Comparison of Virtual HDR Prostate Treatment Plans Created with Fixed and Variable Aperture Collimators

Abstract

We previously compared Virtual HDR prostate CyberKnife System (CK) SBRT plans versus simulated HDR brachytherapy plans, based on identical contour sets for each comparison plan pair, in 10 consecutive patients (Int J Radiat Oncol Biol Phys 2008; 70: 1588-1597). In the current analysis, for the same patient cohort, we assess the same endpoints for plans created using the recently released Iris™ Variable Aperture Collimator. The 10 Virtual HDR prostate CK cases were planned and treated using one or two fixed collimators (“Fixed”). The prescription dose was 38 Gy delivered in 4 fractions. These cases were replanned using the Iris Collimator and the recently released Sequential Optimization inverse planning algorithm (“Iris”). The Iris Collimator allows the use of any of 12 field sizes between 5 and 60 mm for each beam in a single traversal of the robotic manipulator. Replanning objectives included maintaining or improving the conformality and normal tissue sparing of the original plan, increasing extraurethral-intraprostatic dose escalation, and minimizing the number of beams and monitor units (MU). Median (range): Non-zero beams: 266 (234-318), Fixed; 191 (141-208), Iris; median difference 92, 10/10 cases lower with Iris. MU: 101.2K (76.0K-116.6K), Fixed; 76.4K (67.1K-89.2K), Iris; 10/10 cases lower with Iris. Beam-on time: 42.2 min, Fixed (600 MU/min CK platform); 23.9 min, Iris (800 mu/min CK platform). Rx isodose line: 57% (49-67%), Fixed; 53% (47-60%), Iris; 10/10 cases same or lower with Iris. V125: 43.95% (28.4-55.5%), Fixed; 45.6% (35.5-55.9%), Iris; 7/10 cases higher with Iris. V150: 8.45% (0.1-20.5%), Fixed; 18.05% (3.0-22.0%), Iris; 9/10 cases higher with Iris. nCI: 1.14 (1.10-1.23), Fixed; 1.12 (1.10-1.20), Iris; 8/10 cases same or lower with Iris. Urethra Dmax: 44.25 Gy (43.1-46.1 Gy), Fixed; 42.05 Gy (40.6-43.4 Gy), Iris; 9/10 cases lower with Iris. Rectum Dmax, Bladder Dmax, Urethra D50: No significant difference. Treatment plans generated using the Iris Collimator and Sequential Optimization consistently have considerably fewer beams and MU than plans generated using one or two fixed collimators, with a median 43% reduction of beam on time per case. Part of the time reduction is due to a higher dose rate linac (800 vs. 600 MU/min). Simultaneously, the Iris Collimator plans typically have similar or better conformality, higher maximum intraprostatic doses, lower maximum urethral doses, and similar maximum doses to the bladder and rectum. In summary, using multiple field sizes enabled by the Iris Collimator can produce treatment plans with a more tailored, HDR-like dose distribution and that can be delivered more efficiently than plans created with one or two fixed collimators.