Reducing Treatment Time and MUs by using Dynamic Conformal Arc Therapy for SBRT Breath-Hold Patients Timothy Miller, Sebastian Nunez Albermann, Besil Raju, Anton Johnson, Jordan Sutton CMD, Ramaswamy Sadogopan M.S The University of Texas MD Anderson Cancer Center School of Health Professions Abstract The purpose of this study is to decrease the treatment time and number of Monitor Units (MUs) for stereotactic body radiation therapy (SBRT) lung breath-hold (BH) patients by using Dynamic Conformal Arc Therapy (DCAT) and to recommend the use of DCAT to our clinic whenever feasible. DCAT was able to reduce treatment time and MUs by about 20% and was able to obtain the clinically desired coverage, while also keeping the Organs at Risk (OAR) under tolerance. Results Figure 1 show the dose comparison between a DCAT and VMAT plan when the tumor is closely located to the chest-wall. Figure 2 shows the dose comparison between a DCAT and VMAT plan when a tumor is small and centrally located. The treatment time for VMAT cases were expected to be higher than DCAT plans due to the use of beam modulation. This is proven to be true. Table 1 shows the time, MUs, average, and standard deviation between all 12 patients. Figure 4 shows that DCAT plans have lower treatment time than VMAT. The VMAT plans recorded an average of 4.1 minutes for treatment, whereas the average time for the DCAT plans was 3.3 minutes. DCAT was able to reduce treatment time by an average of 20%. According to figure 5, the MUs for the DCAT plans were reduced by an average of 20% from VMAT plans. Figure 6 and 7 show the average treatment time and MUs delivered. The VMAT plans recorded an average of 4.1 minutes for treatment, whereas the average time for the DCAT plans was 3.3 minutes. The average VMAT MUs recorded was 2441, whereas the DCAT was 1957. Discussion A key aspect for radiation therapy to be successful is to have accurate dose delivery to the tumor margins. According to ICRU-50 and 62, the tumor margins that are applied to the presented disease account for subclinical disease, set up errors, internal movements, physical movements, and organs at risk.2 Since thoracic and GI patients are usually treated with both arms up and due to factors such as high dose in sbrt or gating, treatments are long enough to cause significant pain and discomfort which subsequently may lead to patient movement. Reducing treatment time will go a long way to minimize patient discomfort and motion and thus improving treatment delivery accuracy. DCAT reduced both the treatment time and MU by an average of 20% compared to VMAT. Even though all the plans were more efficient, two of them did not meet the clinically acceptable requirements due to close proximity to critical structures or large target size. These two patients had large tumors that were situated near the chest-wall. Our clinical objective demanded the chest-wall meet the dose constraints of V30<30% for 50Gy 4fx and V50<60cm, V40<120cm, and V30<250cm for 70 Gy 10fx. We also had trouble meeting the total lung constraint which had to have a mean dose <6 Gy,V5<30% and V10<17%, V20<12%, and V30<7%. The results are clinically important because the treatment time and MUs were reduced, meaning less patient movement is likely to occur. This allows a more accurate treatment to the desired site. Fig. 1. Shows the comparison between a VMAT and DCAT plan close to the chest-wall. A is VMAT and B is DCAT. Fig. 2. Shows the comparison between a VMAT and DCAT plan centrally located. C is VMAT and D is DCAT. A B C D Introduction One of the challenges in radiation therapy treatment has been the movement of not only the patient, but also the tumors and organs at risk (OAR) due to physiological and voluntary movements.1 By decreasing treatment times, it is possible to assure greater accuracy of dose deliverance to the desired region of interest . DCAT is a treatment method that has a gantry that can rotate up to 360 degrees and uses dynamic multileaf collimators (MLC’s) to conform the radiation to the shape of the target. We will be comparing this technique to VMAT, which is the most common technique used in our clinic. Clinical VMAT Conformal Arc Patient Number Time (Min) MUs 1 4.23 2435 3.24 1998 2 4.51 2808 3.42 2128 3 3.58 2184 3.33 2008 4 4.03 2388 3.25 1958 5 4.53 2811 4.01 2322 6 4.26 2563 4.13 2390 7 5.38 3231 3.4 2080 8 5.1 2925 3.22 1886 9 2.19 1283 2.01 1092 10 2.42 1514 2.2 1258 11 4.49 2639 4.06 2374 12 4.21 2508 3.28 1994 Average 4.1 2441 3.3 1957 STD 1.0 588 0.7 423 Table 1. Shows all the MUs, time, averages, and standard deviations for all twelve patients. Material and Methods In this study, 12 previously treated SBRT breath hold thoracic patients were randomly selected from Mosaiq v2.6 from a date range of 2016-2018. All patients selected for this study were previously treated with VMAT under breath hold. All cases were clinically acceptable and approved for treatment using Philips Pinnacle Software v9.10. The main objectives were to reduce treatment time and MUs while maintaining PTV and GTV coverages. The secondary objective was to keep all critical structure doses within the same constraints that were used for the original clinically acceptable VMAT plans without sacrificing target coverages. Once target coverage and critical structure tolerances were achieved, the treatment times were calculated using the delivery time estimator. .Fig. 4. Compares the number of MUs between DCAT and VMAT. Fig. 3. Compares the treatment time between DCAT and VMAT. Conclusions In conclusion, reducing treatment time will improve treatment delivery accuracy, and reducing MUs will reduce the volume of healthy tissues receiving low level doses which will lead to the reduction in secondary malignancy. Ultimately using DCAT must be considered for clinical use based on these results. Fig. 5. Shows the average number of MUs comparison for both VMAT and DCAT. References 1. Stambaugh C, Nelms BE, Dilling T, Stevens C, Latifi K, Zhang G, et al. Experimentally studied dynamic dose interplay does not meaningfully affect target dose in VMAT SBRT lung treatments. Med Phys [Internet] 2013;40(9). 2. Mackie T, Gregoire V. ICRU Recommendations3. Hall, J. E. Intensity-modulated radiation therapy, protons, and the risk of second cancers. Int. J. Radiat. Oncol. Biol. Phys. 65 (1): 1-7; 2006. Fig. 6. Shows the average time comparison between VMAT and DCAT.