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Purpose: Describe the dosimetry of an episcleral brachytherapy device. Methods: The SMD‐I device is designed to treat exudative age‐related macular degeneration (AMD) and employs a Sr‐90/Y‐90 source encapsulated in a stainless steel cylinder. The source is welded to a flexible wire allowing it to travel from a shielded vault in the SMD‐I handle to the distal end of a curved cannula to deliver a therapeuticdose of radiation through the sclera to the neovascular target in the subchoroidal space. The SMD‐I handle and vault are comprised of Ultem, a lightweight radiation tolerant plastic, which shields the surgeon. Dose calculations were performed using the MCNPXradiation transport code. The absolute dose rate was determined using radiochromic film (GAFChromatic© MD‐55) at a point in solid water 2.0mm from the source center perpendicular to the cannula. Dose rates at several depths were measured using Kodak EDR2 film in water equivalent phantoms to compare with the absolute dose rate measurement and MCNPX calculations. The surgeon's hand dose received while manipulating the device with the source in the vault was measured using standard TL (thermoluminescence) finger ring dosimeters, TL ChipstratesTM, and calculated with MCNPX.Results: The absolute dose rate 2.0mm from the source center is 0.45 Gy/min/mCi. The EDR2 film results agree with the absolute dose measurement and the MCNPX calculations. The dose rate decreases rapidly with depth so that the dose at the target depth (3mm) is approximately 8 times less than at 1mm depth (sclera). The dose distribution is sensitive to the angle between the cannula and the neovascular plane. Both TL methods yield a maximum dose rate of 6 μSv/min mCi to the surgeon's fingers consistent with the MCNPX calculation. Conclusions: The SMD‐I device permits accurate delivery of a therapeuticradiationdose for the treatment of exudative AMD. Russell J. Hamilton is a founder and currently serves on the Scientific Advisory Board of Salutaris Medical Devices, Inc. Wendell Lutz and Thomas Cetas serve on the Scientific Advisory Board of Salutaris Medical Devices, Inc. All authors have received financial support from Salutaris Medical Devices, Inc.

Purpose:Although there is increasing interest in radiosurgery, little quantitative data regarding current patterns of radiosurgery practice are available. We developed a radiosurgery questionnaire to obtain information on radiosurgery practice. Methods and Materials: We distributed the questionnaire to the entire membership of the American Society of Therapeutic Radiology and Oncology in early 1993. Responses were obtained from 74 facilities that practice radiosurgery, corresponding to over 6000 treatments carried out since 1983 by 135 radiation oncologists and 130 physicists. Results: Most respondents were found to work within a multidisciplinary team, consisting of the following specialists (Average hours devoted per patient on day of treatment in parentheses): radiation oncologist (3.8), neurosurgeon (3.2), physicist (6.1), radiologist (0.7), nurse (2.7), other (3.0). On average, neurosurgeons and nurses who perform Gamma Knife radiosurgery devote significantly more time-per-patient on the day of treatment than their peers who perform linac radiosurgery. On average, less experienced radiation oncologists and physicists (≤ 24 months experience, or ≤ 50 patients treated) devote significantly more time-per-patient on the day of treatment than their more experienced peers. Although there are many more linac radiosurgery facilities than Gamma Knife facilities, on average the number of patients treated per month per facility is significantly larger at the latter. On average, follow-up responsibilities are nearly equally shared by radiation oncologists and neurosurgeons, except at Gamma Knife facilities, where neurosurgeons assume a larger percentage of follow-up responsibility. The percentages of patients treated at linac facilities for metastases or primary CNS malignancy are larger than the corresponding percentages at Gamma Knife facilities; the opposite is true for arteriovenous malformation, acoustic neuroma, and meningioma. Conclusion: Current radiosurgery practice usually involves a team approach, with participation of specialists from radiation oncology, neurosurgery, physics, radiology, and nursing. The average number of M.D. and Ph.D. hours required per treatment on the day of radiosurgery is high.

The influence of tissue and applicator heterogeneities on brachytherapydose distributions is not well understood, despite widespread use of shielded applicators in intracavitary therapy. Heterogeneity correction factors (HCF) have been measured using a silicon diode detector arising from bounded heterogeneities consisting of lead, steel, titanium,silver,aluminum, and air cylinders near brachytherapysources of 125I, 137Cs, and 192Ir. In addition, transverse‐axis dose distributions for the three sources in homogeneous water were measured for distances of 0.2 to 16.0 cm. For each point of measurement, relative diode readings were simulated by a Monte Carlophoton transport code utilizing accurate models of the source internal structure, the experimental measure geometry and the source‐strength calibration geometry. Comparison of measured and calculated HCF’s reveals excellent agreement (1%–3% average) over a wide range of materials, diameters, and thicknesses. In addition, Monte Carlo simulation not only accurately reproduced the relative transverse‐axis dose distributions in homogeneous medium, but was able to predict the variation of diode response with photon energy with an accuracy of 3% over the range of 30–662 keV. Our measurements demonstrate that HCF’s vary by as much as 60%–100% with distance and heterogeneity diameter for a fixed thickness. Finally, silicon diode measurements of HCF (defined as reading with heterogeneity/reading in homogeneous medium) is shown to lead to errors of 5%–30% for 137Cs and 192Ir sources in the presence of high‐atomic number shielding materials. This paper concludes, that Monte Carlo simulation is a powerful, convenient and accurate tool for investigating the long‐neglected area of brachytherapy heterogeneity corrections.

