A round-robin gamma stereotactic radiosurgery dosimetry interinstitution comparison of calibration protocols.

PURPOSE: Absorbed dose calibration for gamma stereotactic radiosurgery is challenging due to the unique geometric conditions, dosimetry characteristics, and nonstandard field size of these devices. Members of the American Association of Physicists in Medicine (AAPM) Task Group 178 on Gamma Stereotactic Radiosurgery Dosimetry and Quality Assurance have participated in a round-robin exchange of calibrated measurement instrumentation and phantoms exploring two approved and two proposed calibration protocols or formalisms on ten gamma radiosurgery units. The objectives of this study were to benchmark and compare new formalisms to existing calibration methods, while maintaining traceability to U.S. primary dosimetry calibration laboratory standards. METHODS: Nine institutions made measurements using ten gamma stereotactic radiosurgery units in three different 160 mm diameter spherical phantoms [acrylonitrile butadiene styrene (ABS) plastic, Solid Water, and liquid water] and in air using a positioning jig. Two calibrated miniature ionization chambers and one calibrated electrometer were circulated for all measurements. Reference dose-rates at the phantom center were determined using the well-established AAPM TG-21 or TG-51 dose calibration protocols and using two proposed dose calibration protocols/formalisms: an in-air protocol and a formalism proposed by the International Atomic Energy Agency (IAEA) working group for small and nonstandard radiation fields. Each institution's results were normalized to the dose-rate determined at that institution using the TG-21 protocol in the ABS phantom. RESULTS: Percentages of dose-rates within 1.5% of the reference dose-rate (TG-21+ABS phantom) for the eight chamber-protocol-phantom combinations were the following: 88% for TG-21, 70% for TG-51, 93% for the new IAEA nonstandard-field formalism, and 65% for the new in-air protocol. Averages and standard deviations for dose-rates over all measurements relative to the TG-21+ABS dose-rate were 0.999±0.009 (TG-21), 0.991±0.013 (TG-51), 1.000±0.009 (IAEA), and 1.009±0.012 (in-air). There were no statistically significant differences (i.e., p>0.05) between the two ionization chambers for the TG-21 protocol applied to all dosimetry phantoms. The mean results using the TG-51 protocol were notably lower than those for the other dosimetry protocols, with a standard deviation 2-3 times larger. The in-air protocol was not statistically different from TG-21 for the A16 chamber in the liquid water or ABS phantoms (p=0.300 and p=0.135) but was statistically different from TG-21 for the PTW chamber in all phantoms (p=0.006 for Solid Water, 0.014 for liquid water, and 0.020 for ABS). Results of IAEA formalism were statistically different from TG-21 results only for the combination of the A16 chamber with the liquid water phantom (p=0.017). In the latter case, dose-rates measured with the two protocols differed by only 0.4%. For other phantom-ionization-chamber combinations, the new IAEA formalism was not statistically different from TG-21. CONCLUSIONS: Although further investigation is needed to validate the new protocols for other ionization chambers, these results can serve as a reference to quantitatively compare different calibration protocols and ionization chambers if a particular method is chosen by a professional society to serve as a standardized calibration protocol.

Prognostic factors for stereotactic radiosurgery-treated patients with cerebral metastasis: implications on randomised control trial design and inter-institutional collaboration.

INTRODUCTION: Defining key prognostic factors for patients with cerebral metastases who underwent stereotactic radiosurgery (SRS) treatment will greatly facilitate future clinical trial designs. METHODS: We adopted a two-phase study design where results from one cohort were validated in a second independent cohort. The exploratory analysis reviewed the survival outcomes of 1017 consecutive patients (with 3610 metastases) who underwent Gamma radiosurgery at the University of California, San Diego (UCSD)/San Diego Gamma Knife Center (SDGKC). Multivariate analysis was performed to identify prognostic factors. Results were validated using data derived from 2519 consecutive patients (with 17,498 metastases) treated with SRS at the Katsuta Hospital. RESULTS: For the SDGKC cohort, the median overall survival of patients following SRS was 7 months. Two year follow-up data were available for 85% of the patients. Multivariate analysis found that patient age, Karnofsky Performance Status, systemic cancer status, tumour histology, number of metastasis and cumulative tumour volume independently associated with overall survival (p<0.001). All statistical associations were validated by multivariate analysis of data derived from the Katsuta Hospital cohort. CONCLUSIONS: This is the first integrated study that defined prognostic factors for SRS-treated patients with cerebral metastases using an inter-institutional validation study design. The work establishes a model for collaborative interactions between large volume centers and provides prognostic variables that should be incorporated into future clinical trial design.

