Robustness against interfraction prostate movement in scanned ion beam radiation therapy

Purpose: To assess the robustness of scanned ion beam treatment plans against the interfraction internal target motion and evaluate the limits of validity of target-based isocenter realignment for prostate cancer radiation therapy.

Methods and materials: For 12 prostate patients, scanned beam carbon ion treatment plans were prepared using 2 lateral opposed beams and the raster scanning technique with different clinical target volume (CTV) to planning target volume (PTV) margins (2-10 mm). Internal target motion of 2-15 mm in anteroposterior (AP), superoinferior (SI), and left-to-right (LR) directions was simulated by displacing the CTV contours with respect to the computed tomography data. The plans were recalculated with and without target-based isocenter realignment and the CTV coverage was assessed.

Results: For CTV shifts within the applied planning margin, the CI-98% is greater than 98.0%, both with and without isocenter realignment. Without realignment, because of the sharp lateral gradients, the CI-98% shows rapid fall as soon as the target shift exceeds the applied planning margin for all displacement directions. With isocenter realignment, the coverage improves notably for shifts in AP and SI directions and the CI-98% is restored to >95.0% for plans optimized with a 2-mm margin, >97% with a 3-mm margin, and >98% with larger margins. For large corrections, predominately in AP direction, cold spots in the CTV may occur. Their magnitude is dependent on the patients' individual anatomies.

Conclusions: Within the physiological limits of internal prostate movement, target-based isocenter realignment results in improved CTV coverage for shifts in AP and SI directions exceeding the applied planning margin. Assuming optimal patient setup reproducibility (eg, immobilization, setup error correction, patient preparation protocols), hence negligible interfraction bone and soft-tissue variations, changes in traversed densities resulting from target-based realignment applied with a reduced planning margin do not induce significant dose deterioration in most of the cases.