William F. Hanson

Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations

Since publication of the American Association of Physicists in Medicine (AAPM) Task Group No. 43 Report in 1995 (TG-43), both the utilization of permanent source implantation and the number of low-energy interstitial brachytherapy source models commercially available have dramatically increased. In addition, the National Institute of Standards and Technology has introduced a new primary standard of air-kerma strength, and the brachytherapy dosimetry literature has grown substantially, documenting both improved dosimetry methodologies and dosimetric characterization of particular source models. In response to these advances, the AAPM Low-energy Interstitial Brachytherapy Dosimetry subcommittee (LIBD) herein presents an update of the TG-43 protocol for calculation of dose-rate distributions around photon-emitting brachytherapy sources. The updated protocol (TG-43U1) includes (a) a revised definition of air-kerma strength; (b) elimination of apparent activity for specification of source strength; (c) elimination of the anisotropy constant in favor of the distance-dependent one-dimensional anisotropy function; (d) guidance on extrapolating tabulated TG-43 parameters to longer and shorter distances; and (e) correction for minor inconsistencies and omissions in the original protocol and its implementation. Among the corrections are consistent guidelines for use of point- and line-source geometry functions. In addition, this report recommends a unified approach to comparing reference dose distributions derived from different investigators to develop a single critically evaluated consensus dataset as well as guidelines for performing and describing future theoretical and experimental single-source dosimetry studies. Finally, the report includes consensus datasets, in the form of dose-rate constants, radial dose functions, and one-dimensional (1D) and two-dimensional (2D) anisotropy functions, for all low-energy brachytherapy source models that met the AAPM dosimetric prerequisites [Med. Phys. 25, 2269 (1998)] as of July 15, 2001. These include the following 125 I sources: Amersham Health models 6702 and 6711, Best Medical model 2301, North American Scientific Inc. (NASI) model MED3631-A/M, Bebig/Theragenics model I25.S06, and the Imagyn Medical Technologies Inc. isostar model IS-12501. The 103 Pd sources included are the Theragenics Corporation model 200 and NASI model MED3633. The AAPM recommends that the revised dose-calculation protocol and revised source-specific dose-rate distributions be adopted by all end users for clinical treatment planning of low energy brachytherapy interstitial sources. Depending upon the dose-calculation protocol and parameters currently used by individual physicists, adoption of this protocol may result in changes to patient dose calculations. These changes should be carefully evaluated and reviewed with the radiation oncologist preceding implementation of the current protocol.

AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams

A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between 60Co and 50 MV and electron beams with nominal energies between 4 and 50 MeV. The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, N(60Co)D,w which are traceable to national primary standards, and the equation D(Q)w = MkQN(60Co)D,w where Q is the beam quality of the clinical beam, D(Q)w is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, M is the fully corrected ion chamber reading, and kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a beam of quality Q. Values of kQ are presented as a function of Q for many ion chambers. The value of M is given by M = PionP(TP)PelecPpolMraw, where Mraw is the raw, uncorrected ion chamber reading and Pion corrects for ion recombination, P(TP) for temperature and pressure variations, Pelec for inaccuracy of the electrometer if calibrated separately, and Ppol for chamber polarity effects. Beam quality, Q, is specified (i) for photon beams, by %dd(10)x, the photon component of the percentage depth dose at 10 cm depth for a field size of 10x10 cm2 on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by R50, the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size > or =10x10 cm2 on the surface of the phantom (> or =20x20 cm2 for R50>8.5 cm) at an SSD of 100 cm. R50 is determined directly from the measured value of I50, the depth at which the ionization falls to 50% of its maximum value. All clinical reference dosimetry is performed in a water phantom. The reference depth for calibration purposes is 10 cm for photon beams and 0.6R50-0.1 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a 10x10 cm2 field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup. For electron beams clinical reference dosimetry is performed with a field size of > or =10x10 cm2 (> or =20x20 cm2 for R50>8.5 cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification compared to the AAPMs TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors. Worksheets for various situations are presented along with a list of equipment required.

