Purpose: The aims of this study are to 1) evaluate dosimetric deviations between treatment plans of optimized on the original CT and re-calculated on the metal-artifact-reduced CT for prostate cancer patient with metal-hip-prostheses, and 2) investigate radiobiological deviation of physical metrics.
Methods: Optimized and re-calculated plans of 16 cases were for intensity-modulated-proton-therapy (IMPT) using two-field pencil-beam-scanning proton beams and for volumetric-modulated-arc-therapy (VMAT) using two-photon-arcs. The prescribed dose to clinical-target-volume (CTV) over 44 fractions in optimized plan of each patient was 72 Gy for IMPT and 79.2 Gy for VMAT. Two IMPT plans sets were evaluated, LAT plan with two lateral-fields and HipAvd plan with one-lateral and the other posterior-oblique avoiding hip-prostheses. Obtained IMPT/VMAT plans were evaluated with previously developed Radio-Therapy-Plan-Evaluation-Tool (RTPET). The 1.1 constant radiobiological effectiveness for IMPT target was applied in RTPET. The radiobiological evaluation was conducted with tumor-control-probability (TCP), and normal-tissue-complication-probability (NTCP) with physical metrics e.g. Cumulative DVHs, Conformity-Index (CI), Dose-Homogeneity-Index (DHI).
Results: The presence of metal artifacts barely affected VMAT plans with all metrics within +/- 2.0% differences, while it significantly affected IMPT plans in TCP/NTCP values that could result in tumor control deficiency. Dividing plans with single and bilateral hip-prosthesis, the TCP_CTV for LAT plans varied <10% for single, but > 80% for bilateral. The larger TCP_CTV deviation of bilateral corresponds to > 40 Gy deviation of minimum dose in CTV. The values of NTCP for bladder and rectum for LAT varied < 0.5% for single, but more > 2% for bilateral. The variation of the mean and maximum dose to bladder or rectum were < 2 Gy.
Conclusion: The presence of metal artifacts without any correction in planning could compromise radiobiological outcomes as well as surrounding normal tissue sparing power of IMPT.