Radiation Oncology ??Biology ??Physics October 1984, Volume 10, Sup. 2

RS12 TREATMENT PLANNING DOSIMETRY OF A GAMMAME D II REMOTE AFTERLOADING SYSTEM

James E. Rodgers and Surendra N. Rustgi

Georgetown University Hospital, Washington, D.C.

The GammaMed II automated after-loading system employs a nominal 1OCi Ir 192 "seed" (lmm long by lmm diameter) source for brachytherapy using needle or tube applicators. This high dose rate device can be pro- grammed to deliver "optimal" distributions by specifying the source dwell time in each of 1 to 20 positions along the applicator axis. Control of source movement from the source carrier housing to the applicator attached to a flexible guide tube is executed from a remote console. Our concerns here are related to the calibration and dosimetric quantities required for accurate delivery of radiation dose in a water phantom.

A calibration of the effective initial source activity is required for the computation of decayed exposure rates at later dates and for treatment planning. This was computed from measurements of the expo- sure rate using a calibrated parallel plate chamber placed perpendicular to the applicator axis and far enough away to minimize field nonuniformity over the chamber volume. A layer of polystyrene was placed over the chamber entrance plate so as to eliminate the extraneous contributions of low energy photons and elec- trons. Although a thickness of 100mg/cmz was found to be sufficient to reach exponential drop-off, our chamber was calibrated with a 120mg/cm2 build-up layer. The chamber was calibrated at NBS for three x-ray beam qualities covering the broad photon spectrum of Ir-192. Since the ionization chamber also receives extraneous irradiation during the period (of several seconds) when the source is traveling out but before the timer is activated (and upon retraction also), there is a timer on/off error. This on/off error can vary from less than 1 set to 20 set depending on the calibration set-up geometry and also on whether the chamber is in air or phantom. Although the on/off error can usually be neglected for the dosimetry of points several cm deep in phantom, it must be accounted for in exposure rate calibration.

The dose rate around the Ir-192 source in air is modified in tissue due to attenuation and scatter of photons. The ratios of dose rates to a point in water relative to a mini-phantom in-air at the same location were measured using a 0.1cm3 microchamber (wall thickness 210mg/cm2) controlled by a Therados RFA3 scanner. The resulting TAR curve over the distances l-15cm agrees well with the measurements of Meisberger, et al. (Radial. 90, 953 (1968)). Measurements of dose in water at distances of 5 and 10~x1 from the applicator were also made using LiF TLD chips which were calibrated in a Co-60 beam. Excellent agreement (1.7% and .l%, respectively) was found with predictions based on the in air exposure rate and TAR factors.

The relative isodose distributions in water in several planes parallel to the length of the applicator were measured with the RFA3 scanner using the 0.1~~ microchamber and compared with computer generated dose distributions based on an isotropic point source corrected by the appropriate TAR factor. In a plane 1.5cm from the applicator axis, the radii of measured 80%, 60 and 40% isodose curves differ from the radii of com- puter generated isodose curves by 1, .3, and 1.3mm, respectively. Similar excellent agreement was obtained in other planes parallel to the applicator axis.

RS13 TR04lwmrroFmxnxEAL (7ARcm WIm ~IALTRMEINVOL~,Al~~WPERIBS(B

Cengiz Aygun, M.D., Robert G. Slawson, M.D., F.A.C.R., John Hankins, M.D., Roberta A. Strohl, R.N., M.N. and Cmar M. Salazar, M.D.

University of Maryland Hospital, Department of Radiation Oncology, 22 S. Greene St., Baltimore, Maryland 21201

From 1973-1983, 43 patients were seen with carcinoma of the esophagus involving the tracheobronchial tree. All patients had bronchoscopy. Sixteen patients had positive biopsy with gross disease, 21 had gross intraluminal tumor, and six had tracheoesophageal fistula at presentation.

The wide variety of treatment reflects the confusion existing in the management of this problgn. Treatment consisted of: intralcrminal tuba only (3); intraluminal tube followed by irradiation (5); sup- portive care with hyparalimentation, hydration, and antibiotics (12); radiation alone (13); palliative resection with post-operative irradiation (1); and surgery alone (3).

Mean survival was 12 weeks for radiation with/without intraluminal tube and 10 weeks for intraluminal tube only, but was not significantly different in the aggressively treated groups. Mean survival of pa- tients treated conservatively was only 3 weeks.

Four of 13 patients treated with irradiation alone developed a subsequent fistula (one from tumor progression and three fran tuner regression). High daily dose did not increase the risk of fistula for- mation. Massive bleeding occurred in one radiation patient while massive bleeding and/or superior vena caval s@rane was the cause of death in four unirradiated patients.

Surgical bypass procedures not only maintain continuity of the alimentary tract but also reduce the risk of pulmonary complication from fistula formation either due to natural causes of disease or by re- gression due to irradiation. While irradiation does not increase survival in aggressively treated pa- tients, it is certainly not contraindicated and does decrease the risk of massive bleeding or superior vena caval syndrcme.