HIV-related nephropathy: a South African perspective.

Human immunodeficiency virus (HIV)-associated nephropathy (HIVAN) is an important cause of renal failure in those of African origin. A number of other kidney diseases occur in HIV-positive patients. We conducted a retrospective review of renal biopsies in HIV-positive Black African patients to determine the prevalence of both 'classic HIVAN' and non-HIVAN pathologies in this group. Clinical and laboratory data from HIV-positive patients who underwent renal biopsy from 1st January 2003 to 31st December 2004 were collected. Similar information on HIV-negative patients biopsied during the same period was also recorded by way of comparison to try and assess the influence of the virus on renal histologic patterns. HIV-positive group - 99 biopsies were suitable for study. The main histologic categories were 'classic HIVAN' (27%) and HIV immune complex kidney disease ('HIVICK') (21%). The subepithelial immune deposits in 'HIVICK' induced a newly described 'ball-in-cup' basement membrane reaction. Other glomerulonephritides included membranous, post-infectious disease, mesangial hyperplasia, and immunoglobulin A nephropathy. Overlapping clinical presentations prevented pre-biopsy histologic predictions. HIV-negative group - There were no examples of collapsing focal segmental glomerulosclerosis or nonspecific immune complex disease, but increased numbers of minimal change and membranoproliferative disease. 'Classic HIVAN' accounted for less than a third of the nephropathies occurring in HIV-positive Black South Africans. 'HIVICK' is another important cause of chronic kidney disease in this group. Future research is needed into the earlier detection and treatment of these diseases, which have a high mortality in our context.

Radiosurgery for patients with brain metastases: a multi-institutional analysis, stratified by the RTOG recursive partitioning analysis method.

PURPOSE: To estimate the potential improvement in survival for patients with brain metastases, stratified by the Radiation Therapy Oncology Group (RTOG) recursive partitioning analysis (RPA) class and treated with radiosurgery (RS) plus whole brain radiotherapy (WBRT). METHODS AND MATERIALS: An analysis of the RS databases of 10 institutions identified patients with brain metastates treated with RS and WBRT. Patients were stratified into 1 of 3 RPA classes. Survival was evaluated using Kaplan-Meier estimates and proportional hazard regression analysis. A comparison of survival by class was carried out with the RTOG results in similar patients receiving WBRT alone. RESULTS: Five hundred two patients were eligible (261 men and 241 women, median age 59 years, range 26-83). The overall median survival was 10.7 months. A higher Karnofsky performance status (p = 0.0001), a controlled primary (median survival = 11.6 vs. 8.8 months, p = 0.0023), absence of extracranial metastases (median survival 13.4 vs. 9.1 months, p = 0.0001), and lower RPA class (median survival 16.1 months for class I vs. 10.3 months for class II vs. 8.7 months for class III, p = 0.000007) predicted for improved survival. Gender, age, primary site, radiosurgery technique, and institution were not prognostic. The addition of RS boosted results in median survival (16.1, 10.3, and 8.7 months for classes I, II, and III, respectively) compared with the median survival (7.1, 4.2, and 2.3 months, p <0.05) observed in the RTOG RPA analysis for patients treated with WBRT alone. CONCLUSION: In the absence of randomized data, these results suggest that RS may improve survival in patients with BM. The improvement in survival does not appear to be restricted by class for well-selected patients.

Risk management in a community Gamma Knife unit.