Impact of irradiation technique and tumor extent in tumor control and survival of patients with unresectable non-oat cell carcinoma of the lung. Report by the radiation therapy oncology group

An analysis of intrathoracic tumor control was carried out in 378 patients with histologically proven unresectable non‐oat cell carcinoma of the lung treated with definitive radiotherapy, randomized to one of four treatment regimens: 4000 rad split course (2000 rad in five fractions in one week, two weeks rest and additional 2000 rad in five fractions in one week) or 4000, 5000 or 6000 rad continuous courses, five fractions per week. Between 85 and 101 patients are analyzed in each treatment group. The complete plus partial response was 46–51% in the 4000 rad groups in contrast to 61–66% in the 5000 to 6000 rad groups (P = 0.008). The overall two year survival rate was 10–11% for the patients treated with 4000 rad split or continuous course, and 19% in the patients treated with 5000 to 6000 rad. The complete response in patients with tumors 3 cm or less in diameter was 16% when treated with 4000 rad in contrast to 20–31% in those treated with 5000–6000 rad. In the patients with lesions from 4 to 6 cm in diameter, complete and partial tumor regression was 48% in the 4000 rad group, 67% with 5000 rad, and 71% with 6000 rad. These differences are statistically significant (P = 0.033). Intrathoracic recurrences were correlated with the dose of irradiation given: 52% with 4000 rad, 41% with 5000 rad, and 30% with 6000 rad (P = 0.006). An analysis of protocol compliance was carried out in 301 patients with available data, irradiated at the primary site according to protocol instruction (none or minor variation). Median survival for patients treated to the ipsilateral or contralateral hilar lymph nodes according to the protocol varied from 46–50 weeks in contrast to 20–30 weeks for those with major protocol variations in nodal irradiation. Variations in ipsilateral and contralateral nodal irradiation correlated with significant reductions in tumor control (P = 0.02 and P = 0.009, respectively). In addition to patient and tumor characteristics, the technical factors of irradiation are critical parameters that affect tumor control and survival in patients with non‐oat cell bronchogenic carcinoma. Strict quality assurance criteria in radiotherapy are necessary to achieve optimal treatment results and a careful program to evaluate techniques of irradiation and protocol compliance should be maintained in cooperative group studies in order to enhance the validity of clinical trials.

A Phase II trial of brachytherapy alone after lumpectomy for select breast cancer: Tumor control and survival outcomes of RTOG 95-17

PURPOSE Radiation Therapy Oncology Group 95-17 is a prospective Phase II cooperative group trial of accelerated partial breast irradiation (APBI) alone using multicatheter brachytherapy after lumpectomy in select early-stage breast cancers. Tumor control and survival outcomes are reported. METHODS AND MATERIALS Eligibility criteria included Stage I/II breast carcinoma confirmed to be <3 cm, unifocal, invasive nonlobular histology with zero to three positive axillary nodes without extracapsular extension. APBI treatment was delivered with either low-dose-rate (LDR) (45 Gy in 3.5-5 days) or high-dose-rate (HDR) brachytherapy (34 Gy in 10 twice-daily fractions over 5 days). End points evaluated included in-breast control, regional control, mastectomy-free rate, mastectomy-free survival, disease-free survival, and overall survival. The study was designed to analyze the HDR and LDR groups separately and without comparison. RESULTS Between 1997 and 2000, 100 patients were accrued and 99 were eligible; 66 treated with HDR brachytherapy and 33 treated with LDR brachytherapy. Eighty-seven patients had T1 lesions and 12 had T2 lesions. Seventy-nine were pathologically N0 and 20 were N1. Median follow-up in the HDR group is 6.14 years with the 5-year estimates of in-breast, regional, and contralateral failure rates of 3%, 5%, and 2%, respectively. The LDR group experienced similar results with a median follow-up of 6.22 years. The 5-year estimates of in-breast, regional, and contralateral failure rates of 6%, 0%, and 6%, respectively. CONCLUSION Patients treated with multicatheter partial breast brachytherapy in this trial experienced excellent in-breast control rates and overall outcome that compare with reports from APBI studies with similar extended follow-up.

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 Centers (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.

