Intensity Modulated Proton Therapy (IMPT) - A comparative treatment planning study

Abstract: Purpose: To study Intensity Modulated Proton Therapy (IMPT) with a new treatment planning optimization software. The aim was also to make a comparative study between existing conventional photon plans, Intensity Modulated X-ray Therapy (IMXT) plans and IMPT plans for three different cancer diagnoses. Materials and methods: The conventional photon plans and the IMXT plans were optimized in Oncentra MasterPlan (OMP) version 1.5 by Nucletron (Nucletron B.V., The Netherlands). The IMPT plans were optimized using Orbit Workstation version 1.0 developed by RaySearch (RaySearch Laboratories, Sweden). The IMPT software in Orbit was at the time of this study not clinically released.Three different cancer diagnoses with a total number of seven patients were used, three patients with mammary carcinoma, two with prostate cancer and two with cancer of the parotid. The optimization parameters such as number of beams and beam angles were based on trial and error attempts guided by different publications. Other settings such as number of segments and iterations had already been clinically evaluated and were therefore kept within these limits. The evaluation and comparison of the treatment plans were performed in terms of physical quantities based on Dose Volume Histograms (DVH), target dose uniformity, Radiation Conformity Index (RCI) and Equivalent Uniform Dose (EUD) for both targets and Organs At Risk (OARs) and the Irradiated Volume (IV). Results: In the left sided mammary carcinoma cases, with the ipsilateral lung and the heart as the primary OARs, the IMPT technique rendered the best treatment plans with improved target dose uniformity and RCI. The EUD-values for the left lung and the heart was decreased and the irradiated volume was reduced on average by 24 % compared with the conventional plans and 28 % compared with the IMXT plans.In both prostate cases the doses to the primary OARs, the rectum and the bladder, revealed only small differences between the three treatment techniques. The EUD-values for both targets and OARs, target dose uniformity, RCI and irradiated volume differed only slightly.In the third diagnose, cancer of the parotid, the IMPT plans were superior to both photon techniques. Target dose uniformity, RCI and irradiated volume were improved with the IMPT technique. Furthermore, all delineated OARs received a significantly lower dose and hence a lower EUD than obtained with any of the photon plans. Conclusions: All treatment plans produced with intensity modulated protons resulted in equal or better target dose uniformity with smaller irradiated volumes compared with both the conventional and the IMXT technique. Dose reductions with the IMPT plans were clearly seen in structures located at a distance from the target and not in the primary beam track. Results from this study suggest that the IMPT technique is most suitable for tumors of the head and neck, a region with many critical structures, even though the treatment plans in the mammary carcinoma cases also were improved considerably with the IMPT technique.