TG‐51: Experience from 150 institutions, common errors, and helpful hints

Abstract

The Radiological Physics Center (RPC) is a resource to the medical physics community for assistance regarding dosimetry procedures. Since the publication of the AAPM TG‐51 calibration protocol, the RPC has responded to numerous phone calls raising questions and describing areas in the protocol where physicists have had problems. At the beginning of the year 2000, the RPC requested that institutions participating in national clinical trials provide the change in measured beam output resulting from the conversion from the TG‐21 protocol to TG‐51. So far, the RPC has received the requested data from ~ 150 of the ~ 1300 institutions in the RPC program. The RPC also undertook a comparison of TG‐21 and TG‐51 and determined the expected change in beam calibration for ion chambers in common use, and for the range of photon and electron beam energies used clinically. Analysis of these data revealed two significant outcomes: (i) a large number (~ 1/2) of the reported calibration changes for photon and electron beams were outside the RPC's expected values, and (ii) the discrepancies in the reported versus the expected dose changes were as large as 8%. Numerous factors were determined to have contributed to these deviations. The most significant factors involved the use of plane‐parallel chambers, the mixing of phantom materials and chambers between the two protocols, and the inconsistent use of depth‐dose factors for transfer of dose from the measurement depth to the depth of dose maximum. In response to these observations, the RPC has identified a number of circumstances in which physicists might have difficulty with the protocol, including concerns related to electron calibration at low energies false(R50<2.1emcmfalse), and the use of a cylindrical chamber at 6 MeV electrons. In addition, helpful quantitative hints are presented, including the effect of the prescribed lead filter for photon energy measurements, the impact of shifting the chamber depth for photon depth‐dose measurements, and the impact of updated stopping‐power data used in TG‐51versus that used in TG‐21, particularly for electron calibrations.PACS number(s): 87.53.–j, 87.66.–a

Volumetric modulation arc radiotherapy with flattening filter-free beams compared with conventional beams for nasopharyngeal carcinoma: a feasibility study

“…The Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA) was used for 6 MV beams with or without flattening filter from a TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA). The accelerator was calibrated to deliver 1 cGy/MU to water at a depth of 1.5 cm for a 10 cm × 10 cm field at a source-to-surface distance of 100 cm following the American Association of Physicists in Medicine Task Group 51 report [8] . The dose constraints for the target volumes and OARs used in this study are listed in Table 1 .…”

Insight gained from responses to surveys on reference dosimetry practices

“…Respondents to the LD survey indicate that they still use lead foil for high‐energy photon beam quality measurements. Note that the addendum to the TG‐51 protocol states that the simplified procedure without the use of lead foil can be used as the default method for some beams to avoid operational errors since the use of lead foil only introduces an error in k Q of 0.2% …”

“…The American Association of Physicists in Medicine (AAPM) TG‐106 report on beam data commissioning provides recommendations on scanning procedures and this is a good starting point for depth‐dose determination. Tailor et al and Followill describe guidelines and common sources of error related to the practical clinical implementation of the TG‐51 protocol.…”

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

“…If the lead foil is not used, an error in k Q (and absorbed dose) at the 0.2% level is introduced. 38 The procedure given in TG-51 relies on positioning a lead foil at either 30 cm (±1 cm) or 50 cm (±5 cm) from the phantom surface. In some cases, it is not possible to position the foil at either of these distances, so the foil can be positioned somewhere in between, where possible, and the equation for the closest distance from TG-51 should be used.…”

“…However, the use of the lead foil results in higher accuracy determination of beam quality specifiers for photon beams with energy around 10 MV and above, so should be used. If the lead foil is not used, an error in $$k_{Q}$$ (and absorbed dose) at the 0.2% level is introduced 38 …”