Permanent prostate seed implant brachytherapy: report of the American Association of Physicists in Medicine Task Group No. 64.

There is now considerable evidence to suggest that technical innovations, 3D image-based planning, template guidance, computerized dosimetry analysis and improved quality assurance practice have converged in synergy in modern prostate brachytherapy, which promise to lead to increased tumor control and decreased toxicity. A substantial part of the medical physicist's contribution to this multi-disciplinary modality has a direct impact on the factors that may singly or jointly determine the treatment outcome. It is therefore of paramount importance for the medical physics community to establish a uniform standard of practice for prostate brachytherapy physics, so that the therapeutic potential of the modality can be maximally and consistently realized in the wider healthcare community. A recent survey in the U.S. for prostate brachytherapy revealed alarming variance in the pattern of practice in physics and dosimetry, particularly in regard to dose calculation, seed assay and time/method of postimplant imaging. Because of the large number of start-up programs at this time, it is essential that the roles and responsibilities of the medical physicist be clearly defined, consistent with the pivotal nature of the clinical physics component in assuring the ultimate success of prostate brachytherapy. It was against this background that the Radiation Therapy Committee of the American Association of Physicists in Medicine formed Task Group No. 64, which was charged (1) to review the current techniques in prostate seed implant brachytherapy, (2) to summarize the present knowledge in treatment planning, dose specification and reporting, (3) to recommend practical guidelines for the clinical medical physicist, and (4) to identify issues for future investigation.

103Pd loaded cartridge air kerma strength verification.

Using radioactive 125I or 103Pd seeds in large number to treat prostate cancer has become popular. This procedure's popularity translates to increased physicist and dosimetrist workloads with regard to performing source strength assays. In addition, radiation exposure to the physicists' and dosimetrists' hands as a result of source handling has increased significantly. As a result of these increases, a method of source calibration using applicator cartridges was developed to allow safe handling of sources and expedited source assays. The method uses a holder which allows a cartridge loaded with five to fifteen seeds to be placed in a standard well ionization chamber. Ionization readings are converted to air kerma source strengths which are compared with the manufacturer's stated activity. Excellent agreement between individual seed air kerma strength as well as comparison with the vendor's air kerma strength statement proves the validity of this assay technique. Absorption/geometry correction factors were experimentally derived during the development of this method. These factors allow accurate determination of air kerma source strength from a shipment of Iodine or Palladium seeds.

Personnel exposure rates during simulated biopsies with a real-time CT scanner.

RATIONALE AND OBJECTIVES: Real-time computed tomography (CT) has the potential to expedite and improve CT-guided needle biopsies by allowing cross-sectional images to be viewed in real time as a needle is advanced toward the target lesion. A major concern about this procedure is the scattered and leaked radiation to which the operator is exposed. This study was undertaken to determine the exposure rates around a CT scanner during CT-guided needle biopsies and to identify the areas of greatest personnel exposure. MATERIALS AND METHODS: Pig and human cadavers were used to simulate patients undergoing a CT-guided needle biopsy. Various anatomic biopsy sites were used. The radiologist's exposure was assessed by timing the procedure and measuring the exposure rates around the CT scanner with an ionization-chamber survey meter. Ion-chamber measurements multiplied by the time the radiologist spent performing several mock biopsies were compared with film dosimeter results. Doses to the hands, wrists, and whole body were measured with ring, wrist, body, and collar film dosimeters. RESULTS: The average time required to perform a single biopsy was about 1 minute. The dose to the radiologist performing the simulated biopsies was calculated to be 123 mR, 68 mR, 14 mR, and less than 0.5 mR to the fingers, wrist, collar, and body, respectively, as calculated from ionization-chamber and time measurements. These exposure rates correlate well with the film dosimeter readings accumulated during the mock procedures. CONCLUSION: The dose received by the radiologist performing a CT-guided biopsy was comparable to that of other interventional procedures. In addition, operating from the head of the machine (ie, distal to the bed) appeared to markedly reduce personnel exposure, due to the shielding in the gantry of the CT scanner used in the study.

Tumor perfusion studies using fast magnetic resonance imaging technique in advanced cervical cancer: a new noninvasive predictive assay.

