Dose specification and quality assurance of radiation therapy oncology group protocol 95-17; a cooperative group study of iridium-192 breast implants as sole therapy.

PURPOSE: The Radiation Therapy Oncology Group (RTOG) protocol 95-17 was a Phase I/II trial to evaluate multicatheter brachytherapy as the sole method of adjuvant breast radiotherapy for Stage I/II breast carcinoma after breast-conserving surgery. Low- or high-dose-rate sources were allowed. Dose prescription and treatment evaluation were based on recommendations in the International Commission on Radiation Units and Measurements (ICRU), Report 58 and included the parameters mean central dose (MCD), average peripheral dose, dose homogeneity index (DHI), and the dimensions of the low- and high-dose regions. METHODS AND MATERIALS: Three levels of quality assurance were implemented: (1) credentialing of institutions was required before entering patients into the study; (2) rapid review of each treatment plan was conducted before treatment; and (3) retrospective review was performed by the Radiological Physics Center in conjunction with the study chairman and RTOG dosimetry staff. RESULTS: Credentialing focused on the accuracy of dose calculation algorithm and compliance with protocol guidelines. Rapid review was designed to identify and correct deviations from the protocol before treatment. The retrospective review involved recalculation of dosimetry parameters and review of dose distributions to evaluate the treatment. Specifying both central and peripheral doses resulted in uniform dose distributions, with a mean dose homogeneity index of 0.83 +/- 0.06. CONCLUSIONS: Vigorous quality assurance resulted in a high-quality study with few deviations; only 4 of 100 patients were judged as representing minor variations from protocol, and no patient was judged as representing major deviation. This study should be considered a model for quality assurance of future trials.

The US radiation dosimetry standards for 60Co therapy level beams, and the transfer to the AAPM accredited dosimetry calibration laboratories.

This work reports the transfer of the primary standard for air kerma from the National Institute of Standards and Technology (NIST) to the secondary laboratories accredited by the American Association of Physics in Medicine (AAPM). This transfer, performed in August of 2003, was motivated by the recent revision of the NIST air-kerma standards for 60Co gamma-ray beams implemented on July 1, 2003. The revision involved a complete recharacterization of the two NIST therapy-level 60Co gamma-ray beam facilities, resulting in new values for the air-kerma rates disseminated by the NIST. Some of the experimental aspects of the determination of the new air-kerma rates are briefly summarized here; the theoretical aspects have been described in detail by Seltzer and Bergstrom ["Changes in the U.S. primary standards for the air-kerma from gamma-ray beams," J. Res. Natl. Inst. Stand. Technol. 108, 359-381 (2003)]. The standard was transferred to reference-class chambers submitted by each of the AAPM Accredited Dosimetry Calibration Laboratories (ADCLs). These secondary-standard instruments were then used to characterize the 60Co gamma-ray beams at the ADCLs. The values of the response (calibration coefficient) of the ADCL secondary-standard ionization chambers are reported and compared to values obtained prior to the change in the NIST air-kerma standards announced on July 1, 2003. The relative change is about 1.1% for all of these chambers, and this value agrees well with the expected change in chambers calibrated at the NIST or at any secondary-standard laboratory traceable to the new NIST standard.

Dosimetry characteristics of four fast neutron generators involved in RTOG interinstitutional clinical trials.

PURPOSE: To demonstrate the consistency of dosimetry data used for four fast neutron generators involved in inter-institutional clinical trials. METHODS AND MATERIALS: Central-axis dosimetry characteristics (field size dependence, percentage depth dose, beam modifier factors) at four institutions were measured by independent physicists from the Radiological Physics Center. These measurements were made in water with a single set of dosimetry equipment using tissue equivalent ionization chambers. Measurements were made with the chamber cavities filled with stationary air and flowing tissue-equivalent gas. All measurements were performed using techniques developed by the Radiological Physics Center for conventional radiotherapy equipment. The results of our measurements were compared to the data used by the institution to determine the consistency of patient doses reported to the Radiation Therapy Oncology Group. RESULTS: The agreement of the Radiological Physics Center and institution data is similar to what is typically found with conventional radiotherapy equipment. CONCLUSIONS: The doses reported by these institutions to the Fast Neutron Working Group of the Radiation Therapy Oncology Group are consistent, within accepted clinical criteria.

