Radiotherapy of prostate cancer with multileaf collimators (MLCs). Optimization of the undulating dose distribution at the MLC edge

Background and Purpose: A technical modification for radiotherapy of prostate cancer is presented to smooth the scalloped dose pattern that occurs at treatment field edge, when a multileaf collimator (MLC) has been used. Material and Methods: Ten patients with prostate cancer receiving postoperative, adjuvant irradiation were studied prospectively. By a three-dimensional planning system (TMS, Helax 6.1B) the irradiation was planned for an 18-MV linear accelerator (Primus 1, Siemens). The volumes of interest (VOI) were the planning target volume (PTV; the region of the prostate including the seminal vesicles), the volume of rectum (Vrectum) and urinary bladder (Vbladder). Two four-field techniques (0 , 90 , 180 , 270 ) were planned using 'beam's eye view' for setting the leaf position of the MLC. For technique A the MLC was adapted to the PTV using a 0 collimator angle for the lateral fields. For technique B the collimator angle of the lateral fields was optimized to compensate the cascade field shape. Dose-volume histograms of PTV, Vrectum and Vbladder were analyzed. The dose was prescribed for the reference point according to ICRU 50. Film dosimetry was used to show the dose pattern at the field edge produced by the two techniques. Results: Dose to PTV did not differ between technique A and B. Median dose to Vrectum was 82.6% for technique A and 77.3% for technique B (p<0.001). Technique A irradiates a larger Vrectum than technique B being significant for all isodose levels tested. Median dose to Vbladder did not differ for technique A and B (p>0.05). Conclusion: The presented technical modification is an effective method to blur the staggered dose distribution that results, when the MLC is conventionally stepped to adapt to the dorsal, irregular PTV broder in irradiation of prostate. Especially for irradiation to escalated dose levels, this modification may reduce the dose to the rectum and thus the rectal side effects in comparison to the conventional MLC fields. (orig.)