Abutment of adjacent intraoperative radiotherapy electron fields to treat large tumor volumes

Abutment of unmodified Intraoperative Radiotherapy (IORT) electron fields to irradiate large volumes can lead to dose inhomogeneities at the junction site of the matched fields. Although precise field matching is difficult to achieve in the IORT setting, we have fabricated special cones and dosimetrically evaluated their use. Based on these results, we have established guidelines for the routine use of multiple IORT fields that require field matching. This study examines the abutment of adjacent IORT electron fields for various cone sizes and beam energy combinations, using film dosimetry and a scanning densitometer. Results indicate that for abutting two adjacent 9 x 9 cm square cones, an optimum field separation at the depth of the 90% isodose line (d90) can be obtained as follows: For 6, 9, and 12-MeV electron beams, abut the inner wall of the second IORT cone to the inner wall of the first IORT cone. For 16- and 20-MeV electron beams, abut the inner wall of the second IORT cone to outer wall of the first IORT cone.

Measurement and estimation of organ Bremsstrahlung radiation dose

Bremsstrahlung radiation doses were measured in an anthropomorphic phantom using thermoluminescent dosimeters. A single source of 90Y (beta-ray range less than or equal to 1.0 cm) was inserted in the bladder region and dosimeters were placed at distances greater than or equal to 3 cm to preclude detection of decay betas. Doses were corrected so as to represent the case of no biologic clearance. By comparing dosimeter location with the standard MIRD human geometry, sample organ doses could be determined. Representative results were 432 +/- 76 mrad/mCi at 3 cm (bladder), 260 +/- 60 mrad/mCi (uterus), 71 +/- 4 mrad/mCi (lower large intestine), and 1.4 +/- 0.7 mrad/mCi (liver). An estimation method, based on absorbed fraction tables, gave organ doses that were within the errors of measurement for all tissues with the exception of the bladder site. We conclude that organ bremsstrahlung radiation doses are not negligible and that they can be estimated using an integration over both the brake and beta-ray spectra.

Changes in heating patterns of interstitial microwave antenna arrays at different insertion depths

The changes in heating patterns of interstitial microwave antennas at different insertion depths were investigated in a static phantom at 915 MHz. Antennas for the Clini-Therm Mark VI system were inserted 5-15 cm into muscle-equivalent material, through nylon catheters. The phantom was heated with arrays of antennas at 2 cm spacings for 60 s at 15 W per antenna. Midplane and transverse heating patterns were determined thermographically with the antennas inserted parallel or perpendicular to the split of the phantom. Hot spots, independent of heating near the junction plane, were observed in the midplane of the 2 x 2 and 2 x 4 arrays at 2.8 cm from the insertion end. The magnitudes of these hot spots were reduced by 40-45 per cent as insertion depth was increased from 7 to 10.5 cm. With insertion depths of more than 11.5 cm the hot spots gradually diminished and heating occurred mostly near the junction plane. The observed heating patterns were caused by changes in impedance of the antenna arrays at different insertion depths. The impedance mismatch had resulted in different wave propagation within the tissue material which produced different radiation patterns. During treatments, because heating varies with insertion depth, great care must be exercised in monitoring temperatures.

Intracatheter hyperthermia and iridium-192 radiotherapy in the treatment of bile duct carcinoma

We report a case of a patient with locally advanced bile duct carcinoma treated with 4500 cGy external beam radiotherapy, followed 3 weeks later by intracatheter 915 MHz microwave hyperthermia and radiotherapy delivered through a biliary U-tube placed at the time of surgery. Heating was to 43-45 degrees C for 1 hour followed immediately by intracatheter Iridium-192 seeds to deliver 5000 cGy over a 72 hour period. Prior to treatment, a thermal dosimetry study in phanton was conducted, using the same type of U-tube catheter tubing as in the patient. Orthogonal X rays of the patient's porta hepatis region were used to reconstruct the catheter geometry in the phantom. Proper insertion depth was determined thermographically to obtain maximum heating at the center of the tumor. The maximum SAR was 8.8 watts per kilogram per watt input. During the treatment, the average power applied was 30 W. Six months after therapy, the patient is asymptomatic. Although alkaline phosphatase, SGOT and SGPT have remained elevated, bilirubin has returned to normal and computerized tomographic scans and cholangiograms remain stable. A duodenal ulcer developed after therapy and is healing well with conservative medical management. This case demonstrates that hyperthermia applied through biliary drainage catheters is technically feasible and clinically tolerated. We believe the use of intracatheter hyperthermia in conjunction with external and/or intracatheter radiotherapy in selected patients with unresectable bile duct carcinomas warrants further study.

Dosimetry of a dual photon energy linear accelerator

We describe the dosimetric characteristics of a newly introduced dual photon energy linear accelerator, the Varian Clinac 1800. Depth doses are compared with other accelerators of the same nominal accelerating potentials (6 and 10 MV). Field flatness at dmax and at 10 cm depth, depth of dmax, wedge characteristics, output factors, and doses in the buildup region are presented.

Routine quality control of Ir-192 sources: results of 46 months of surveillance

Documented problems with loose and inert Ir-192 seeds in nylon ribbons prompted us to develop a quality control program. Several problem cases are illustrated. A sample of ribbons from each shipment is measured in a radioisotope dose calibrator. Results of the measurements over a 46-month period are presented.

Dosimetry of single fraction high dose total body irradiation as measured by thermoluminescent dosimeters

Eighty-five patients with acute myelogenous or acute lymphoblastic leukemia were treated at the City of Hope National Medicine Center with chemotherapy, total body irradiation, and bone marrow transplant. The average mid-line dose to these patients was 1002 rad with a uniformity of 8%.