PURPOSE: This study investigated sequential changes in tumor blood supply using magnetic resonance (MR) perfusion imaging and assessed their significance in the prediction of outcome of patients with advanced cervical cancer. The purpose of this project was to devise a simple, noninvasive method to predict early signs of treatment failure in advanced cervical cancer treated with conventional radiation therapy. METHODS AND MATERIALS: Sixty-eight MR perfusion studies were performed prospectively in 17 patients with squamous carcinomas (14) and adenocarcinomas (3) of the cervix, Stages bulky IB (1), IIB (5), IIIA (1), IIIB (8), and IVA (1), and recurrent (1). Four sequential studies were obtained in each patient: immediately before radiation therapy (pretherapy), after a dose of 20-22 Gy/ approximately 2 weeks (early therapy), after a dose of 40-45 Gy/ approximately 4-5 weeks (midtherapy), and 4-6 weeks after completion of therapy (follow-up). Perfusion imaging of the tumor was obtained at 3-s intervals in the sagittal plane. A bolus of 0.1 mmol/kg of MR contrast material (gadoteridol) was injected intravenously 30 s after beginning image acquisition at a rate of 9 ml/s using a power injector. Time/signal-intensity curves to reflect the onset, slope, and relative signal intensity (rSI) of contrast enhancement in the tumor region were generated. Median follow-up was 8 months (range 3-18 months). RESULTS: Tumors with a higher tissue perfusion (rSI > or = 2.8) in the pretherapy and early therapy (20-22 Gy) studies had a lower incidence of local recurrence than those with a rSI of < 2.8, but this was not statistically significant (13% vs. 67%; p = 0.05). An increase in tumor perfusion early during therapy (20-22 Gy), particularly to an rSI of > or = 2.8, was the strongest predictor of local recurrence (0% vs. 78%; p = 0.002). However, pelvic examination during early therapy (20-22 Gy) commonly showed no appreciable tumor regression. The slope of the time/signal-intensity curve obtained before and during radiation therapy also correlated with local recurrence. Follow-up perfusion studies did not provide information to predict recurrence. CONCLUSION: These preliminary results suggest that two simple MR perfusion studies before and early in therapy can offer important information on treatment outcome within the first 2 weeks of radiation therapy before response is evident by clinical examination. High tumor perfusion before therapy and increasing or persistent high perfusion early during the course of therapy appear to be favorable signs. High perfusion suggests a high blood and oxygen supply to the tumor. The increase in tumor perfusion seen in some patients early during radiation therapy suggests improved oxygenation of previously hypoxic cells following early cell kill. Radiation therapy is more effective in eradicating these tumors, resulting in improved local control. Our technique may be helpful in identifying early-while more aggressive therapy can still be implemented-those patients who respond poorly to conventional radiation therapy.

Dose behind various immobilization and beam-modifying devices.

PURPOSE: To quantify the degradation of skin sparing associated with using beam modifiers such as compensators, immobilization devices, and custom blocks for high energy photon beams. METHODS AND MATERIALS: The degradation of skin sparing was quantified by measuring dose build-up curves with an extrapolation chamber for 6 and 15 MV photon beams. Uniform thickness compensators made of gypsum and lead, thermoplastic mask material, immobilization cradle foam, and cerrobend custom blocks were placed in geometries that mimic relevant clinical situations. RESULTS: Compensators, whether made of gypsum or lead, placed in the linear accelerator's wedge slot did not significantly effect the depth dose curve's build-up region. Immobilization devices such as cradle foam or thermoplastic placed in contact with the patient degrade the skin sparing expected from high energy photon beams proportional to their thickness and density. Measurements behind custom blocks show that surface and near surface doses for a blocked field are best described by build-up curves for an equivalent size open field. CONCLUSIONS: These results allow explanation and possibly prediction of skin reactions on patients in which compensators, foam immobilization cradles, thermoplastic masks, or custom blocks are used. These results also provide a baseline by which substitute materials can be evaluated.

Total scalp treatment of mycosis fungoides: the 4 x 4 technique.

