Purpose/Objective(s): We investigated the dosimetric quality and de- livery efficiency of head and neck (HN) plans achieved by a novel jawless 6 MV flattening-filter free (6FFF) linac with fast MLC collimation. Materials/Methods: Five challenging HN cases treated with simultaneous integrated boost (SIB) were retrospectively selected. There were two to three prescription dose levels with the high dose of 60-70 Gy and the low dose of 54-59.4 Gy. The treatment goal was that 3 95% of both high dose PTV (HD_PTV) and low-dose PTV (LD_PTV) will receive the prescrip- tion doses. The OARs included brainstem, spinal cord, parotids, esoph- agus, larynx, oral cavity, and mandible. The original 9-field IMRT clinical plans were done using a treatment planning system for conventional linacs. Each plan was replanned with a 9-field IMRT technique using an unre- leased version of the treatment planning system for the novel jawless linac with 1-cm-wide dual-layer MLCs. The lower layer MLCs were used for beam modulation while the upper layer MLCs were used to block inter- leaf leakage. The original clinical plans and the newly generated replans were compared using dosimetric endpoints (such as PTV coverages and OAR max/mean doses), homogeneity index (HI), conformity index (CI) and total MU. Paired t-tests were performed and the results were consid- ered significant if the p-values were  0.05. Results: The population averaged dosimetric endpoints, HI, CI, and total MU are summarized in the table below. All newly generated replans for the jawless linac met the clinical dose limit requirements. The PTV coverage, OAR doses, HI, and CI in the replans were all comparable to those in the clinical plans. The total MUs in the replans (2584  407) were signifi- cantly greater than those in the clinical plans (1495  207). However, the delivery times for the replans were still shorter than that for the clinical plans because the jawless linac has a much higher dose rate (800 MU/min) compared to the conventional linac (300 MU/min). Conclusion: For challenging HN cases, the novel jawless linac with fast MLC collimation achieved similar plan quality and improved delivery efficiency compared to the conventional linacs. Author Disclosure: Z.L. Shen: None. P. Kosterin: None. P.T. Teo: None. R.J. Lalonde: None. K. Plakan: None. D. Peskorski: None. D.E. Heron: No personal compensation; Accuray Exchange in Radiation Oncology. Partnership; Cancer Treatment Services International. In this role, I am a Board Member for UPMC CancerCenter; UPMC CancerCenter. Vice Chairman of Clinical Affairs for the Department of Radiation Oncology; University of Pittsburgh School of Medicine. M.S. Huq: Honoraria; Varian Medical Systems; American Association of Physicists in Medicine. Responsible for the management of the activities of TPC; AAPM. TU_35_3245 A Continuum Approach to Account for Uncertainties in Defining the Clinical Target Volume N. Shusharina, 1 D. Craft, 1 Y.L.E. Chen, 2 H.A. Shih, 1 and T. Bortfeld 1 ; 1 Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, 2 Department of Radiation Oncology, Massachusetts General Hospital, Boston, MA Purpose/Objective(s): The purpose of this study is to put forward a new approach that deals with delineation uncertainties of the clinical target volume (CTV) definition. Instead of a standard contour-based CTV we introduce a continuum distribution of probability of finding tumorous cells at a certain distance from the gross tumor volume (GTV) and call it the clinical target distribution (CTD). Materials/Methods: We test the new approach in clinical cases of glioma, and bone sarcoma patients. The CT scans of previously treated patients along with the contours of GTV and CTV were presented to radiation oncologists, one specializing in brain tumors and another in sarcoma. They were asked to draw the lines that would correspond to their levels of delineation uncertainty, or, equivalently, to the probabilities of tumor spread. Both physicians expanded the previously delineated CTV by approximately 20% by drawing the line of 0% probability to find