Clarification of the AAPM Task Group 21 protocol

Schulz, Robert; Almond, Peter R.; Kutcher, Gerald J.; Loevinger, Robert; Nath, Ravinder; Rogers, D. W. O.; Suntharalingam, N.; Wright, Kayla; Kahn, Faiz M.

doi:10.1118/1.595843

Cited by 36 publications

(23 citation statements)

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“…Using

k_{Q} = 1.0

and assuming

P_{pol} \approx 1.0

, the following relationship for a

{}^{60}{normalC} o

beam can be obtained by taking the ratio between Eqs. (2) and (5):

\frac{D_{w}}{D_{water}} = \frac{N_{D, w}}{N_{k} {(0.8791)}^{- 1} (\frac{N_{gas}}{N_{x}}) P_{repl} P_{wall} {(\frac{\overset{L}{true}}{ρ})}_{air}^{water}},

where the value 0.8791 is the conversion factor from exposure to air‐kerma (kinetic energy released in air); 3

N_{k}

and

N_{x}

are the air‐kerma and exposure calibration factors, respectively. If no systematic difference exists in both calibration standards and protocols, Eq.…”

Section: Methodsmentioning

confidence: 99%

Comparison between TG‐51 and TG‐21: Calibration of photon and electron beams in water using cylindrical chambers

Cho

Lowenstein

Balter

et al. 2000

J Applied Clin Med Phys

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A new calibration protocol, developed by the AAPM Task Group 51 (TG‐51) to replace the TG‐21 protocol, is based on an absorbed‐dose to water standard and calibration factor (ND,w), while the TG‐21 protocol is based on an exposure (or air‐kerma) standard and calibration factor (Nx). Because of differences between these standards and the two protocols, the results of clinical reference dosimetry based on TG‐51 may be somewhat different from those based on TG‐21. The Radiological Physics Center has conducted a systematic comparison between the two protocols, in which photon and electron beam outputs following both protocols were compared under identical conditions. Cylindrical chambers used in this study were selected from the list given in the TG‐51 report, covering the majority of current manufacturers. Measured ratios between absorbed‐dose and air‐kerma calibration factors, derived from the standards traceable to the NIST, were compared with calculated values using the TG‐21 protocol. The comparison suggests that there is roughly a 1% discrepancy between measured and calculated ratios. This discrepancy may provide a reasonable measure of possible changes between the absorbed‐dose to water determined by TG‐51 and that determined by TG‐21 for photon beam calibrations. The typical change in a 6 MV photon beam calibration following the implementation of the TG‐51 protocol was about 1%, regardless of the chamber used, and the change was somewhat smaller for an 18 MV photon beam. On the other hand, the results for 9 and 16 MeV electron beams show larger changes up to 2%, perhaps because of the updated electron stopping power data used for the TG‐51 protocol, in addition to the inherent 1% discrepancy presented in the calibration factors. The results also indicate that the changes may be dependent on the electron energy.PACS number(s): 87.66.–a, 87.53.–j

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“…Using

k_{Q} = 1.0

and assuming

P_{pol} \approx 1.0

, the following relationship for a

{}^{60}{normalC} o

beam can be obtained by taking the ratio between Eqs. (2) and (5):

\frac{D_{w}}{D_{water}} = \frac{N_{D, w}}{N_{k} {(0.8791)}^{- 1} (\frac{N_{gas}}{N_{x}}) P_{repl} P_{wall} {(\frac{\overset{L}{true}}{ρ})}_{air}^{water}},

where the value 0.8791 is the conversion factor from exposure to air‐kerma (kinetic energy released in air); 3

N_{k}

and

N_{x}

are the air‐kerma and exposure calibration factors, respectively. If no systematic difference exists in both calibration standards and protocols, Eq.…”

Section: Methodsmentioning

confidence: 99%

Comparison between TG‐51 and TG‐21: Calibration of photon and electron beams in water using cylindrical chambers

