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A Dosimetric Comparison of Treating High-Risk Prostate Cancer Using Sequential Versus Simultaneous Integrated Boost Planning Techniques in Photon and Proton Radiotherapy
K Greco, B.Sc., H Guerrero, B.Sc., C McKenzie, B.Sc., M Fagundes, M.D., M Rodrigues, M.D., A Garcia Serra, M.D., A Gutierrez, Ph.D, MBA
Department of Radiation Oncology, Miami Cancer Institute, Miami, Florida 33176, USA
Introduction
High-risk prostate cancer is an aggressive disease that carries an increased risk of distant metastases and fatality. At our institution with the use of image guided radiotherapy (IGRT), the target is more easily localized allowing patients to be dose escalated to a total of 78 Gy. With the use of intensity modulated radiotherapy (IMRT) and intensity modulated proton therapy (IMPT), delivery of different total doses and doses per fraction to different targets within the radiation field are possible with a simultaneous integrated boost technique (SIB). A sequential three-phase technique can also be used and is a common practice in radiotherapy planning for high-risk prostate cancer. The purpose of this study is to dosimetrically quantify the impact of the SIB technique for both photon and proton patients compared to sequential delivery with comparisons of target coverage and organs at risk sparing (OARs).
Results
When comparing the SIB to sequential technique within protons, the differences in OAR doses were insignificant except for the femurs (lower for SIB with a difference of 25.17% for the right and 23.93% for the left when looking at the V45 Gy). Homogeneity and the PITV for both dose levels (54 and 48.6 Gy) were statistically significant with a difference of .96%, 4.59%, 9.17% and 9.40%, respectively. In regards to comparing the SIB technique between protons and photons, the homogeneity index (54 Gy), target coverage and PITV for both dose levels were statistically significant with a difference of -2.80%, 4.35%, 6.52%, 10.10% and 17.00%. Differences in OAR doses were insignificant except for the rectum V60 Gy, bladder V80 Gy, V70 Gy, small bowel V15 Gy, V45 Gy and the penile bulb mean (see Table 2).
Table 2: Dose statistics for OAR constraints between Proton and Photon SIB planning.
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Figure 1: Sagittal Isodose distribution (A. Proton-SIB) (B. Proton-Sequential, first phase) (C. Proton-Composite plan SIB)
(D. Photon-SIB) (E. Photon-Sequential, first phase) (F. Photon-Composite plan SIB). A A B C
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Table 1: Dose statistics for homogeneity index, target coverage and PITV between Proton and Photon SIB planning
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Conclusions
Both the SIB and sequential technique offer similar target coverage and doses to OARS. Overall, proton plans were more homogenous with better target coverage, but had slightly higher OAR doses (although still clinically acceptable per QUANTEC constraints). The conformity was slightly better for photons due to proton planning accounting for robustness which generates a larger dose cloud. By treating with an SIB technique, the overall number of fractions are reduced and requires less quality assurance while still maintaining acceptable target coverage and OAR doses.