TITLE:
Effectiveness of a Patient-Specific Immobilization and Positioning System to Limit Interfractional Translation and Rotation Setup Errors in Radiotherapy of Prostate Cancers
AUTHORS:
Gilbert Law, Ronnie Leung, Frankle Lee, Hollis Luk, Ka Chai Lee, Frank Wong, Matthew Wong, Steven Cheung, Venus Lee, Wing Ho Mui, Mark Chan
KEYWORDS:
Immobilization, Prostate, CBCT, Margin, Interfractional Motion
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.5 No.3,
August
22,
2016
ABSTRACT: 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.