PURPOSE: A technique for treating mycosis fungoides confined to the scalp using a method known as the 4 x 4 technique is presented. METHODS AND MATERIALS: Uniform dose distribution on the scalp and acceptable "hot spots" along five match lines is accomplished by using four sets of four fields (i.e., 4 x 4) on the patient. Precise and reproducible patient and field alignment was accomplished with a solid thermoplastic mask, which is the surface on which match lines are drawn. In-vivo dosimetry (thermoluminescent dosimeters and film) are easily attached to the mask which also provides a portion of the 7 mm bolus used to shift the characteristic 6 MeV electron depth dose toward the skin surface. RESULTS: In-vivo dosimetry demonstrated that single fraction match line doses are within 25% to 30% of central axis dose. Shifting these match lines to four locations reduces these "hot spots" to satisfactory levels (less than 120%). Three patients have been treated with this technique and each patient continues to have a complete clinical response at 14 to 21 months post treatment. In addition, each patient has excellent cosmetic results with no evidence of acute or chronic side effects at the match lines. CONCLUSION: The 4 x 4 technique has proven to be useful in the treatment of mycosis fungoides confined to the scalp.

A policy for radiotherapy in patients with implanted pacemakers.

As the prevalence of implanted pacemakers increases, the likelihood that cancer requiring radiotherapy will develop in pacemaker-bearing patients increases proportionately. A management policy for pacemaker-dependent patients who require radiotherapy helps assure that they will be safely and efficiently treated with external beam radiation. Guidelines for external beam treatment have been set forth in the ASTRO Newsletter. These guidelines are the basis of the specific policy that was developed and implemented. This policy (1) requires a cardiology consultation and discussion with the manufacturer before treatment, (2) prohibits the use of a betatron, (3) suggests that attempts be made to limit the dose received by the pacemaker to 2-Gy, (4) requires that pacemaker dose be calculated, measured, and recorded in the patient chart, (5) requires cardiac monitoring in accordance with the degree of patient pacemaker dependence, and (6) requires a follow-up cardiology consultation after the patient's final treatment. Adherence to this policy is assured by use of a worksheet that is included in each pacemaker patient's chart.

Determination of build-up region over-response corrections for a Markus-type chamber.

Over-response corrections for a widely used parallel plate ionization chamber were determined using contemporaneous measurement of build up for 4, 6, 10 and 18-MV photon beams utilizing a commercially available extrapolation chamber (PTW model 23392). The resultant over-response corrections were essentially independent of field size (5 X 5 to 30 X 30 cm) and were less for increased depth (from the surface) and higher x-ray energy. The over-response of the parallel plate chamber (Markus-type, PTW model 329) was 13.8, 10.7, 6.2 and 4.7 percent (absolute) at the surface for 4, 6, 10 and 18-MV x-ray energies. At only 2 mm depth, the over-response of the Markus chamber under investigation decreased to 50% of surface value over-response. These corrections, so derived, are applied to Markus parallel plate chamber build-up measurements by simple subtraction of the derived corrections. For example, a 6-MV percentage depth dose of 37.5% measured at the surface with the Markus chamber is reduced by 10.7% so to agree with the surface dose indicated by the PTW extrapolation chamber for this x-ray field, that is, 26.8%.

Acceptance testing of an automated scanning water phantom.

The setup of an automated scanning water phantom must be concluded with a methodical functional acceptance test. This test must include mechanical, radiological, and calculational facets. Only after an acceptance test is successfully completed can data collected with a new or repaired system be trusted. An acceptance test outline was written and applied to a newly purchased Wellhofer scanning system. This testing procedure and its results are presented in this paper.

A needed modification to vendor supplied oversized 45 degree wedges.

A recent change in the source-to-blocking tray distance on at least one new linear accelerator restricts simultaneous use of the vendor supplied oversized (i.e., 20 cm) 45 degree wedge and 7.5 cm or higher custom cerrobend blocks. Truncating the "heel" end of the vendor supplied steel wedge as well as substituting a 45 degree lead wedge (designed and supplied by the vendor) allows simultaneous wedge and 7.5 cm high block usage. Central axis depth dose, transverse dose profile, wedge transmission factor, and surface dose measurements were made for each wedge for 6 through 18 MV x-rays. Dosimetric data for the truncated steel wedge is insignificantly different from that of the untruncated steel wedge, whereas the lead substitute wedge dosimetric data is substantially different and requires remeasurement.

