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

AAPM WGTG51 Report 385: Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy electron beams

An Addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water is presented for electron beams with energies between 4 MeV and 22 MeV (1.70 cm ≤ R 50 ≤ 8.70 cm $1.70\nobreakspace {\rm cm} \le R_{\text{50}} \le 8.70\nobreakspace {\rm cm}$ ). This updated formalism allows simplified calibration procedures, including the use of calibrated cylindrical ionization chambers in all electron beams without the use of a gradient correction. Newk Q $k_{Q}$ data are provided for electron beams based on Monte Carlo simulations. Implementation guidance is provided. Components of the uncertainty budget in determining absorbed dose to water at the reference depth are discussed. Specifications for a reference-class chamber in electron beams include chamber stability, settling, ion recombination behavior, and polarity dependence. Progress in electron beam reference dosimetry is reviewed. Although this report introduces some major changes (e.g., gradient corrections are implicitly included in the electron beam quality conversion factors), they serve to simplify the calibration procedure. Results for absorbed dose per linac monitor unit are expected to be up to approximately 2 % higher using this Addendum compared to using the original TG-51 protocol.

Investigation of triaxial cables and microdetectors in small field dosimetry

Background. Small field dosimetry presents unique challenges with source occlusion, lateral charged particle equilibrium and detector size. As detector volume decreases, signal strength declines while noise increases, deteriorating the signal-to-noise ratio (SNR). This issue may be compounded by triaxial cables connecting detectors to electrometers. However, effects of cables, critical for precision dosimetry, are often overlooked. There is a need to evaluate triaxial cable and detector impacts on SNR in small fields. The purpose of this study is to evaluate the influence of triaxial cables and microdetectors on signal-to-noise ratios in small-field dosimetry. This study also aims to establish the importance of cable quality assurance for measurement accuracy.Methods. Six 9.1 m length triaxial cables from different manufacturers were tested with six microdetectors (microDiamond, PinPoint, EDGE, Plastic scintillator, microSilicon, SRS-Diode). A 6 MV photon beam (TrueBeam) was used, with a water phantom at 5 cm depth with 0.5 × 0.5 cm2to 10 × 10 cm2fields at 600 MU min-1. Readings were acquired using cable-detector permutations with a dedicated electrometer (except the scintillator which has its own). Cables had differing connector types, conductor materials, insulation, and diameters. Detectors had various sensitive volumes, materials, typical signals, and bias voltages.Results. Normalized field output correction factors (FOFs) relative differences of 13.4% and 4.6% between the highest and lowest values across triaxial cables for 0.5 × 0.5 cm2and 1 × 1 cm2fields, respectively. The maximum difference in FOF between any cable-detector combinations was 0.2% for the smallest field size. No consistent FOF trend was observed across all detectors when increasing cable diameter. Additionally, the non-normalized FOF differences of 0.9% and 0.3% were observed between cables for 0.5 × 0.5 cm2and 1 × 1 cm2fields, respectively.Conclusions. Regular triaxial cable quality assurance is critical for precision small field dosimetry. A national protocol is needed to standardize cable evaluations/calibrations, particularly for small signals ( Collapse

Key Words

• QA
• SNR
• microdetectors
• small field dosimetry
• triaxial cable

Collapse

MESH Headings

• Signal-To-Noise Ratio
• Radiometry/instrumentation
• Radiometry/methods
• Phantoms, Imaging
• Equipment Design
• Photons
• Humans

Collapse

Grants Collapse

Affiliation(s)

James J Sohn Department of Radiation and Cellular Oncology, University of Chicago, Chicago, IL, 60637, United States of America
Indra J Das Department of Radiation Oncology, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, IL, 60611, United States of America

Collapse

Monte Carlo evaluation of uncertainties in photon and electron TG-51 absorbed dose calibration

PURPOSE

The accuracy of dose delivery to all patients treated with medical linacs depends on the accuracy of beam calibration. Dose delivery cannot be any more accurate than this. Given the importance of this, it seems worthwhile taking another look at the expected uncertainty in TG-51 photon dose calibration and a first look at electron calibration. This work builds on the 2014 addendum to TG-51 for photons and adds to it by also considering electrons. In that publication, estimates were made of the uncertainty in the dose calibration. In this paper, we take a deeper look at this important issue.

METHODS

The methodology used here is more rigorous than previous determinations as it is based on Monte Carlo simulation of uncertainties. It is assumed that mechanical QA has been performed following TG-142 prior to beam calibration and that there are no uncertainties that exceed the tolerances specified by TG-142.

RESULTS/CONCLUSIONS

Despite the different methodology and assumptions, the estimated uncertainty in photon beam calibration is close to that in the addendum. The careful user should be able to easily reach a 95% confidence interval (CI) of ± 2.3% for photon beam calibration with standard instrumentation. For electron beams calibrated with a Farmer chamber, the estimated uncertainties are slightly larger, and the 95% CI is ±2.6% for 6 MeV and slightly smaller than this for 18 MeV. There is no clear energy dependence in these results. It is unlikely that the user will be able to improve on these uncertainties as the dominant factor in the uncertainty resides in the ion chamber dose calibration factorN D , w 60 Co $N_{D,w}^{{}^{60}{\mathrm{Co}}}$ . For both photons and electrons, reduction in the ion chamber depth uncertainty below about 0.5 mm and SSD uncertainty below 1 mm have almost no effect on the total dose uncertainty, as uncertainties beyond the user's control totally dominate under these circumstances.