Dosimetric prerequisites for routine clinical use of new low energy photon interstitial brachytherapy sources

The dose distributions around higher energy photon-emitting brachytherapy sources ~e.g., Ir and Cs!, are relatively insensitive to small differences in internal structure. In contrast, the dosimetric characteristics of low energy sources, such as I and Pd, are much more sensitive to the details of encapsulation geometry and source internal structure due to self-absorption and filtration effects. Dosimetrically significant differences between different seed models containing the same radionuclide may result from relatively minor differences in design specifications or in manufacturing processes. It is therefore important to individually evaluate the dosimetric characteristics of each new low energy ~less than 50 keV!, photon-emitting brachytherapy source product. It is inappropriate to apply the dose-rate constants, radial dose functions, anisotropy functions, anisotropy factors, and geometry functions published in the Task Group 43 report for currently-available I ~Amersham models 6711 and 6702! and Pd ~Theragenics model 200! interstitial sources to other low-energy seed products. The recommendations below are intended as guidance to manufacturers and regulatory agencies involved in development and approval of new sealed brachytherapy sources. They reflect the consensus views of the AAPM as to what dosimetric measurements should be made and should be available to users before releasing the sources for routine patient treatments. These recommendations do not apply to investigational brachytherapy sources used in human clinical studies approved by FDA and/or the institution’s IRB.

Guidance to users of Nycomed Amersham and North American Scientific, Inc., I‐125 Interstitial Sources: Dosimetry and calibration changes: Recommendations of the American Association of Physicists in Medicine Radiation Therapy Committee Ad Hoc Subcommittee on Low‐Energy Seed Dosimetry

Dose calculations to patients undergoing implantation of 125I interstitial brachytherapy sources are affected by two recent changes in low-energy seed dosimetry: (a) implantation of a new primary air-kerma strength standard at the National Institute of Standards and Technology (NIST) on 1 January 1999 and (b) publication of revised dose-rate distributions in AAPMs Task Group 43 Report. The guidance herein represents AAPMs recommendations for users of 125I interstitial seed products marketed prior to 1 January 1999 (Nycomed Amersham models 6711 and 6702 and North American Scientific, Inc. models 3631 A/S and 3631 A/M. Implementation of Task Group 43 (TG43) 125I dose calculations involves revising data stored in files of radiation treatment planning software and lowering the prescribed dose to be delivered to patients by as much as 15% to avoid modifying the dose actually delivered to patients. The magnitude of the dose prescription change depends on the dosimetry data used prior to TG43 and the implant geometry. Adapting to the revised NIST calibration standard requires the user to increase the dose-rate constant (or its equivalent by 11.5%) but does not require modification of the prescribed dose. Failure to correctly implement these modifications can result in 20% or even 30% errors.

Findings from NSABP protocol no. B‐04‐comparison of radical mastectomy with alternative treatments for primary breast cancer. I. Radiation compliance and its relation to treatment outcome

Between 1971 and 1974, 1,665 women with primary operable breast cancer were entered into a prospective randomized clinical trial (NSABP Protocol No. B‐04) in order to compare the worth of radical mastectomy with alternative treatments. Six‐hundred forty‐six of the women, 352 clinically nodenegative and 294 node‐positive, were randomized so that they were to have been treated with total mastectomy and postoperative radiation. Due to a meticulous comprehensive program of radiation monitoring involving close cooperation between the NSABP Headquarters, the NSABP Radiation Monitoring Committee, the Radiological Physics Center at M. D. Anderson Cancer Center and participating institutions, it has been possible to determine protocol compliance of the radiation administered and to correlate any variation of radiation employed with treatment outcome. At the onset of the study and prior to any evaluation of treatment results, the Radiation Monitoring Committee defined minor variations from the protocol which were acceptable and those variations which were more major and unacceptable. While it was found that 53% of the 543 evaluable patients had been treated by radiation having some degree of variation from the protocol, it was ascertained that 77% of the 2.172 irradiated fields of those patients (82% in clinically node‐negative and 71% in clinically node‐positive patients) received radiation per protocol. Only 6.7% of all sites were the recipients of a major (unacceptabel) variation in radiation. Analysis of data demonstrated that there was a remarkable similarity in the incidence and rate of treatment failure (TF) or mortality between patients having some radiation variation, regardless of its degree or extent and those who were treated with no protocol variation. Results were similar when comparisons were made taking into consideration either the degree (minor or major low) or the extent (one or more than one field) of the radiation variation. A substantial number of clinically positive axillary node patients received only the radiation to the axillary field intended for those with clinically negative nodes, i.e., they failed to receive the specified radiation boost. Despite that deviation, there was no difference in TF or survival from those receiving the prescribed treatment. Moreover, findings were not different when the axilla was the only site of radiation deviation or there were deviations to other fields as well. Of singular importance was an inability to associate the localregional location of a recurrence with a low‐dose radiation deviation at the site. Almost all recurrences in clinically node‐negative patients were at sites treated per protocol. In positive‐node patients, supraclavicular and chest wall recurrences occurred in patients having acceptable radiation to those sites and axillary recurrences were equivalent in patients treated with or without deviations at that site. The findings from these investigations are not presented in order to indicate that the quality and quantity of radiation may be unimportant. They have been obtained under careful monitoring and the variations are within a relatively narrow range. Consequently, more general extrapolations from these must be made with circumspection. The observations do suggest, however, that there is some acceptable leeway in the use of radiation, at least for treatment of the extent of disease present in patients in this study. Moreover, they refute any consideration that the failure to demonstrate a significant advantage following the use of postoperative radiation in Protocol No. B‐04 could be related to the variation and inadequacy of radiation employed in a proportion of the patients. They tend to substantiate the contention that factors other than the type of operative procedure employed or the precision of administration of radiation are important in determining the survival of women with breast cancer. Finally, the mechanism whereby the radiation employed was quality controlled serves as a model for the mandatory monitoring of all modalities of therapy in cooperative clinical trials. Such monitoring, by providing more credible dat, will favorably affect the management of cancer patients. Cancer 46:1–13, 1980.

