1. Optimization of Volumetric Modulated Arc Therapy (VMAT) Planning Strategy Using Ring-shaped ROI for Localized Prostate cancer Kentaro Ishii, Masako Hosono, Daisaku Tatsumi, Ryosei Nakada, Shinichi Tsutsumi, Ryo Ogino, Yoshie Takada, Takuhito Tada, Yukio Miki Department of Radiology, Osaka City University Hospital, Japan Introduction Among the various IMRT techniques currently used, VMAT requires less treatment time and fewer monitor units (MU) when delivering planned doses. The aim of this study was to optimize dose distribution in VMAT planning using ring-shaped regions of interest (ROIs) as compared with five-field IMRT. Methods and Materials Ten CT data sets of localized prostate cancer cases Elekta Synergy with 1 cm multi-leaf collimator (VMAT: 6MV, IMRT: 10MV) TPS & optimization algorithm: VMAT: ERGO++ version 1.71, AMOA (Arc Modulation Optimization Algorithm) IMRT : Pinnacle 3 version 8.0m, DMPO (Direct Machine Parameter Optimization ) 74 Gy in 37 fractions to the PTV were used for plan comparison CTV=entire prostate, PTV=CTV + 7 mm (posteriorly), + 10 mm (another directions) Techniques: VMAT (single rotation), IMRT (five coplanar equidistant fields) Dose constraints ： VMAT-1 PTV + Ring-shaped ROI VMAT-2 PTV + rectum + bladder + bil. femoral heads Plan comparison (VMAT-1 vs VMAT-2 vs IMRT) ・ DVH parameters (PTV, rectum, bladder, femoral heads, and whole tissue) ・ Homogeneity index (HI=D max /D prescribed ) ・ Conformity index (CI=V tissue D95% /V PTV ) ・ Monitor units (MU), total treatment time Fig. 1 Rotation technique for the VMAT approach. The rectum was shielded only when it was in front of the PTV in the beam’s-eye view with conformal beam delivery. Fig.2 Ring-shaped ROIs. Ring- shaped ROIs were dummy ROIs placed on tissues surrounding the PTV with a thickness of 1 cm at a distance of 2-3 cm from the PTV and were incorporated in the planning. *Both VMAT plans were transferred to the Pinnacle 3. For all techniques, the final dose calculation was performed using the superposition algorithm. Results Compared to VMAT-2, VMAT-1 resulted in more concentric dose distribution characterized by dynamic arc therapy (Fig. 3). PTV: VMAT-1 and -2 provided comparable conformality with IMRT (Fig. 4). Although IMRT plan presented better CI and HI than both VMAT plans, VMAT plans were clinically acceptable (Table 1). OARs: OARs were preserved in VMAT-1 and -2 as well as in IMRT (Fig. 4). Differences between VMAT-1 and VMAT-2 were observed in terms of the rectum V70 and V40, however, the differences were clinically not significant (Table 1). The whole tissue: The whole tissue V30 was significantly reduced for VMAT-1 compared with VMAT-2 and IMRT, although the whole tissue V50 and V70 were similar for all treatment techniques. On the other hand, the whole tissue V10 was the most significantly reduced for IMRT (Table 2, Fig. 5). Compared to the IMRT plans, both VMAT plans achieved a 11% relative reduction in MUs required for RT delivery. The mean treatment time was 1.9 min for VMAT-1 and -2 versus 7.3 min for IMRT (p=0.006) (Table 1). (a)(b) (c) PTV Volume, % Dose, Gy Rectum Dose, Gy Volume, % Bladder Dose, Gy Volume, % VMAT-1 VMAT-2 IMRT Wilcoxon matched-pair signed rank p VMAT-1VMAT-2IMRT VMAT-1 VMAT-2 vs VMAT-2vs IMRT TOTAL MU 378±14377±18433±230.36 * 0.006 TREATING TIME [ s ]114±13113±14440±20 0.41 * 0.006 PTV MEA N DOS E [Gy]75.3±0.675.2±0.874.8±0.10.610.0320.15 D 95% [Gy]70.8±0.471.4±0.472.0±0.50.019 * 0.0050.032 RECTUMV 70Gy [ % ]6.2±2.18.7±2.49.0±1.7 * 0.0060.0140.92 V 40Gy [ % ]38.2±3.435.3±3.141.2±0.8 * 0.0060.025 * 0.006 BLADDERV 70Gy [ % ]17.9±8.117.4±8.013.0±4.20.0740.025 V 40Gy [ % ]37.1±14.238.4±16.135.9±9.30.410.760.54 FEMORAL HEAD V 40Gy [ % ]0.7±2.00.1±0.24.7±5.90.280.012 * 0.006 CONFORMITY INDEX 1.48±0.061.47±0.061.34±0.110.76 * 0.008 HOMOGENEITY INDEX 1.07±0.011.06±0.021.05±0.010.41 * 0.0060.15 Table 1: Detailed comparison of mean values for different plan parameters. Dose-volume parameters, CI, HI, total MU, and treatment time for each treatment plan were compared with the two-sided Wilcoxon matched-pair signed-rank test (significant at p ≦ 0.01). Wilcoxon matched-pair signed rank p VMAT-1VMAT-2IMRT VMAT-1 VMAT-2 vs VMAT-2vs IMRT V 70Gy [cc]154±44153±47148±260.650.540.76 V 50Gy [cc]290±73289±79292±480.330.840.76 V 30Gy [cc]794±250909±2431053±159 * 0.006 0.025 V 10Gy [cc]2859±3422850±3442702±3540.68 * 0.006 Table 2: Summery of dosimetric results for whole tissue. VMAT-1 VMAT-2 IMRT Dose, Gy Volume, cc Fig.5: Patient averaged dose-volume histogram for whole tissue. Discussion In the treatment of prostate cancer, we have shown that ERGO-VMAT is capable of producing DVHs similar to those with fixed field IMRT. Both VMAT plans significantly reduced the MU and total treatment time compared to IMRT plans. Reduced MU in VMAT may decrease risk of secondary cancer. In contrast, volume receiving lower dose is larger in VMAT than IMRT. As it is reported that secondary cancer risk may increase when low dose volume increases, it is arguable whether VMAT may really lower risk of secondary cancer. Reduced total treatment time lead to a significant impact on the clinical throughput. ERGO-VMAT is a new treatment option and therefore optimization of procedures has not been fully achieved. We have proposed an optimization method utilizing a ring-shaped ROI. Our validation test has demonstrated a nearly concentric dose distribution well characterized by dynamic arc treatment without impairing DVHs for the rectum and bladder. Dose constraints were required only for PTV and the ring-shaped ROI, which shortened the calculation time down to 30 minutes excluding contouring time. The exposure dose to whole tissue was also reduced compared to a plan without using the ring-shaped ROI. These findings have suggested that the ring-shaped ROI is an effective tool to create a VMAT plan. Conclusion The VMAT plans significantly reduced MU and treatment time as compared to IMRT. VMAT optimization employing ring ROIs yielded lower normal tissue exposure with preserved target coverage, which potentially lessens the risk of secondary malignancy. Fig. 3: Representative axial computed tomography slices showing isodose disteibutions for (a)VMAT-1, (b)VMAT-2, and (c)IMRT. Isodose lines are: red for 100%, yellow for 95%, blue for 50%, and green for 30% of the prescribed dose. Fig.4: Comparison of patient-averaged dose volume histograms of the PTV, rectum, and bladder for the two VMAT plans and the IMRT plan.