Transperineal 125iodine implantation for treatment of clinically localized prostate cancer: 5-year tumor control and morbidity.

PURPOSE: To evaluate the efficacy and toxicity of transperineal 125I implants for clinically localized prostate cancer in elderly men in a community cancer setting. METHODS AND MATERIALS: From 1988 to 1993, 206 patients, median age 77 years, with localized (Stage T1 and T2), low-grade (Gleason score < or = 7) prostate cancer were treated using pre-planned 125I transperineal implants. Patients were followed for biochemical freedom from disease, overall survival, and treatment-associated morbidity. RESULTS: The 5-year actuarial biochemical freedom from failure rate for all patients available for follow-up was 63%. Specifically, biochemical freedom from failure was 76% in patients with pretreatment PSA < or = 10 ng/ml, compared to 51% of patients with values > 10 ng/ml (median observation time 35 months). Actuarial freedom from failure for patients with PSA < or = 4 ng/ml was 84%. Stage and Gleason score did not predict outcome. PSA nadir was the strongest predictor of long-term biochemical disease-free survival (p < 0.001) with only 2 failures in 62 patients who achieved a posttreatment PSA nadir < or = 0.5 ng/ml. CONCLUSION: Transperineal 125I implants for early prostate cancer are efficacious and feasible for certain populations of elderly patients with favorable prognostic indicators in the community cancer setting. Patients with poor prognostic indicators at diagnosis do not appear to be candidates for treatment with implant alone. ( 1999 El.vit r 'Cio;noo lnc

Electron beam central axis depth dose measurements.

Central axis depth dose measurements were made by the Radiological Physics Center on over 70 electron-producing machines used in radiation therapy. These data were consistent for each machine model and nominal energy. However, the data show that depth dose relations can vary significantly among different machine models for electron beams having the same nominal energy. Analysis shows that both the method used to achieve beam flatness and the mean incident electron energy determine the central axis depth dose curve past the depth of maximum dose. A linear relation of depth dose versus mean incident electron energy is used to predict depth dose to within 2 mm for most electron beams used clinically at depths greater than d95.

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 comparison of high-energy accelerator depth dose data.

Accurate depth dose information is necessary for the use of high-energy radiotherapy photon beam units. It would be useful, therefore, to have one set of published data available for each different type unit manufactured to which physicists can compare their measured data. Pertinent questions are raised regarding the similarity between accelerators and their central axis depth dose characteristics, the availability of adequate published central axis depth dose data, and the minimum amount of data needed to determine the applicability of published data to a particular machine. Data taken by the Radiological Physics Center (RPC) for 4-10 MV units are analyzed and compared with published data in an attempt to answer these questions.

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.

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.

A review of the reliability of chamber factors used clinically in the United States (1968--1976).

One of the principal concerns of a physicist responsible for calibrating megavoltage radiotherapy equipment is the validity and stability of the 60Co exposure correction factor assigned to his ionization-chamber and electrometer system. It is the practice of the AAPM Radiological Physics Center (RPC) to perform an intercomparison between the RPC chamber and electrometer system and the chamber and electrometer in use at each of the various institutions visited by the RPC. The results of 202 such intercomparisons are reviewed to determine (1) the consistency in the assignment of exposure correction factors by a calibrating agency with itself and with other calibrating agencies, and (2) the dependence of the reliability of the exposure correction factors upon the type of field instrument and the time since calibration.

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.