Design, development, and implementation of the Radiological Physics Center’s pelvis and thorax anthropomorphic quality assurance phantoms

The Radiological Physics Center (RPC) developed two heterogeneous anthropomorphic quality assurance phantoms for use in verifying the accuracy of radiation delivery: one for intensity-modulated radiation therapy (IMRT) to the pelvis and the other for stereotactic body radiation therapy (SBRT) to the thorax. The purpose of this study was to describe the design and development of these two phantoms and to demonstrate the reproducibility of measurements generated with them. The phantoms were built to simulate actual patient anatomy. They are lightweight and water-fillable, and they contain imageable targets and organs at risk of radiation exposure that are of similar densities to their human counterparts. Dosimetry inserts accommodate radiochromic film for relative dosimetry and thermoluminesent dosimetry capsules for absolute dosimetry. As a part of the commissioning process, each phantom was imaged, treatment plans were developed, and radiation was delivered at least three times. Under these controlled irradiation conditions, the reproducibility of dose delivery to the target TLD in the pelvis and thorax phantoms was 3% and 0.5%, respectively. The reproducibility of radiation-field localization was less than 2.5 mm for both phantoms. Using these anthropomorphic phantoms, pelvic IMRT and thoracic SBRT radiation treatments can be verified with a high level of precision. These phantoms can be used to effectively credential institutions for participation in specific NCI-sponsored clinical trials.

Recommendations of the American association of physicists in medicine on 103Pd interstitial source calibration and dosimetry: Implications for dose specification and prescription

The National Institute of Standards and Technology (NIST) introduced a national standard for air kerma strength of the ThreaSeed Model 200 103Pd source (the only 103Pd seed available until 1999) in early 1999. Correct implementation of the NIST-99 standard requires the use of dose rate constants normalized to this same standard. Prior to the availability of this standard, the vendors calibration procedure consisted of intercomparing Model 200 seeds with a 109Cd source with a NIST-traceable activity calibration. The AAPM undertook a comprehensive review of 103Pd source dosimetry including (i) comparison of the vendor and NIST-99 calibration standards; (ii) comparison of original Task Group 43 dosimetry parameters with more recent studies; (iii) evaluation of the vendors calibration history; and (iv) evaluation of administered-to-prescribed dose ratios from the introduction of 103Pd sources in 1987 to the present. This review indicates that for a prescribed dose of 115 Gy, the administered doses were (a) 124 Gy for the period 1988-1997 and (b) 135 Gy for the period 1997-1999. The AAPM recommends that the following three steps should be undertaken concurrently to implement correctly the 1999 dosimetry data and NIST-99 standard for 103Pd source: (1) the vendor should provide calibrations in terms of air kerma strength traceable to NIST-99 standard, (2) the medical physicist should update the treatment planning system with properly normalized (to NIST-99) dosimetry parameters for the selected 103Pd source model, and (3) the radiation oncologist in collaboration with the medical physicist should decide which clinical experience they wish to duplicate; the one prior to 1997 or the one from 1997 to 1999. If the intent is to duplicate the experience prior to 1997, which is backed by the long-term follow-up and published outcome studies, then the prior prescriptions of 115 Gy should be replaced by 124 Gy to duplicate that experience.