Mailable TLD system for photon and electron therapy beams.

The mailable TLD system developed by the Radiological Physics Center for monitoring calibration of photon beam energies from cobalt 60 to 25 MV and electron beam energies from 6 to 20 MeV has been in use since 1977 for photons and since 1982 for electron beams. Design considerations, proper use of the system and calibration techniques are detailed. The accuracy of the system is comparable to that of ion chamber measurements made in a water phantom, although it shows less precision.

Report of the ad hoc committee of the AAPM radiation therapy committee on 125I sealed source dosimetry.

PURPOSE: Two developments in 125I-sealed source dosimetry have necessitated swift and accurate implementation of TG43 dosimetry in clinic: (a) the dosimetry constants of 125I endorsed by the AAPM Task Group 43 Report result in calculated dose rate that deviates by as much as 15% from currently accepted dose-rate distributions, and (b) The National Institute of Standards and Technology (NIST) has proposed modifying the 125I air-kerma strength standard by approximately 10%. METHODS AND MATERIALS: The ad hoc committee of AAPM Radiation Therapy Committee describes specific procedures to implement these two developments without causing confusion and mistakes. CONCLUSIONS: Confusion and mistakes may be avoided when the following two general steps are taken: 1) STEP I, TG-43 implementation, and 2) STEP II, new air-kerma strength standard implementation when available from NIST.

Effects of plastic protective caps on the calibration of therapy beams in water.

Plastic 60Co buildup caps have been widely used to protect ionization chambers when calibrating high-energy x-ray and electron beams in water, and have been used consistently by the Radiological Physics Center. Recent calibration protocols base their calculations on a theory that assumes that no protective cap is used during calibration in phantom. The change in ionization within the chamber due to the presence of a protective cap has been investigated for acrylic and polystyrene caps of various wall thicknesses, using 60Co and x-ray beams from 6-25 MV and electron beams from 7-18 MeV. The change has been shown to be small, no more than 0.5% for x rays and 0.7% for electrons using acrylic 60Co caps. The change for polystyrene is seen to be as much as twice that for acrylic. Empirical correction factors to compensate for this effect have been determined. A basis in theory for photons is suggested by an extension of the theory in recent protocols. The effect for electrons is explained only qualitatively.

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

Task Group 51 (TG51), of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM), has developed a calibration protocol for high-energy photon and electron therapy beams based on absorbed dose standards. This protocol is intended to replace the air-kerma based protocol developed by an earlier AAPM task group (TG21). Conversion to the newer protocol introduces a change in the determined absorbed dose. In this work, the change in dose is expressed as the ratio of the doses (TG51/TG21) based on the two protocols. Dose is compared at the TG-51 reference depths of 10 cm for photons and d(ref) for electrons. Dose ratios are presented for a variety of ion chambers over a range of photon and electron energies. The TG51/TG21 dose ratios presented here are based on the dosimetry factors provided by the two protocols and the chamber-specific absorbed dose and exposure calibration factors (N60Co(D,w) and Nx) provided by the Accredited Dosimetry Calibration Laboratory (ADCL) at The University of Texas, M. D. Anderson Cancer Center (MDACC). As such, the values presented here represent the expected discrepancies between the two protocols due only to changes in the dosimetry parameters and the differences in chamber-specific dose and air-kerma standards. These values are independent of factors such as measurement uncertainties, setup errors, and inconsistencies arising from the mix of different phantoms and ion chambers for the two protocols. Therefore, these ratios may serve as a guide for institutions performing measurements for the switch from TG21-to-TG51 based calibration. Any significant deviation in the ratio obtained from measurements versus those presented here should prompt a review to identify possible errors and inconsistencies. For all cylindrical chambers included here, the TG51/TG21 dose ratios are the same within +/-0.6%, irrespective of the make and model of the chamber, for each photon and electron beam included. Photon beams show the TG51/TG21 dose ratios decreasing with energy, whereas electrons exhibit the opposite trend. The dose ratio for photons is near 1.00 at 18 mV increasing to near 1.01 at 4 mV while the dose ratio for electrons is near 1.02 at 20 MeV decreasing only 0.5% to near 1.015 at 6 MeV. For parallel-plate chambers, the situation is complicated by the two possible methods of obtaining calibration factors: through an ADCL or through a cross-comparison with a cylindrical chamber in a high-energy electron beam. For some chambers, the two methods lead to significantly different calibration factors, which in turn lead to significantly different TG51/TG21 results for the same chamber. Data show that if both N60Co(D,w) and Nx are obtained from the same source, namely an ADCL or a cross comparison, the TG51/TG21 results for parallel-plate chambers are similar to those for cylindrical chambers. However, an inconsistent set of calibration factors, i.e., using N60Co(D,w) x k(ecal) from an ADCL but Ngas from a cross comparison or vice versa, can introduce an additional uncertainty up to 2.5% in the TG51/TG21 dose ratios.