the tumor at further distances. The levels of 80%, 60%, and 40% were drawn taking into account anatomic structures surrounded the GTV. IMRT treatment plans were created and optimized to deliver prescription dose to the space between the drawn levels with the coverage objective weighted with the corresponding probability. These plans were compared with the plans optimized to deliver the same dose to traditionally defined CTV. Results: The CTD-based plan for glioma was optimized using three different levels of the mean brain dose. The physician appreciated a naturally occurring option of adjusting the coverage to different levels of tumor probability by choosing an appropriate tradeoff between the coverage and organ sparing. The CTV- and CTD-based plans with similar coverage were different by the mean brain dose being lower for the CTD by 23%. For the sarcoma case, both plans achieved the goal of sparing the OARs, although the dose distribution within extended target was different. The CTD-based approach was superior to the conventional CTV approach in covering regions marked with higher probabilities of finding tumor cells by shaping the lower dose regions around OARs so that the dose fall-off followed the decrease of tumor probability. Conclusion: CTD allows to find the most suitable tradeoff between target coverage and sparing of surrounding healthy organs at the treatment planning stage, without having to modify or redraw a CTV. Owing to the variable probabilities afforded by the CTD, a more flexible and more clinically meaningful sparing of critical structure becomes possible. Author Disclosure: N. Shusharina: None. D. Craft: None. Y.L.E. Chen: Employee; Beth Israel Deaconess Medical Center. H.A. Shih: Employee; Dartmouth Hitchcock. Honoraria; UpToDate. Hospital site residency program director; Harvard Radiation Oncology Program. Clinical opera- rions director; MGH Proton Therapy Center. Clinical operational leader; Massachusetts General Hospital. T. Bortfeld: Research Grant; RaySearch AB, Stockholm Sweden. TU_35_3246 What Dose Specification Should be used for NRG Radiation Therapy Trials, Dose-to-Medium or Dose-to-Water? M. Cabanas, 1 C. Yan, 1 R.J. Lalonde, 2 D.E. Heron, 3 and S. Huq 4 ; 1 UPMC, Pittsburgh, PA, 2 D3 Oncology Solutions, Pitttsburgh, PA, 3 UPMC Hillman Cancer Center, Pittsburgh, PA, 4 Department of Radiation Oncology, UPMC Hillman Cancer Center, University of Pittsburgh School of Medicine, Pittsburgh, PA Purpose/Objective(s): Monte Carlo dose calculation is currently the most accurate algorithm, but it is still too computationally intensive and slow to be used routinely in clinics. An implementation of the Linear Boltzmann Transport Equation, Acuros XB dose calculation algorithm, has similar accuracy to Monte Carlo but is much faster, and has already been used in many clinics. Like Monte Carlo, Acuros allows the user to report dose to water or dose to medium. The main aim of this study is to compare and evaluate both Acuros XB dose-to-medium D m,m and dose-to-water D w,m algorithms with the widely used AAA algorithm. Materials/Methods: Twenty one lung patients that previously received IMRT or VMAT treatments at our institution were analysed by recalcu- lating plans for each one with AAA algorithm (reviewed and approved by our radiation oncologists) and with both reporting modes of Acuros XB Abstract TU_35_3244: Table 1 Plan comparison (nZ5) Clinical Replan Clinical vs. Replan (p-value) HD_PTV V HD (%) 96.6  1.9 96.9  1.6 0.639 LD_PTV V LD (%) 96.0  1.7 97.3  1.8 0.166 Brainstem D max (Gy) 31.1  15.6 29.2  13.7 0.290 Spinal_cord D max (Gy) 39.6  3.9 38.4  1.1 0.541 Parotid_L D mean (Gy) 29.0  6.1 27.5  6.6 0.099 Parotid_R D mean (Gy) 26.4  3.8 25.5  4.1 0.484 Esophagus D mean (Gy) 52.9  18.5 52.5  19.4 0.553 Larynx D mean (Gy) 52.3  22.2 53.5  22.3 0.438 Oral Cavity D mean (Gy) 31.2  11.0 30.4  9.4 0.497 Mandible D mean (Gy) 30.5  13.6 29.4  11.6 0.530 HI 1.09  0.01 1.09  0.02 0.541 CI 0.96  0.02 0.97  0.02 0.109 MU 1495  207 2584  407 0.002 Volume 102  Number 3S  Supplement 2018 Poster Viewing Q&A Sessions E531