OBJECTIVE: The records of the San Diego Gamma Knife Center were retrospectively reviewed for unprogrammed events as part of a risk management assessment. MATERIALS AND METHODS: Review was made of the physicist notes of the first 1,000 patients successfully treated at the center. This encompassed 1,020 stereotactic frame placement procedures, accompanied by Gamma Knife radiosurgery in 98.0% of intended cases. A total of 7,145 Gamma Knife shots were delivered to 1,509 lesions. RESULTS: Of the 43 unprogrammed events documented, 8 were patient related and 14 were related to lesion growth or location; these were considered unavoidable. Further, one event was related to dose administration, 5 to diagnosis, 15 to technique; these were considered potentially avoidable. This yielded an avoidable error rate of 2.1% per patient, 1.4% per lesion treated, and 0.29% per shot. It was clear that more avoidable errors occurred early in the center's operation, consistent with a learning curve. Review of individual physician's cases revealed none appeared more likely to have an avoidable event. CONCLUSIONS: Consistently high quality may be achieved at a community-based Gamma Knife center if sufficient multidisciplinary involvement is achieved.

Physics of rotating gamma systems for stereotactic radiosurgery.

PURPOSE: The purpose of this study was to evaluate the characteristics of a new rotating gamma system for stereotactic radiosurgery by comparison with a well accepted system. METHODS AND MATERIALS: A novel gamma unit for stereotactic radiosurgery has been developed and distributed to 15 hospitals in China. The unit contains 30 cobalt-60 gamma radiation sources with initial activity of 200 Ci (7.4 x 10(12) Bq) each. The sources are positioned along 30 arcs, and rotate continuously as a group in an axis orthogonal to the patient's body. Measurements have been made on a representative unit installed in the Auhai Radiosurgery Center at the Beijing Navy General Hospital in the People's Republic of China. Ionization chambers calibrated by an American accredited dosimetry calibration laboratory were used for these measurements, as well as radiochromic film and thermoluminescent dosimeters. The unit tested utilizes collimators of nominal diameters of 4, 8, 14, and 18 mm. Radiochromic film samples from a Leksell Model U Gamma Knife were evaluated by the same laboratory and are presented for comparison. The treatment planning system was not evaluated. RESULTS: Radiation-absorbed dose rates and profiles measured for this unit are comparable to those previously measured with the same techniques for the Leksell Model U Gamma Knife units in San Diego and Atlanta. CONCLUSION: This unit is capable of producing well collimated beams of high energy photons, suitable for stereotactic radiosurgery. It has similar physical characteristics to those previously reported for the Leksell Model U Gamma Knife unit.

The open Gamma Knife Center concept.

A freestanding Gamma Knife radiosurgery center has been open for just over 1 year in La Jolla, Calif., USA, located in the grounds of a medium-sized community hospital. The center employs a full-time medical physicist and neurosurgical nurse together with clerical personnel. The neurosurgeons and radiation oncologists are drawn from the entire community and bill their fees separately. Written protocols for each indication to be treated govern patient eligibility and suggested treatment algorithms. Additionally, each patient to be treated is presented in conference, and a treatment strategy (i.e., radiosurgery, craniotomy, conventional radiation therapy) is decided. Twenty new physicians and physicists have begun training to use the Gamma Knife in the first year. Certification must be achieved before a clinician can be physician of record.

Hodgkin's disease following successful treatment of gastric teratoma in a neonatal female.

Teratomas of gastric origin are rare. We are aware of 90 such tumors having previously been reported, all of which have been benign and only three of which have arisen in females. We report a gastric teratoma that presented as an exophytic gastric tumor in an 8-week-old female, the fourth such recorded tumor in a female neonate. Complete surgical excision was followed by clinical and biochemical surveillance without adjuvant treatment. At 1 year the patient was well without evidence of residual or recurrent disease. The child developed Hodgkin's disease 3 years later, and we believe this to be the first recorded association of these two tumors.

A new re-entrant ionization chamber for the calibration of iridium-192 high dose rate sources.

A re-entrant (well-type) ionization chamber has been designed and fabricated at the University of Wisconsin for use with iridium-192 high dose-rate (HDR) remote after-loading brachytherapy devices. The chamber was designed to provide an ionization current of about 10(-8) ampere with a nominal 10 curie iridium-192 source. A narrow opening is provided into the sensitive volume of the chamber to insert a Nucletron MicroSelectron catheter, or catheters with similar diameters from other HDR manufacturers. The chamber exhibits a flat response (+/- 0.1%) for any source position within +/-4 mm of the chamber center. A 300 volt chamber bias yields a 99.96% ion collection efficiency. The chamber is capable of being calibrated directly with an iridium-192 source which has in turn been calibrated with thimble-type ion chambers. Reproducibility for readings in the current mode for 10 consecutive insertions of the MicroSelectron iridium-192 HDR source is within 0.02% or less. Two thimble chambers calibrated by the U.S. National Institute of Standards and Technology provide calibration traceability of iridium-192 HDR sources and re-entrant chambers to a primary national standards laboratory. Results of activity measurements of 6 commercial iridium-192 HDR sources are reported.

