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

Abstract

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

Calculated absorbed‐dose ratios, TG51/TG21, for most widely used cylindrical and parallel‐plate ion chambers over a range of photon and electron energies

“…In addition, Khan calculates N D,w from N gas , essentially from N X , rather than using a measured value. The difference between the two may be up to 1-1 1/2% as shown in our Cobalt 60 results in Table II and in Sang et al 5 Last, Khan compares dose at d max for electrons, while we compare at d ref . Once these differences are accounted for, we agree to better than 0.5% for the PTW N23333 chamber used by Khan.…”

“…According to Shortt et al, 16 a difference of ϳ1.1% exists in the ratio N D,w /N X from the two standards laboratories. In addition, Cho et al 5 report difference of the order of 1% between a calculated N D,w 60 Co /N X and ADCL supplied values.…”

“…The measured dosedifference between the two protocols, for some selected ion chambers, have been presented in poster presentations 4 and publications. [5][6][7][8] Also, calculated differences have been presented for selected chambers in two publications. 6,9 In these studies, a maximum difference of about 2% has been reported for both high-energy photon beams and electron beams.…”

Comparison of high‐energy photon and electron dosimetry for various dosimetry protocols

“…For example, the beam quality of photon beams is specified by the percentage depth-dose instead of the nominal accelerator potential used in the JARP and TG-21 protocols or TPR 10 20 used in the TRS-277 and TRS-398 protocols. The TG-51 beam quality factor is defined by %dd (10) x , which is the photon component of the percentage depth-dose at 10 cm depth in a 10ϫ10 cm 2 field on the surface of a water phantom at an SSD of 100 cm. Clinical reference dosimetry is performed in either an SSD or SAD setup with a 10ϫ10 cm 2 field size defined on the surface of the phantom for SSD setup or at the detector position for an SAD setup.…”

“…To date, comparisons of absolute dose using the TG-51 and TG-21 protocols have been reported by Cho et al, 10 Ding et al, 11 and Huq and Andreo. 12 Their results show that TG-51 increases the absorbed-dose to water by approximately 1% for photon beams and 1-3% for electron beams in comparison to TG-21 when calibration factors (N D,w 60 Co and N x ) are traceable to the National Institute of Standards and Technology ͑NIST͒ in the United States.…”