3603 Toxicity Profile in Non-Small Cell Lung Cancer Patients Treated With Lung Stereotactic Body Radiation Therapy Using the Modified Dynamic Conformal Arc (MDCA) Technique E. Ku, 1 A.Y. Desai, 1 D. Fang, 2 C. Lawrence, 2 C.M. Iannuzzi, 2 and C. Shi 2 ; 1 Frank H. Netter M.D. School of Medicine at Quinnipiac University, North Haven, CT, 2 St. Vincent’s Medical Center, Bridgeport, CT Purpose/Objective(s): This is one of the first studies to review follow-up data on patients treated for stage I/II non-small cell lung cancer (NSCLC) with stereotactic body radiation therapy (SBRT) using the modified dynamic conformal arc (MDCA) planning technique. Currently, only a few centers use the MDCA technique for lung SBRTand publications tend to focus on the technical aspects rather than clinical outcomes. The purpose of our study was to report clinical and radiographic toxicity as well as local control rate on patient follow-ups in order to evaluate the effect of the MDCA technique on posttreatment toxicity and radiographic changes over time. Materials/Methods: We retrospectively reviewed data from 15 patients treated for lung lesions with SBRT between October 2011 and July 2014 utilizing the MDCA technique. This technique uses coplanar beams, the body center for beam isocenter, and 6 partial rotation conformal arcs to target the tumor. Treatment planning parameters were based on Radiation Therapy Oncology Group (RTOG) guidelines. Patients received 3 to 7 fractions with a minimum of 48 hours between fractions. The majority of patients received 5 fractions in total. Median total dose was 60 Gy (range: 45 to 70 Gy). Clinical and radiograph data was gathered from electronic patient health records (EHR). Results: Median follow-up was 18 months after treatment (range: 6-51 months). Median age was 72.5 years (range: 48-90 years). Patient cancer staging was nonbiopsied (n Z2), unspecified t1 (nZ2), t1a (nZ7), t1b (nZ2), and t2 (nZ2). Histology was adenocarcinoma (nZ9) and squa- mous cell carcinoma (nZ3). Treatment-related clinical findings included fatigue (nZ5) and chronic chest wall pain (nZ1). Seven patients reported pulmonary/upper respiratory symptoms, which included cough (nZ4), dyspnea (nZ5), and hemoptysis (nZ1). No patient deaths or toxicity grade 4 events related to treatment were recorded. Computed tomogra- phy (CT) imaging showed radiographic scarring in 6 patients. Local control rate was 97% (nZ14), and 1 patient had local recurrence. Conclusion: Our results using the MDCA technique with SBRT for stage I/ II NSCLC revealed an excellent toxicity profile consistent with previous RTOG 0236 findings. Our local control rate, 97%, was very close to their estimated 3-year primary tumor control rate of 97.6%. Toxicity rates were also similardRTOG 0236 constitutional symptoms and pulmonary/upper respiratory symptoms rates were 36.4% and 60.0%, respectively, while ours were 33.3% and 46.7%, respectively. We were limited by a small sample size and relatively short follow-up, but our findings support the use of the MDCA technique for lung SBRT. Author Disclosure: E. Ku: None. A.Y. Desai: None. D. Fang: None. C. Lawrence: None. C.M. Iannuzzi: None. C. Shi: None. 3604 Withdrawn 3605 Novel Dosimetric Comparison of Intensity Modulated Radiation Therapy Versus Volumetric Modulated Arc Therapy Delivery When Using Dental Stents for Protection of Oral Mucosa E. Allan, 1 H. Hamedani, 2 M. Van Putten, 3 and D.M. Blakaj 1 ; 1 The James Cancer Hospital and Solove Research Institute, Wexner Medical Center at The Ohio State University, Department of Radiation Oncology, Columbus, OH, 2 The Ohio State University Wexner Medical Center, Columbus, OH, 3 The James Cancer Hospital and Solove Research Institute, Wexner Medical Center at The Ohio State University, Department of Maxillofacial Prosthodontics, Columbus, OH Purpose/Objective(s): Oral mucositis is a nearly universal toxicity observed during head and neck radiation therapy. Incident X rays liberate short-range electrons that deposit excess dose into adjacent mucosa, causing deep, painful ulcers. This contributes to patient morbidity and possibly leads to breaks in therapy, decreasing treatment