Off-axis beam quality change in linear accelerator x-ray beams.

The effective energy of the x-ray beam from linear accelerators changes as a function of the position in the beam due to nonuniform filtration by the flattening filter. In this work, the transmittance through a water column was measured in good geometry and the beam quality characterized in units of HVL in water. Measurements were made on a variety of linear accelerators from 4 to 10 MV. The beam energy decreased with increasing distance from the central ray for all accelerators measured.

Calculative technique to correct for the change in linear accelerator beam energy at off-axis points.

The change in energy of linear accelerator x-ray beams from the central ray to off-axis points causes errors in the dose calculated by conventional techniques for large, irregularly shaped fields. A modification of conventional calculative methods to correct for the change in beam energy is presented. The results of measurements in irregular fields on a Clinac-4 are reported which verify the validity of the calculative method. A discussion of the clinical significance will point out errors of 3% to 4% in conventional dose calculations.

Verification of total body photon irradiation dosimetry techniques.

A method of verifying the dosimetry of patients undergoing total body irradiation (TBI) with photon beams having energies from cobalt-60 to 25 MV is presented. A simple set of spot checks at the TBI axis has been used to verify data used for TBI dosimetry. Calculations to verify dose delivered to TBI patients are done in the same manner as those irradiated at standard treatment distances. A simple method of effective field size determination for various anatomical locations in a typical adult is presented. Measurements in an Alderson phantom with thermoluminescent dosimeters and an ion chamber at several anatomical locations indicate that this calculational method can predict the dose along the patient axis to within 4% for 60Co and 18-MV photon beams, provided the dosimetry data are appropriate (as determined by the spot checks). Results of intercomparisons of TBI beam calibration, off-axis and depth-dose data at various institutions visited by the Radiological Physics Center are also presented.

TG-21 versus TG-25: a comparison for electrons.

Since 1984, the Radiological Physics Center (RPC) has used the American Association of Physicists in Medicine Task Group 21 (TG-21) protocol (absorbed dose determination) as the basis of its On-site Dosimetry Review visits to institutions participating in the National Cancer Institute's cooperative clinical trials. Subsequent to the TG-21 protocol, the Task Group 25 (TG-25) report on electron-beam dosimetry was published. The TG-25 report was not intended to supercede the TG-21 protocol, but to supplement it for depths other than dmax. However, both reports included measurement techniques and data regarding the calibration of electron beams. TG-25 was not intended for absolute calibrations made clear by the fact that it does not present all of the data required for plastic phantom calibrations, i.e., unrestricted stopping power ratios. As a result, some confusion has arisen at various institutions as to which protocol should be used for machine calibration. In this study, possible discrepancies that arise when using TG-21, a version of TG-21 modified by the RPC, and TG-25 are compared. The differences in the results are calculated as a function of energy (6 and 20 MeV), chamber type (cylindrical or parallel plate), and the type of phantom material (water, polystyrene, or acrylic). The largest discrepancies noted were between TG-25 and the two TG-21 methods for low-energy electrons in either water or polystyrene. The mean difference for all conditions was 0.8% with a maximum value of 3.3% in polystyrene. The definition of the effective point of measurement; determination of the mean nominal incident energy (E0), mean energy at depth (EZ) and most probable energy at the surface (Ep,0) for each protocol, and subsequent stopping power ratio, chamber replacement factor, and electron fluence correction factor are the major contributors to the calculated differences.

