First online real-time motion-including prostate and bladder dose reconstruction during prostate radiotherapy.

BACKGROUND AND MOTIVATION: Organ motion can distort prostate radiotherapy doses. This study presents the first real-time calculation of the motion-induced dose distortions performed online during prostate radiotherapy.

METHODS

Twenty patients were treated with stereotactic prostate radiotherapy of 35 Gy or 40 Gy in 5 fractions using intrafractional image guidance for real-time prostate localization and patient repositioning upon prostate misalignments exceeding 1.5  mm. In-house developed software performed motion-including prostate and bladder dose reconstruction during treatment. The reconstructed doses were retrospectively validated against a clinical treatment planning system (TPS) where motion was encoded in treatment plans as multiple 3D isocenter shifts. Hypothetical doses delivered without intra-treatment repositioning were reconstructed post-treatment for comparison to illustrate how the dose reconstruction allows easy assessment of the effectiveness of the used motion mitigation method.

RESULTS

Dose reconstruction was performed for 91 fractions either online during treatment (n = 41) or retrospectively using recorded motion (n = 50). The real-time calculated doses (calculated by the in-house software using a simplified algorithm) agreed with TPS calculations with mean (±std) differences of 0.1 % (±0.9 %) for clinical target volume (CTV) D95% and 0.2 % (±0.2 %) for bladder V36Gy. The mean time per online dose-reconstruction (±std) was 336 ± 86 ms, proving the real-time applicability of the proposed method. The average (±std) motion-induced dose distortions for individual fractions with intrafractional image guidance were -0.5 % (±1.0 %) for CTV D95% and +0.1 % (±0.5 %) for bladder V36Gy for individual fractions. Accumulated across all fractions of each patient, these deviations decreased to 0.0 % (±0.7 %) for the CTV and +0.1 % (±0.2 %) for the bladder. In contrast, without intratreatment repositioning, deviations would have been -1.3 % (±5.0 %) for CTV D95% and +0.5 % (±1.5 %) for bladder V36Gy, with individual fractions exhibiting clinically unacceptable CTV D95% decreases of up to 42.5 %.

CONCLUSION

This study marks the first clinical realization of real-time motion-including dose reconstruction for both target and organ-at-risk structures paving the way for real-time dose-guided radiotherapy.