efficacy. Avariety of protective dental measures have been proposed, and some have been tested in simple phantoms using single-beam arrangements. Our objective was to generate the first quantitative assessment of electron scatter and stent efficacy using an anatomically realistic phantom and a modern beam configuration and delivery method in order to better address our patients’ quality of life during treatment. Materials/Methods: A phantom was created to simulate a complete oral cavity with 2 sets of removable gold caps on opposing molars. A set of 4- mm upper and lower ethylene copolymer dosimetric stents was fabricated to provide space between the mucosae and crowns. A water bath was used to simulate facial soft tissue. A linear accelerator delivered 6-MV photons in opposed lateral, IMRT, and VMAT configurations. We used self- developing dosimetry film for dosimetric measurement in both the occlusal plane and the vertical plane to simulate mucosal surfaces of the tongue and the cheek. A dose of 200 cGy was prescribed to the tongue region. Measurements were made both with and without the stents in place. Film data were digitized using linear accelerator quality assurance software, and ImageJ open source software was used to measure the areas encompassed by isodose lines at 20-cGy intervals. Results: Both the IMRT as well as VMAT delivery demonstrated re- ductions in maximum dose when using dental stents. However, the improvement was more marked in the VMAT scenario (40% vs 27%). In the IMRT setting, the dental stents were also not as effective at reducing the area of film receiving 50% of the prescription dose, with a 23% versus 29% reduction (6 cm 2 vs 7.5 cm 2 ) in occlusal plane and an 8% versus 17% (2.7 cm 2 vs 5.5 cm 2 ) reduction in buccal plane. Conclusion: Dosimetric dental stents are valuable tools when treating head and neck cancer patients with dental crowns, as our results imply the oral mucosa in these patients may receive in excess of 100 Gy in certain configurations. Dosimetric dental stents may provide a more effective mucosal protection when using VMAT treatment delivery as compared to IMRT. This has important implications for patient quality of life and treatment efficacy as IMRT and VMAT treatment delivery become increasingly prevalent. Author Disclosure: E. Allan: None. H. Hamedani: None. M. Van Putten: None. D.M. Blakaj: None. 3606 XACT: A Novel Imaging Technique for Radiation Therapy Guidance L. Xiang, 1 , 2 S. Tang, 1 M. Ahmad, 2 and L. Xing 3 ; 1 Oklahoma University, Norman, OK, 2 Stanford University, Stanford, CA, 3 Department of Radiation Oncology, Stanford University, Stanford, CA Purpose/Objective(s): We report a new imaging modality, X ray-induced acoustic tomography (XACT), which takes advantage of high sensitivity to x-ray absorption and high ultrasonic resolution in a single modality. This new imaging modality will help improve the precision in radiation therapy and optimize treatment strategies. Materials/Methods: In XACT, we detect the acoustic signals generated by X rays. The initial pressure rise p 0 induced by X ray can be written as p 0 ZGh th mF. Where G is Grueneisen parameter, h th is the percentage of absorbed energy that is converted to heat, m denotes the x-ray absorption coefficient, and F denotes the x-ray fluence. When focusing on the variation in the local absorption coefficient, we have Dp 0 /p 0 ZDm/m. Considering that, where r is the mass density, s is the absorption cross- section, and A are Avogadro number and atomic number, then the acoustic pressure variation Dp 0 is proportional to the variation of tissue characteristics DsDr. Dr denotes the change in tissue density, while Ds reflects the change in tissue compositions. Therefore, any fractional change in tissue characteristic translates into an equal amount of frac- tional change in the XA signal. The XACT system comprises 3 major hardware components: an x-ray tube to produce ultrashort pulsed X rays for generating the XA signals; a low noise ultrasound detector; and a signal processing system. The x-ray generator can provide ultrashort International Journal of Radiation Oncology  Biology  Physics E654