Uncertainty analysis of absorbed dose calculations from thermoluminescence dosimeters.

Thermoluminescence dosimeters (TLD) are widely used to verify absorbed doses delivered from radiation therapy beams. Specifically, they are used by the Radiological Physics Center for mailed dosimetry for verification of therapy machine output. The effects of the random experimental uncertainties of various factors on dose calculations from TLD signals are examined, including: fading, dose response nonlinearity, and energy response corrections; reproducibility of TL signal measurements and TLD reader calibration. Individual uncertainties are combined to estimate the total uncertainty due to random fluctuations. The Radiological Physics Center's (RPC) mail out TLD system, utilizing throwaway LiF powder to monitor high-energy photon and electron beam outputs, is analyzed in detail. The technique may also be applicable to other TLD systems. It is shown that statements of +/- 2% dose uncertainty and +/- 5% action criterion for TLD dosimetry are reasonable when related to uncertainties in the dose calculations, provided the standard deviation (s.d.) of TL readings is 1.5% or better.

How water equivalent are water-equivalent solid materials for output calibration of photon and electron beams?

The water equivalency of five "water-equivalent" solid phantom materials was evaluated in terms of output calibration and energy characterization over a range of energies for both photon (Co-60 to 24 MV) and electron (6-20 MeV) beams. Evaluations compared absorbed doses calculated from ionization measurements using the same dosimeter in the solid phantom materials and in natural water (H2O). Ionization measurements were taken at various calibration depths. The Radiological Physics Center's standard dosimetry system, a Farmer-type ion chamber in a water phantom, was used. Complying with the TG-21 calibration protocol, absorbed doses were calculated using eight measurement and calculational techniques for photons and five for electrons. Results of repeat measurements taken over a period of 2 1/2 years were reproducible to within a +/- 0.3% spread. Results showed that various combinations of measurement techniques and solid phantom materials caused a spread of 3%-4% in the calculation of dose relative to the dose determined from measurements in water for all beam energies on both modalities. An energy dependence of the dose ratios was observed for both photons and electrons.

A first order approximation of field-size and depth dependence of wedge transmission.

The Radiological Physics Center, through its dosimetry review visits to participating institutions, is aware that many institutions ignore the field-size and depth dependence of wedge transmission values. Reference wedge transmission values are normally measured by the Radiological Physics Center for a 10 cm x 10 cm field at the calibration depth of 5 or 7 cm. Recently, additional measurements (1) for a 10 cm x 10 cm field at 20-cm depth and (2) for a 20 cm x 20 cm field at the calibration depth were included. The transmission under these two conditions was compared with that under reference conditions. The relative transmission values for 138 photon beams from 88 separate linear accelerators (4-25 MV) and 60Co units were measured. Our data suggest that the dependence of the wedge transmission on field-size and depth, in the first approximation, depends on the absolute value of the transmission under reference conditions. For wedges with a transmission value greater than 0.65%, field-size dependence and change in depth dose are typically less than 2%. However, for wedges with transmission values less than 0.65%, field-size dependence increases with decreasing reference wedge transmission. The change in wedge transmission with depth is significant (> 2%) only for photon energies less than or equal to 10 MV and can exceed 5% for thick wedges. Failure to include the depth and field-size dependencies of wedge transmission in patient dosimetry calculations can result in significant tumor-dose discrepancies.

Equilibration of air temperature inside the thimble of a Farmer-type ion chamber.

Ionization chambers are frequently moved from one environment to another, sometimes with significant differences in temperature between the chamber and measurement phantom. To obtain reliable ionization data, the temperature of the air in the chamber must be allowed to equilibrate with the measuring phantom. The air temperature inside a thimble of a Farmer-type ion chamber was measured as a function of time for various phantom materials (air, water, and plastic). Equilibration rates for the various conditions are presented. Heat-diffusion theory is presented to explain the characteristics of the measured data. Waiting times for temperature equilibration down to 10% of the initial temperature difference ranges from 1 to 18 min, depending on the phantom material and use of bare or covered thimble. Radiation measurements confirm the temperature data.

