The potential of optimized intensity modulation to escalate prostate dose to 86.4 Gy

Purpose: At our institution, doses of 75.6 and 81 Gy have been successfully delivered to patients with locally advanced prostatic cancer using 3-dimensional conformal technology, carried out with manually designed uniform-intensity and wedged beams. Although our experience has demonstrated that such treatments are safe, there are gains to be made by further escalation of dose and a concomitant improvement in normal tissue sparing. The objective of this work was to determine the optimal prostate treatment technique to permit the 'safe delivery' of dose to levels above 81 Gy for patients involved in a dose escalation study. Materials and Methods: The criteria for optimization of intensity distributions were specified in terms of desired homogeneous dose to the planning target volume (PTV) and upper limits of dose to rectum and bladder as well as to the regions of overlap between these organs and the PTV. The optimization was carried out using the inverse planning technique. To obtain acceptable 86.4 Gy plans, numerous alternatives were explored. Attempts were made to obtain dose distributions by balancing the often conflicting constraints that (1) the 86.4 Gy isodose line be as close as possible to the interface between the PTV and the anterior rectal wall, (2) the smallest possible volume of rectum receive dose above 81 Gy, (3) no more than 25% of the rectum receive 75.6 Gy, (4) the dose distribution of the PTV be as homogeneous as possible. Constraints for the bladder were also considered but they were less severe. The treatment scheme which resulted in the most favorable dose distributions was the one employing the same set of intensity-modulated fields (IM) for the entire course of treatment. The resulting plans were compared with a manually-developed, uniform-intensity (UI) conformal plan in which a two-phase scheme employed in our 81 Gy phase of the dose escalation study was extrapolated to 86 Gy. In the first phase of this strategy, the entire PTV was planned to receive 72 Gy using a six-field conformal treatment plan. In the second phase, the remaining 14.4 Gy boost was intended to be delivered using a different set of 6 to 8 conformal beams to a boost volume which excluded the rectum and seminal vesicles with the constraint that the region of overlap between the target and the rectum receive no more than 3.6 Gy additional dose. Comparison of two types of plans considered dose distributions, dose-volume histograms, target dose homogeneity, the fractional volumes of the target receiving 75.6, 81 and 86.4 Gy, and the fractional volume of rectum and bladder receiving 50, 75.6 and 81 Gy. Results: Using the results of a systematic study of optimization parameters and constraints, it was possible to achieve acceptable 86 Gy intensity-modulated treatment plans for which the specified rectum and bladder constraints are met and the target dose distributions were significantly superior than the manually-designed plans. For the first patient planned, the fractional volumes of PTV receiving 75.6, 81 and 86.4 Gy was 93, 84 and 70% respectively for the two-phase UI plan, compared to 99, 96 and 83% respectively for the IM plan. The fractional volumes of the rectum receiving 50, 75.6 and 81 Gy was 36, 16 and 0.5% respectively for the two-phase UI plan, compared to 40, 11 and 2% respectively for the IM plan. The fractional volume of the bladder receiving 50 and 75.6 Gy was 53 and 39% respectively for the two-phase UI plan, compared to 54 and 35% respectively for the IM plan. Comparison of these two treatment plans for two additional patients yielded similar results. The quality of plans depended upon the number of intensity-modulated beams. For 86.4 Gy treatments, seven beams were considered to be the minimum to achieve the desired dose distributions. Conclusions: Compared to the two-phase UI plans, IM based treatment plans for planned doses of 86.4 Gy provide superior coverage of the PTV with acceptable doses to the rectum and bladder. We plan to implement this technique in the treatment of locally advanced prostatic cancer to doses of 86.4Gy as part of our ongoing dose escalation trial