Dose Gradient Near Target–Normal Structure Interface for Nonisocentric CyberKnife and Isocentric Intensity-Modulated Body Radiotherapy for Prostate Cancer

Purpose The treatment planning quality between nonisocentric CyberKnife (CK) and isocentric intensity modulation treatment was studied for hypofractionated prostate body radiotherapy. In particular, the dose gradient across the target and the critical structures such as the rectum and bladder was characterized. Methods and Materials In the present study, patients treated with CK underwent repeat planning for nine fixed-field intensity-modulated radiotherapy (IMRT) using identical contour sets and dose–volume constraints. To calculate the dose falloff, the clinical target volume contours were expanded 30 mm anteriorly and posteriorly and 50 mm uniformly in other directions for all patients in the CK and IMRT plans. Results We found that all the plans satisfied the dose–volume constraints, with the CK plans showing significantly better conformity than the IMRT plans at a relative greater dose inhomogeneity. The rectal and bladder volumes receiving a low dose were also lower for CK than for IMRT. The average conformity index, the ratio of the prescription isodose volume and clinical target volume, was 1.18 ± 0.08 for the CK plans vs. 1.44 ± 0.11 for the IMRT plans. The average homogeneity index, the ratio of the maximal dose and the prescribed dose to the clinical target volume, was 1.45 ± 0.12 for the CK plans vs. 1.28 ± 0.06 for the IMRT plans. The average percentage of dose falloff was 2.9% ± 0.8%/mm for CK and 3.1% ± 1.0%/mm for IMRT in the anterior direction, 3.8% ± 1.6%/mm for CK and 3.2% ± 1.9%/mm for IMRT in the posterior direction, and 3.6% ± 0.4% for CK and 3.6% ± 0.4% for IMRT in all directions. Conclusion Nonisocentric CK was as capable of producing equivalent fast dose falloff as high-number fixed-field IMRT delivery.