Cho

Lowenstein

Balter

et al. 2000

J Applied Clin Med Phys

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“…Only generic data for chambers with a TE wall (A‐150) were provided without taking into account any proton energy dependence (except for a distinction made between the SOBP region and the plateau region) of the required quantities. Clarifications that were made to the AAPM TG‐21 protocol thus also applied to this protocol for ion beam dosimetry . The overall relative standard uncertainty was estimated to be 5%, mainly dominated by the uncertainty on

W_{air}

for ion beams.…”

Section: Detectors For Measurements Of Absorbed Dose In Reference Conmentioning

confidence: 99%

Dose detectors, sensors, and their applications

Giordanengo

2018

Medical Physics

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This article is intended to present the different types of particle and radiation detectors available for applications in particle therapy. Several types of detectors and sensors exist for measurements of absorbed dose in reference and nonreference conditions and the ones in use for beam monitoring. Therefore, this manuscript focuses on the following applications: (a) primary methods for dosimetry in ion beams, (b) measurements of absorbed dose in realistic patient‐specific scenarios with different dose delivery configurations, and (c) beam monitoring. Water and graphite calorimeters, Faraday cups, ionization based gas detectors are described first, followed by the description of the protocols for proton and ion dosimetry. Then films, scintillator and solid‐state detectors are reviewed. Finally, several types of ionization chambers (large, pinpoint, arrays of chambers arranged in regular 1D or 2D pattern, parallel‐plate configuration with large integral electrodes or with anode segmented in strips or pixels, multi‐wire, and the multi‐gap prototype) have been considered. New beam monitors to deal with a wide range of intensity and pulsed beams expected from new accelerators, different dose fractionation and advanced delivery techniques are presented. The existing detectors available for particle therapy have been described taking into account the different requirements for devices used in reference and nonreference conditions and the ones designed for beam monitoring.

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“…Thus, at a given polarizing voltage V the inverse of the saturation current, I sat Ϫ1 , and the collection efficiency f can be determined by plotting the measured 1/I(V) vs 1/V and extrapolating the data for f Ͼ0.90 to 1/Vϭ0. A simplified version of this extrapolation is the so-called ''twovoltage'' technique 9,10 which is used to determine the collection efficiency and saturation current with ionization chamber currents measured in pulsed beams at only two polarizing voltage points in the near-saturation region. If I H and I L are the currents measured at a high polarizing voltage V H and a low polarizing voltage V L , respectively, then f (V H ), the collection efficiency at V H , is given by…”

Section: A Ion Collection Efficiency For Pulsed Radiation Beamsmentioning

confidence: 99%

Saturation current and collection efficiency for ionization chambers in pulsed beams

2000

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Saturation currents and collection efficiencies in ionization chambers exposed to pulsed megavoltage photon and electron beams are determined assuming a linear relationship between 1/I and 1/V in the extreme near-saturation region, with I and V the chamber current and polarizing voltage, respectively. Careful measurements of chamber current against polarizing voltage in the extreme near-saturation region reveal a current rising faster than that predicted by the linear relationship. This excess current combined with conventional "two-voltage" technique for determination of collection efficiency may result in an up to 0.7% overestimate of the saturation current for standard radiation field sizes of 10X10 cm2. The measured excess current is attributed to charge multiplication in the chamber air volume and to radiation-induced conductivity in the stem of the chamber (stem effect). These effects may be accounted for by an exponential term used in conjunction with Boag's equation for collection efficiency in pulsed beams. The semiempirical model follows the experimental data well and accounts for both the charge recombination as well as for the charge multiplication effects and the chamber stem effect.

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Clarification of the AAPM Task Group 21 protocol

Abstract: In light of recent questions and comments from the physics community, a review is made of the AAPM protocol for high-energy x-ray and electron beam dosimetry.

Cited by 36 publications

References 0 publications

Comparison between TG‐51 and TG‐21: Calibration of photon and electron beams in water using cylindrical chambers

Comparison between TG‐51 and TG‐21: Calibration of photon and electron beams in water using cylindrical chambers

Dose detectors, sensors, and their applications

Saturation current and collection efficiency for ionization chambers in pulsed beams

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