A quality assurance program for monitor unit calculators.

A quality assurance protocol has been developed to assess the relative accuracy of computerized monitor unit calculations. Results from this testing reveal that errors (defined as the difference between the computer results and those obtained using a standard formula with manual calculation) in monitor unit calculators are possible when they are tested over a wide range of clinically relevant field sizes (including blocked fields), source-to-surface distances, and depths. It is suggested that computerized monitor unit calculators be checked against hand calculations and that this be permanently documented both at the time of initial implementation and following any subsequent changes in the program.

Radionuclide production and partial radiochemical characterization following fast neutron irradiation of mouse spleens.

When radiotherapy patients are irradiated with fast neutron beams (energies greater than 20 MeV), positron emitting radionuclides (15O, 13N, 11C) are created in their tissues. Capillary blood flow can be determined in the irradiated tissue by measuring the washout of the 15O. The chemical form of these positron emitting nuclides is important when assumptions about their transport in the development of a blood flow model are being made. In the present work, normal mouse spleen tissue was used as a model system for these studies. The mouse spleen was activated by whole mouse irradiation using the M.D. Anderson Hospital neutron beam produced by 42 MeV protons impinging on a beryllium target. Results of cellular studies and large molecule precipitation measurements (1) show that at least 65% of 15O created in situ in mouse spleen is capable of being transported out of the spleen by the blood supply, (2) suggest that 13N may be 100% biologically mobile, and (3) indicate that 11C appears to be evenly divided between a population associated with small biologically mobile molecules and a population associated with larger, biologically trapped or immobile molecules.

15O and 11C production in neutron radiotherapy patients.

In order to establish the feasibility of performing blood flow measurements following therapeutic neutron irradiation by determining increased 15O disappearance rate from a volume of tissue, knowledge of the probabilities of 15O, 11C and 13N production in tissue by a p(42)Be neutron-beam irradiation is required. Isotope production probability per unit dose (defined as the isotope-yield coefficient) is determined from the measured production of 15O, 13N and 11C in irradiated H2O. Measured isotope-yield coefficients average 1.20 x 10(-16), 2.85 x 10(-18) and 5.56 x 10(-18) Gy-1 per 16O nucleus, respectively, at depths between 1 and 15 cm in a water phantom. The isotope yield coefficients for 11C production from 12C (polyethylene) average 1.04 x 10(-16) Gy-1 per 12C nucleus at depths between 1 and 15 cm in a water phantom. From these yield coefficients, the creation of 9700 Bq 15O and 450 Bq 11C per cubic centimetre of human soft tissue (chemical composition of soft tissue of Reference Man) is calculated assuming a 0.65 Gy neutron irradiation given at a dose rate of 40 cGy min-1 and assuming no perfusion of the activity. These results agree with those calculated using published neutron cross sections and neutron energy spectra, indicating that (n, 2n) reactions are the predominant activation mechanisms.

Dosimetric evaluation of a pencil-beam algorithm for electrons employing a two-dimensional heterogeneity correction.

The accuracy of a pencil-beam algorithm for electrons employing a two-dimensional heterogeneity correction is demonstrated by comparing calculation with measurement. Ionization measurements have been made in a water phantom for a variety of non-standard geometries. Geometries to demonstrate the effect of an extended treatment distance, a sloping skin surface, and an irregular skin surface have been selected. Additionally, thermoluminescent dosimeters have been used to measure distributions in tissue-substitute phantoms, which were designed from individual patient computerized tomographic scans. Three patient scans have been selected: (1) diffuse hystiocytic lymphoma of the left buccal mucosa and retromolar trigone; (2) squamous cell carcinoma of the nose at the columnella ; and (3) carcinoma of the maxillary antrum. Results demonstrate the algorithm's ability to simultaneously account for the isodose shifting as a result of internal heterogeneities and for sidescatter non-equilibrium caused by lateral discontinuities of the skin surface and internal anatomy. The algorithm is shown to generally be accurate to within +/- 4% in the treatment volume or +/- 4 mm in regions of sharp dose gradients as found in the penumbra and distal edge of the beam. Examples of greater disagreement are shown and their physical interpretation discussed.