Clinical electron‐beam dosimetry: Report of AAPM Radiation Therapy Committee Task Group No. 25

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

“…for the TG‐21 protocol 2 were obtained from the institution's clinical dosimetry data. Note

d_{max}

+ 300 V

) to determine

P_{pol}

and

P_{ion}

and an appropriate correction was applied for temperature and pressure

(P_{T, P})

.…”

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

“…– 3 As the beam passes through the electron applicator, a cohort of electrons is traveling along rays that emanate from the virtual source, and a cohort is traveling at various scattered angles due to collisions with the jaw faces, the inside of the applicator, as well as with the edges of the field shaping insert 3 8 The distance the scattered electrons travel laterally will be greater than that of the divergent electrons after passing through the shape‐defining aperture, especially if there is a large distance between aperture and skin surface.…”

Section: Discussionmentioning

confidence: 99%

Potential therapeutic misadministration due to inappropriate electron beam field shaping

Olch

Fallen

Conrad

et al. 2000

Lead or cerrobend blocking strips are used to shape electron treatment fields when an appropriate custom insert is not available. For the Varian 2100C accelerator, the structural supports of the electron applicators impede the free placement of these field‐shaping strips on the open custom insert frame while placement at the top of the applicator is unimpeded. We have investigated the dosimetric ramifications of placing field shaping strips at the top level of the 15×15 applicator for 6, 9, and 16 MeV electrons. Our results demonstrate as much as a 30% dose decrease and 2 cm penumbral increase when this is done compared to field shaping at the insert level. The magnitude of this dosimetric error qualifies as a therapeutic misadministration in many states depending on how many treatments are delivered in this manner. Based on this finding, we recommend that routine use of lead strip blocking be discouraged in favor of custom inserts due to the potential for inappropriate placement on some linear accelerators.PACS number(s): 87.53.–j, 87.52.–g

“…Next, a phantom consisting of solid water and incorporating a lead shield of dimensions

4 \times 10 {cm}^{2}

and a thickness dependent on the incident beam energy, as recommended in the AAPM Task Group 25 report, was constructed (18) . Lead dimensions were selected such that the length of 10 cm extended beyond the field boundaries and the width of 4 cm removed any scattered dose contributions from points perpendicular to the off‐axis profile, centered at depth under the lead shield, for all energies investigated.…”

Section: Methodsmentioning

confidence: 99%

Evaluation of lens dose from anterior electron beams: comparison of Pinnacle and Gafchromic EBT3 film

Sonier

Wronski

Yeboah

2015

Lens dose is a concern during the treatment of facial lesions with anterior electron beams. Lead shielding is routinely employed to reduce lens dose and minimize late complications. The purpose of this work is twofold: 1) to measure dose profiles under large‐area lead shielding at the lens depth for clinical electron energies via film dosimetry; and 2) to assess the accuracy of the Pinnacle treatment planning system in calculating doses under lead shields. First, to simulate the clinical geometry, EBT3 film and 4 cm wide lead shields were incorporated into a Solid Water phantom. With the lead shield inside the phantom, the film was positioned at a depth of 0.7 cm below the lead, while a variable thickness of solid water, simulating bolus, was placed on top. This geometry was reproduced in Pinnacle to calculate dose profiles using the pencil beam electron algorithm. The measured and calculated dose profiles were normalized to the central‐axis dose maximum in a homogeneous phantom with no lead shielding. The resulting measured profiles, functions of bolus thickness and incident electron energy, can be used to estimate the lens dose under various clinical scenarios. These profiles showed a minimum lead margin of 0.5 cm beyond the lens boundary is required to shield the lens to ≤10% of the dose maximum. Comparisons with Pinnacle showed a consistent overestimation of dose under the lead shield with discrepancies of ∼25% occurring near the shield edge. This discrepancy was found to increase with electron energy and bolus thickness and decrease with distance from the lead edge. Thus, the Pinnacle electron algorithm is not recommended for estimating lens dose in this situation. The film measurements, however, allow for a reasonable estimate of lens dose from electron beams and for clinicians to assess the lead margin required to reduce the lens dose to an acceptable level.PACS number(s): 87.53.Bn, 87.53.Kn, 87.55.‐x, 87.55.D‐