] [8].…”