Fred G. Abrath

Dosimetry of CT-guided volumetric IR-192 brain implant

Over the past 2 years, an afterloading technique has been developed and refined to implant radioactive Ir-192 sources into brain tumors. The implantation procedure integrates a stereotaxic system with computerized tomography (CT), which provides tumor position, volume, and guides the placement of catheters. A radiolucent ring-frame immobilizes the head as holes are made at 1 cm intervals with the aid of a template. Catheters containing dummy sources 1 cm apart are then inserted to the desired depth, and their position verified in three dimensions to insure complete coverage of visible tumor volume as defined by contrast enhancement. Once catheters are secured, the anesthetized patient is moved to the intensive care unit where the dummy sources are replaced by ribbons of Ir-192 seeds (specific activity 0.6-1.0 mg Ra eq). CT scans with the dummy sources in place are used to designate spatial coordinates of the active sources. A computer program converts position data and source strength into isodose contours in any plane. The implant duration (70-100 hours) for the desired dose to the tumor periphery (60-120 Gy) is then calculated. Dose rate contours are superimposed on preimplant CT scans. Maximum and minimum doses are determined in each of the various planes. Verification dosimetry has been carried out with thermoluminescent dosimeters placed in a catheter located in a plane along the tumor periphery. In vivo isodose values compared to idealized plans agree within +/-5%-10%.

Variation in output factor caused by secondary blocking for 7-16 MeV electron beams.

The effect of secondary blocking on the output for 7-16 MeV electron beams is investigated for the two most widely used methods of field shaping, i.e.,(a) shield on surface, and (b) shield at end of cone with subsequent air gap. For those two methods differences in output on the order of 10% are observed. These effects are explained in terms of the blocking geometry and its influence on scattered radiation.

Effects of sequencing of the total course of combined hyperthermia and radiation on the RIF-1 murine tumor.

The optimal sequence for clinical utilization of combined radiotherapy and hyperthermia is not known. The clinical trials have resulted in similar responses whether hyperthermia is given before or after radiation. Moreover, studies addressing the best sequence for an entire course of multifractionated hyperthermia and radiation are lacking. In these experiments, the importance of sequencing of heat and irradiation in a multifractionated treatment regimen in RIF‐1 murine tumors was studied. It was observed that a close sequence of heat and irradiation is more beneficial than separate cytotoxic action. When heat and irradiation were given simultaneously, (within 1 hour) 67% to 75% of the tumors were cured. Heat and irradiation given sequentially (the entire course of one following the entire course of the other, each separated by 72 hours) cured 20% of the tumors. No tumors were cured when treated with heat or irradiation alone. The tumor regrowth time (mean tumor doubling time) is much longer in simultaneous treatment than in sequential treatment. It appears that heating first decreases the effectiveness of subsequent irradiation, causing a shorter growth delay than the opposite sequence. Heat alone does not alter the tumor bed permanently, but irradiation seems to do so, resulting in a slower rate of growth upon recurrence.

Attenuation of Primary and Scatter Radiation in Concrete and Steel for 18 Mv X-rays from a Clinac-20 Linear Accelerator

With the increasing number of high-energy linear accelerators being installed in radiation-therapy facilities, an increasing need exists for carefully measured data concerning the shielding parameters of photon radiation in the energy range above 10 MeV. In this study, the 18 MV X-ray beam from a Varian Clinac 20 linear accelerator was employed. Transmission parameters were measured for the primary X-ray beam in concrete and steel. The tenth value layer for steel was 11.3 +/- 0.6 cm and for concrete 45.0 +/- 2.0 cm. Also measured were the transmission parameters for scattered radiation vs angle in concrete and steel.

25 MV X-ray beam characteristics from a 35 MeV linear accelerator

Abstract This manuscript presents characteristic dosimetry measurements for the 25 MV X-ray beam from the Varian Associates Clinac ® 35 linear accelerator. The quality of the beam is expressed by its attenuation as a narrow beam in lead. Percentage depth dose tables are given as a function of field size for square fields. Tissue-phantom ratios are tabulated for square field sizes. A study of the variation of the dose in the buildup region demonstrates that surface dose and the depth of maximum dose is a strong function of the field size. Other beam parameters measured and discussed include field flatness and symmetry, penumbra, and applicability of inverse square law.

Study of the characteristics of iridium-192 wire used in interstitial implants.

Poor cosmetic results have ocurred in 14% of the skin-cancer patients who underwent treatment with implants of iridium-192 wires that had been used more than twice. Physical characteristics of the iridium wire and the effects of handling on that wire have been studied. Heterogeneities in the core were found, and an increase in ..beta.. radiation was observed when the wire was manipulated. Both factors could explain the poor results in skin-cancer treatment that are of particular concern in facial cancers.