Calibration of 192Ir high-dose-rate afterloading systems.

A method is described for calibration of 192Ir high-dose-rate (HDR) brachytherapy afterloading systems. Since NIST does not offer calibration of ionization chambers with the gamma-ray spectrum of iridium-192, an interpolation procedure is employed, using calibrations above (137Cs, 662 keV) and below (250 kVcp, 146-keV x rays) the exposure-weighted average 192Ir energy of 397 keV. The same total wall + cap thickness must be used for both calibrations, and for the 192Ir measurements. A wall + cap thickness of 0.3 g/cm2 is recommended to assure charged particle equilibrium and to exclude secondary electrons emitted from the source encapsulation. Procedures are described for determining the corrections for source-chamber distance and room scatter during the source calibration in inverse-square-law geometry. A new well-type ionization chamber has been designed specifically for convenient routine use with the HDR afterloading system. It can be calibrated by means of a previously calibrated 192Ir source, and offers a simple means for verifying the decay rate and for calibrating 192Ir replacement sources.

Surface dose rate calibration of Sr-90 plane ophthalmic applicators.

Calibration of an imported strontium-90 ophthalmic applicator at the U.S. National Bureau of Standards (now the National Institute of Standards and Technology) has disclose a significant discrepancy in dose rate calibration (32%-35%) with that quoted by the manufacturer. The University of Wisconsin has investigated this discrepancy and found that both laboratories use similar techniques and a version of the Bragg-Gray equation to yield dose rate estimates. Experimental results indicate a strong relationship between the size of the collecting electrode used in the extrapolation chamber and the resulting estimate of absorbed dose rate. Calibration of these applicators is reviewed and suggestions for improvement and further research are proposed.

Evaluation of a commercial diode monitor for mailed quality control of therapy units.

A commercial diode dosimeter was evaluated for use in a mailed radiotherapy quality control package. The quality control package contained the commercial diode dosimeter and four thermoluminescent dosimeters to measure the given dose, and it also contained a photographic film to measure the light/radiation field coincidence. The film also served as a backup dosimeter. Twenty small Midwestern hospitals participated in field measurements; approximately half of the hospitals had a consulting physicist and the others had one full-time physicist. Each hospital was asked to deliver 1.5 Gy (150 rads) to the device, which was returned to our laboratory for evaluation. Reliable results were obtained from three measurements at each hospital at about 3-month intervals. The diode dosimeter was more precise than thermoluminescent dosimeters or film.

Calibration and use of the Wisconsin kVp test cassette.

The Ardran-Crooks kVp test cassette is widely used in diagnostic radiology to provide a rapid, simple, noninvasive measurement of x-ray tube potential. A modified version of this cassette called the Wisconsin kVp Test Cassette was introduced commercially in the U. S. in 1972. Since then, the method of calibration of these cassettes has changed significantly. Wisconsin kVp Test Cassettes calibrated by the manufacturer prior to August 1982 may yield underestimates of kVp measurements, particularly when using the 90-110 and 110-130 kVp regions with single-phase units. In August 1982 significant improvements in the calibration methods were implemented. The resultant change in calibration is demonstrated by data from the Centers for Radiological Physics. Present calibration methods are believed to be accurate within the greater of +/- 2 kVp or 2% of actual peak tube potential. Proper use of the cassette is necessary to achieve this level of accuracy.

A method for matching NBS x-ray beam qualities with a half- or full-wave rectified generator.

A method has been devised to enable users of half- and full-wave rectified x-ray sets to match the revised National Bureau of Standards (NBS) x-ray beam qualities obtained with a constant potential generator. The method provides a rapid technique for determining the added filter necessary to attain the same first half-value layers and nearly the same homogeneity coefficients as NBS. Beam qualities from 100 to 250 kVcp have been duplicated and used to perform blind calibrations at four beam qualities on each of two ion chambers owned by the National Bureau of Standards.