A 3-FIELD-ARC-TECHNIQUE (3-FAT) FOR TREATING PROSTATE-CANCER

Purpose: We have previously designed two external beam radiotherapy te chniques for treating prostate cancer. The seven-field, coplanar fixed beam technique resulted in dose distributions that were superior to o ther coplanar plans studied. The other technique using bilateral block ed arcs produced slightly higher doses to normal tissues but was far s impler to execute. We combined aspects of both these plans to produce a technique that was less complicated yet resulted in an improved dose distribution, i.e., to improve dose delivery to the clinical target v olume (CTV) while minimizing doses to the rectum, bladder, and femoral heads. Methods and Materials: Twenty patients, previously treated at the University of California, San Francisco (UCSF) with radiotherapy f or adenocarcinoma of the prostate, were studied. Each patient was trea ted with an immobilizer, urethrogram, and a preplanning CT scan. A pre viously employed, seven-field, coplanar, fixed beam technique was comp ared with a newly designed three-field, are technique (3-FAT). This 3- FAT was designed using two equally weighted rotational beams, with non uniform blocks, beginning in the lateral gantry position and spanning anteriorly 35 degrees. The two beams became noncoplanar by turning the table 20 degrees, bringing the patient's feet toward the gantry (infe rior oblique arcs). An anterior inferior oblique (AIO), angled 20 degr ees to the inferior of anterior was included for 10% of the daily trea tment. Dose-volume histograms (DVH) were used to evaluate doses to adj acent critical structures. The dose to each critical structure was ave raged and tabulated for the 20 patients. In addition, we compared norm alized doses to adjacent structures using 3-FAT and seven-coplanar, fi xed beams vs. a technique using four noncoplanar, fixed beams. Results : The three-field are technique produced favorable dose distributions for the rectum, bladder, and femoral heads. Compared to the seven-held plan, employing the 3-FAT resulted in a 13% lower dose to 40% of the rectum, and 25% lower dose to 40% of the bladder. Compared to the four -field plan, employing the 3-FAT resulted in a 23% lower dose to 40% o f the rectum, and 1% decrease in dose to 40% of the bladder. The three -field are technique reduced the dose delivered to 40% of the femoral heads by approximately 45% when compared to the other techniques. Comp ared to other standard treatment techniques, the improvement in dose d istribution was even greater. Conclusions: The 3-FAT represents a tech nical improvement in the treatment of cancer of the prostate and semin al vesicles by minimizing the dose delivered to adjacent critical stru ctures. The 3-FAT can incorporate the advances of multileaf collimatio n and digitally reconstructed radiographs to deliver treatment with co st effectiveness and technological efficiency. (C) 1998 Elsevier Scien ce Inc.