A Phase I clinical trial has been initiated to determine the feasibility, tolerance, and toxicity of interstitial thermoradiotherapy in the treatment of high-grade supratentorial brain gliomas. Hyperthermia was delivered by means of thermally-regulating ferromagnetic implants afterloaded into stereotactically placed plastic catheters. Heat treatments were given immediately before interstitial irradiation; in addition, five patients received a second heat treatment at the completion of brachytherapy. The desired target temperature for the 60-minute hyperthermia session was between 42 degrees C and 45 degrees C. Following hyperthermia, the catheters were afterloaded with Ir-192, which delivered a variable radiation dose of 14-50 Gy depending on the clinical situation. Interstitial irradiation was supplemented with external beam radiotherapy (40-41.4 Gy) in patients with previously untreated tumors. A total of 14 patients (4 males, 10 females) have been treated to date on this protocol. Eleven of the patients had a diagnosis of glioblastoma multiforme, whereas three had anaplastic astrocytoma. The mean implant volume was 61.5 cm3 (range: 9-119 cm3); the median number of interstitial treatment catheters implanted was 19 (range: 7-33). Continuous temperature monitoring was performed by means of multisensor thermocouple probes inserted in the center as well as in the periphery of the tumor. Of the 175 monitored intratumoral points, 83 (47%) had time-averaged mean temperatures of greater than 42 degrees C, and only 12 sensors (7%) exceeded a temperature of 45 degrees C. Among the 19 heat treatments attempted, there have been four minor acute toxicities, all of which resolved with conservative medical management and one major complication resulting in the demise of a patient. These preliminary results indicate that ferromagnetic implants offer a promising new approach to treating brain tumors with hyperthermia.

Because of the proximity of the spinal cord, effective radiotherapy of paraspinal tumors to high doses requires highly conformal dose distributions, accurate patient setup, setup verification, and patient immobilization. An immobilization cradle has been designed to facilitate the rapid setup and radiation treatment of patients with paraspinal disease. For all treatments, patients were set up to within 2.5 mm of the design using an amorphous silicon portal imager. Setup reproducibility of the target using the cradle and associated clinical procedures was assessed by measuring the setup error prior to any correction. From 350 anterior/posterior images, and 303 lateral images, the standard deviations, as determined by the imaging procedure, were 1.3 m, 1.6 m, and 2.1 in the ant/post, right/left, and superior/inferior directions. Immobilization was assessed by measuring patient shifts between localization images taken before and after treatment. From 67 ant/post image pairs and 49 lateral image pairs, the standard deviations were found to be less than 1 mm in all directions. Careful patient positioning and immobilization has enabled us to develop a successful clinical program of high dose, conformal radiotherapy of paraspinal disease using a conventional Linac equipped with dynamic multileaf collimation and an amorphous silicon portal imager.

A small field irradiation technique to deliver high doses of single fraction photon radiation to small, precisely located volumes (0.5 to 8 cm3) within the brain has been developed. Our method uses a modified Brown-Roberts-Wells (BRS), CT-guided, stereotactic system and a 6 MV linear accelerator equipped with a special collimator (diameters of 12.5 mm to 30.0 mm projected to isocenter) located 23 cm from isocenter. Target localization via planar angiography has been added. Treatment consists of a series of arcing beams using both gantry and couch rotations. During treatment, the patient's head is immobilized independently of the radiotherapy couch and is precisely positioned without reference to room lasers or light field. A precise verification of alignment precedes each treatment. Extensive performance tests have shown that a target, localized by CT, can be irradiated with a positional accuracy of 2.4 mm in any direction with 95% confidence. If angiography is used for localization, the results are better. The dose 1.0 cm outside the target volume is less than 20% of the prescribed dose for a medium sized collimator.

A new system has been developed for stereotactically delivering prescribed high doses of radiation to precisely located volumes of approximately 0.6 to 10.0 ml within the brain. A Brown-Roberts-Wells stereotactic apparatus and a 6-MeV linear accelerator equipped with a special collimator (12.5 to 30 mm in diameter) have been adapted. The 20-mm collimator allows treatment of a nearly spherical volume of 2.1 ml. Outside the treatment field, the dosage declines to 80% of the dose prescribed for the periphery of the lesion over a distance of 1.8 mm and to 50% over the next 3.4 mm. Localization can be accomplished via computed tomography or cerebral angiography. Treatment is accomplished with an arcing beam of photon radiation with the turntable (couch) in each of four positions. The entire system has been extensively tested for accuracy in alignment and distribution of radiation. Errors have been measured for the alignment of the apparatus and for the process of localization. Safety of operation was emphasized throughout the design and testing phase. (Neurosurgery 22:454-464, 1988)

This refresher course will discuss primarily TBI treatment methods and dosimetry. Treatment methods are generally dictated by the constraints of existing equipment. Factors such as photon beam eriergy, field flatness, room geometry and available field sizes influence whether patients are to be treated AP-PA or laterally and with horizonal or vertical beams. These factors, along with the desired dose homogeneity, also determine whether compensation, bolus, and/or lung shielding are necessary. Several representative treatment approaches will be used to illustrate the interplay of these factors. Several dedicated TBI facilities exist and will be discussed briefly. Some thoughts will be presented on how an "inexpensive", dedicated facility could be built. Once a treatment technique has been determined, dosimetry data must be measured with a geometry closely approximating the treatment geometry. This generally involves measuring percent depth doses or tissue maximum ratios in a water tank of suitable size located at the treatment position. These measurements can then be related in an absolute way to a calibration procedure preferably at the treatment position but without the necessity of the water tank. In addition, such water tank measurements need to be related to realistic patient contours and sizes. If compensators, bolus or lung shields are used, careful measurements of their effects on the dose distribution are needed. In the end, a given treatment technique should be capable of delivering a reasonably accurate dose to some specific reference point in a patient and with known dose heterogeneities at other parts of the body. Current recommendations on absolute accuracy and maximum dose heterogeneity thrpughout the body will be discussed. Lastly, various approaches to in vivo measurements will be discussed. _