Author(s)

Gilbert Law, Ronnie Leung, Frankle Lee, Hollis Luk, Ka Chai Lee, Frank Wong, Matthew Wong, Steven Cheung, Venus Lee, Wing Ho Mui, Mark Chan

Department of Clinical Oncology, Tuen Mun Hospital, Hong Kong, China.

Objective: To evaluate the effectiveness of a patient-specific immobilization and positioning device in prostate radiotherapy. Methods: Eighty patients were immobilized and positioned by a patient-specific device. Prostate translations and rotations were estimated from daily cone beam computed tomography scans using a contour-based approach assisted by auto-registration and quantified by the group mean GM, systematic Σ and random σ' errors. Dosimetric impacts of residual prostate rotations where the translation errors were corrected were evaluated by robustness plan analysis. Results: Using the patient-specific immobilization alone without online image-guidance, the GM, Σ and σ' of the prostate translations were 0.8, 1.7, and 1.5 mm (left-right; LR), 0.8, 2.1, and 1.9 mm (superior-inferior; SI), and 0.5, 1.7 and 1.5 mm (anterior-posterior; AP), while for the prostate rotations they were 0.0°, 0.6°, and 0.7° (pitch), 0.2°, 0.5°, and 0.6° (roll), and 0.2°, 0.5°, and 0.6° (yaw). The resulting van Herk’s margin was 5.8 (LR), 7.3 (SI) and 5.8 (AP) mm. With adaptive online image-guidance based on estimates from the first 5 fractions, Σ were reduced by 0.7 - 1.2 mm for the prostate translations, resulting in a margin reduction by 2 - 3.5 mm. Changes of Σ and σ' in the prostate rotations were insignificant regardless of translation corrections. Dosimetric impacts of residual rotation errors were negligible if a 2 mm margin was applied. Conclusions: Our patient-specific immobilization system can effectively limit the prostate translations and rotations, which is important without 6D treatment couches or using ultrasound image-guidance without rotational corrections.

Immobilization, Prostate, CBCT, Margin, Interfractional Motion

Law, G. , Leung, R. , Lee, F. , Luk, H. , Lee, K. , Wong, F. , Wong, M. , Cheung, S. , Lee, V. , Mui, W. and Chan, M. (2016) Effectiveness of a Patient-Specific Immobilization and Positioning System to Limit Interfractional Translation and Rotation Setup Errors in Radiotherapy of Prostate Cancers. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 5, 184-195. doi: 10.4236/ijmpcero.2016.53020.