Analysis of prostate bed motion using daily cone-beam computed tomography during postprostatectomy radiotherapy

Area of Science:

• Radiation Oncology
• Medical Imaging
• Prostate Cancer Treatment

Background:

• Accurate patient positioning is crucial for effective radiotherapy.
• Prostate bed radiotherapy requires precise targeting to minimize toxicity.
• Interfraction variations in patient setup can compromise treatment accuracy.

Purpose of the Study:

• To quantify interfraction positioning errors in prostate bed radiotherapy.
• To differentiate between bony misalignment (BM) and prostate bed motion (PBM).
• To evaluate the impact of an adaptive radiotherapy (ART) schedule on target margins.

Main Methods:

• Retrospective analysis of 547 daily kilovoltage cone-beam CT (CBCT) scans from 15 patients.
• Rigid co-registration of CBCT with planning CT to assess total positioning error.
• Automatic bony pelvis co-registration to separate BM and PBM components.
• ART protocol determined by initial CBCT data to adjust margins.

Main Results:

• Total positioning errors yielded systematic errors of 2.69, 2.00, 2.65 mm and random errors of 1.99, 1.49, 2.25 mm (LR, SI, AP).
• Required planning target volume (PTV) margins were 8, 6, 8 mm without adaptive planning.
• ART reduced PTV margins by 40-54% across directions, with significant anteroposterior PBM noted.

Conclusions:

• Significant interfraction errors necessitate substantial PTV margins (6-8 mm) without verification.
• Prostate bed motion, particularly in the anteroposterior direction, is a key contributor to positioning uncertainty.
• Implementing an adaptive radiotherapy protocol effectively reduces PTV margins, enhancing treatment precision.