Determination of thePrpand radial dose correction factor in reference dosimetry

Objectives.In an addendum to AAPM TG-51 protocol, McEwenet al, (DOI:10.1118/1.4866223) introduced a new factorPrpto account for the radial dose distribution of the photon beam over the detector volume mainly in flattening filter free (FFF) beams.Prpand its extension to non-FFF beam reference dosimetry is investigated to see its impact in a clinical situation.Approches.ThePrpwas measured using simplified version of Sudhyadhomet al(DOI:10.1118/1.4941691) for Elekta and Varian FFF beams with two commonly used calibration detectors; PTW-30013 and Exradin-A12 ion chambers after acquiring high resolution profiles in detectors cardinal coordinates. For radial dose correction factor, the ion chambers were placed in a small water phantom and the central axis position was set to center of the sensitive volume on the treatment table and was studied by rotating the table by 15-degree interval from -90 to +90 degrees with respect to the initial (zero) position.Main results.The magnitude ofPrpvaries very little with machine, detector and beam energies to a value of 1.003 ± 0.0005 and 1.005 ± 0.0005 for 6FFF and 10FFF, respectively. The radial anisotropy for the Elekta machine with Exradin-A12 and PTW-30013 detector the magnitudes are in the range of (0.9995±0.0011 to 1.0015±0.0010) and (0.9998±0.0007 to 1.0015±0.0010), respectively. Similarly, for the Varian machine with Exradin-A12 and PTW-30013 ion chambers, the magnitudes are in the range of (1.0004±0.0010 to 1.0018±0.0018) and (1.0006±0.0009 to 1.0027±0.0007), respectively.Significance.ThePrpis ≤ 0.3% and 0.5% for 6FFF and 10FFF, respectively. The radial dose correction factor in regular beams also does not impact the dosimetry where the maximum magnitude is ±0.2% which is within experimental uncertainty.

Quantification of the role of lead foil in flattening filter free beam reference dosimetry

PURPOSE

To quantify the potential error in outputs for flattening filter free (FFF) beams associated with use of a lead foil in beam quality determination per the addendum protocol for TG-51, we examined differences in measurements of the beam quality conversion factor k_Q when using or not using lead foil.

METHODS

Two FFF beams, a 6 MV FFF and a 10 MV FFF, were calibrated on eight Varian TrueBeams and two Elekta Versa HD linear accelerators (linacs) according to the TG-51 addendum protocol by using Farmer ionization chambers [TN 30013 (PTW) and SNC600c (Sun Nuclear)] with traceable absorbed dose-to-water calibrations. In determining k_Q , the percentage depth-dose at 10 cm [PDD(10)] was measured with 10×10 cm² field size at 100 cm source-to-surface distance (SSD). PDD(10) values were measured either with a 1 mm lead foil positioned in the path of the beam [%dd(10)_Pb ] or with omission of a lead foil [%dd(10)]. The %dd(10)x values were then calculated and the k_Q factors determined by using the empirical fit equation in the TG-51 addendum for the PTW 30013 chambers. A similar equation was used to calculate k_Q for the SNC600c chamber, with the fitting parameters taken from a very recent Monte Carlo study. The differences in k_Q factors were compared for with lead foil vs. without lead foil.

RESULTS

Differences in %dd(10)x with lead foil and with omission of lead foil were 0.9 ± 0.2% for the 6 MV FFF beam and 0.6 ± 0.1% for the 10 MV FFF beam. Differences in k_Q values with lead foil and with omission of lead foil were -0.1 ± 0.02% for the 6 MV FFF and -0.1 ± 0.01% for the 10 MV FFF beams.

CONCLUSION

With evaluation of the lead foil role in determination of the k_Q factor for FFF beams. Our results suggest that the omission of lead foil introduces approximately 0.1% of error for reference dosimetry of FFF beams on both TrueBeam and Versa platforms.

Therapeutic radiation beam output and energy variation across clinics, technologies, and time

Over the past several decades, a medical physics service group covering 35 clinical sites has provided routine monthly output and energy quality assurance for over 75 linear accelerators. Based on the geographical spread of these clinics and the large number of physicists involved in data acquisition, a systematic calibration procedure was established to ensure uniformity. A consistent measurement geometry and data collection technique is used across all machines for every calendar month, using a standardized set of acrylic slabs. Charge readings in acrylic phantoms are linked to AAPM's TG-51 formalism via a parameter denoted k_acrylic , used to convert raw charge readings to machine output values. Statistical analyses of energy ratios and k_acrylic values are presented. Employing the k_acrylic concept with a uniform measurement geometry of similar acrylic blocks was found to be a reproducible and simple way of referencing a calibration completed in water under reference conditions and comparing to other machines, with the ability to alert physicists of outliers.

Reflections on a life with Monte Carlo in Medical Physics

The author reminisces about some of his experiences working with Monte Carlo techniques for Medical Physics applications.