DESIGN AND DOSIMETRIC CHARACTERISTICS OF A HIGH-DOSE-RATE REMOTELY AFTERLOADED ENDOCAVITARY APPLICATOR SYSTEM

Purpose: An applicator is described for endocavitary treatment of rect al cancers using a high dose rate (HDR) remote afterloading system wit h a single high-intensity Ir-192 source as an alternative to the 50 kV p x-ray therapy contact unit most frequently used in this application. Methods and Materials: The applicator consists of a tungsten-alloy co llimator with a 45 degrees beveled end, placed in a proctoscope with a n elliptical cross-section. The resultant 3 cm diameter circular treat ment aperture, located in the beveled face of the proctoscope, is irra diated by circular array of dwell positions located about 6.5 mm from the applicator surface. This beveled end allows patients with posterio r wall tumors to be treated in the dorsal lithotomy position. The dose -rate distributions about the applicator were determined using a combi nation of thermoluminescent dosimetry (TLD-100 detectors) and radiochr omic film dose measurement techniques along with Monte Carlo dosimetry calculations. TLD-100 (3 x 3 x 0.9 mm(3) chips) measurements were use d to measure the distribution of dose over the proctoscope surface as well as the central axis dose-rate distribution. Relative radiochromic film measurements were used to measure off-axis ratios (flatness and penumbra width) within the treatment aperture. These data were combine d with Monte Carlo simulation results to obtain the final dose distrib ution. Results: The tungsten collimator successfully limits the dose t o the tissue in contact with the proctoscope walls to less than 12% of the prescribed dose. These results indicate that the HDR applicator s ystem has slightly more penetrating depth-dose characteristics than th e most widely used contact therapy x-ray machine. Flatness characteris tics of the two treatment delivery systems are comparable, although th e HDR endocavitary applicator has a significantly wider penumbra. Fina lly, the HDR applicator has a lower surface dose rate (1.5-4 Gy/min of dwell time) compared to 9-10 Gy/min for the x-ray unit. Conclusions: An applicator system has been developed for endocavitary treatment of early stage rectal carcinoma that uses a single-stepping source HDR re mote afterloading system as a radiation source. The advantages of the HDR-based system over x-ray therapy contact units currently used in th is clinical application are (a) enhanced flexibility in applicator des ign and (b) widespread availability of single-stepping source HDR remo te afterloading systems.