Utilizing A Multifield Optimization Intensity Modulated Proton Technique (Mfo-Impt) To Deliver A Simultaneous Integrated Boost (Sib) To The Dominant Intraprostatic Lesion For Localized Prostate Cancer

To evaluate the dose distribution of delivering a simultaneous integrated boost (SIB) to the dominant intraprostatic lesion (DIL) for patients with localized prostate cancer using a multifield optimized intensity modulated proton beam technique (MFO-IMPT). We performed a comparison analysis on 6 consecutive proton treatment plans all utilizing the scanning beam treatment delivery system for patients diagnosed with localized prostate cancer. Each proton plan compared a simultaneous integrated boost (SIB) delivering 85.8 CGE at 2.2 CGE per fraction to the DIL (BED = 148.2 Gy with α/β = 3) concomitantly with 70.2 CGE at 1.8 CGE to the entire PTV to the standard 78 CGE (BED = 130 Gy with α/β = 3) delivered in 39 fractions prescribed to the PTV. The DIL (defined by MRI without any margin) and PTV (defined as the prostate with a 4 mm expansion posteriorly, 11 mm laterally, and 5 mm in all other directions) volumes were the same for both SIB and standard plan. The SIB treatment plan was designed with a MFO-IMPT technique and the standard fractionation plan with a single-field optimized-IMPT technique. The average PTV D98 for the SIB IMPT compared to the standard fractionation plans was 68.0 CGE vs 74.6 CGE (p<0.001), PTV median dose was 73.5 CGE vs 81.6 CGE (p<0.0001), PTV D02 was 91.3 CGE vs 83.4 CGE (p<0.001), and PTV TDI (target dose homogeneity) was 0.5 vs 0.2 (p<0.0001) respectively. The average rectal V60 was 9% vs 11.5% (p = 0.047) and V70 was 3.2% vs 6.5% (p<0.0001) for the SIB IMPT compared to the standard treatment plan. The average V60 was 35.1% vs 40% (p<0.0001) and V70 was 18.6% vs 29.8% (p<0.001) to the anterior rectal wall for SIB IMPT compared to standard treatment plan. The average bladder V60 was 7.2% vs 8.3% (p = 0.519) and V70 was 3.9% vs 5.8% (p = 0.150) for the SIB IMPT compared to the standard treatment plan. The V50 to the femoral heads for both plans were 0%. For patients with localized prostate cancer, the simultaneous integrated boost technique allowed an increased biologically equivalent dose delivered to the DIL while sparing high doses to the rectum due to the ability to deliver a lower fractionated dose to the entire prostate gland. Further clinical analysis is necessary; however, this may result in an improved biological advantage for tumors with low α/β such as prostate cancer and minimize dose to normal tissues.