Determination of the tissue attenuation factor along two major axes of a high dose rate (HDR) 192Ir source.

Quantitative information on photon scattering around brachytherapy sources is needed to develop dose calculation formalisms capable of predicting dosimetric parameters with minimal empiricism. Photon absorption and scatter around brachytherapy sources can be characterized using the tissue attenuation factor, defined as the ratio of dose in water to water kerma in free space. In this study, the tissue attenuation factor along two major axes of a high dose rate (HDR) 192Ir source was determined by TLD measurements and MCNP Monte Carlo calculations. A calculational method is also suggested to derive the tissue attenuation factor along the longitudinal source axis from the factor along the transverse axis, using published anisotropy data as input. TLD and Monte Carlo results agreed with each other for both source axes within the statistical uncertainty (approximately +/- 5%) of Monte Carlo calculations. Comparison with published data, available only for the transverse source axis, also showed good agreement within +/- 5%. The shape and magnitude of the tissue attenuation factor are found to be remarkably different between the two axes. The tissue attenuation factor reaches a maximum value of about 1.4 at 8 cm from the source along the longitudinal source axis, while a maximum value of about 1.04 occurs at 3-4 cm from the source along the transverse axis. The calculated tissue attenuation factor along the longitudinal source axis generally reproduced the TLD and Monte Carlo results within +/- 5% at most radial distances.

Contamination of ionization chambers by talcum powder.

This is a word of caution to anyone using ionization chambers protected by thin rubber sheaths in water. Four Farmer-type ionization chambers contaminated with talcum powder were received for calibration by the Accredited Dosimetry Calibration Laboratory at the University of Texas M. D. Anderson Cancer Center. The chambers show a marked energy dependence (5% to 20%) to soft orthovoltage x rays. The response of the contaminated chambers is compared with the chambers' response before contamination and after cleaning. Techniques for identifying contaminated chambers and suggestions for cleaning them are presented.

The implementation of the AAPM Task Group 21 protocol by the Radiological Physics Center and its implications.

The Radiation Therapy Committee of the American Association of Physicists in Medicine appointed Task Group 21 to write a new protocol for the calibration of high-energy photon and electron therapy beams. This protocol updates the physical parameters used in the calculations and is intended to account for differences in ionization chamber design and some differences between phantom materials that were not considered in previous protocols. This paper discusses how the Radiological Physics Center (RPC) intends to implement the new protocol, the changes required in the RPC calibration techniques, and the magnitude of the change in the RPC calculations of absorbed dose resulting from the implementation of the new protocol. Although the change in the RPC absorbed-dose calculations will be only 0%-2% over the range of photon and electron energies of interest, some institutions using specific dosimetry systems may find their absorbed-dose calculations changing by 4% or more.

A review of the discrepancy between the in-air and in-water calibration of cobalt-60 machines.

Causes for the discrepancy noted by Grant et al. between the in-water and in-air calibration of 60Co are discussed. Data are presented from measurements with a set of ionization chambers with thimbles of 0.5, 1.0, and 1.5 cm outside radii. These data include measurements of percentage depth dose, backscatter factors, and displacement factors. The results show that the discrepancy noted by Grant et al. is caused by a combination of small errors both in depth dose data and in the displacement factor incorporated into C lambda.

An analysis of discrepancies encountered by the AAPM radiological physics center.

The AAPM Radiological Physics Center has reviewed 188 institutions and has evaluated such parameters as coincidence of radiation field and light field, timer error (end effect), beam flatness and symmetry, transmission through blocking trays, wedges and compensators, and central-axis depth-dose data. In previous papers these data had been presented in combination as they resulted in discrepancies in tumor dose. The individual sources of discrepancies were listed only as frequency and maximum deviation. A detailed analysis is now presented which may help define criteria of recommended practice.