Evaluating the Utility and Accuracy of CT Imaging for Decision Making in Laryngectomy Cases

E612

International Journal of Radiation Oncology  Biology  Physics

3534

reduce the dose to nearby critical structures. Previous studies of digital patients with lung tumors showed that EGRT can deliver high doses to the gross tumor volume (GTV) while minimizing doses to organs at risk. Here we created EGRT plans for two patients with thoracic malignancies and compared doses from these plans with those from intensity-modulated RT (IMRT), volumetric modulated arc therapy (VMAT), and intensitymodulated proton (IMPT) therapy. Materials/Methods: Initial treatment volumes were contoured on positron emission tomography/computed tomography (PET/CT) images from two patients. Patient 1 received VMAT for two bronchoalveolar carcinomas in the right middle lobe (50 Gy/10 fractions & 60 Gy/10 fractions) and presented with a recurrence in a right lower paratracheal node. The GTV was expanded by 6 mm to create the clinical target volume (CTV), which was then expanded by 5 mm to form the planning target volume (PTV). VMAT and IMPT plans were created to administer 60 Gy in 10 fractions to the PTV. Patient 2 had a newly diagnosed 2.5 cm  2.5 cm peripheral left upper lobe nodule consistent with adenocarcinoma. The GTV was expanded by 8 mm to create the PTV. VMAT and IMPT plans were created to treat the PTV to 50 Gy in 4 fractions. EGRT plans were generated to treat the same doses. List-mode emission data from FDG-PET scans were used in EGRT simulations per a published method. 4D PET/CT and breathing trace data were used to estimate emission detection and dose for each respiratory phase to arrive at point-of-view dose estimates for each target and structure. No motion model was used, as EGRT does not assume motion periodicity. Results: For the first patient, EGRT produced maximum doses to 10 cm3 of aorta (4875 cGy EGRT, 5460 cGy VMAT, and 4748 cGy IMPT); corresponding bronchus V50 values were 3.8 cm3, 7.6 cm3, and 7.6 cm3, and mean GTV doses were 7474 cGy, 6263 cGy, and 6647 cGy. For the second patient, EGRT led to lower chest wall V30 (12.1 cm3 vs. 74.1 cm3 VMAT, 37.7 cm3 IMPT), lower left lung V20 (171.6 cm3, 309.5 cm3, 261.3 cm3), and higher dose to the GTV (5967 cGy, 5325 cGy, 5587 cGy). Conclusion: EGRT’s ability to provide image-guided RT in real time has the potential to reduce treatment-related uncertainty. This was the first planning study to compare EGRT with VMAT and IMPT for thoracic tumors, which are greatly affected by tumor motion. EGRT reduced the radiation dose to proximal critical structures while providing escalated doses to the GTV. Further work is needed to validate these findings in phantoms. Author Disclosure: S.N. Seyedin: None. O.R. Mawlawi: None. L.M. Turner: None. S. Mazin: Stock; RefleXion Medical. President; RefleXion Medical. Y. Voronenko: Stock; RefleXion Medical. P.D. Olcott: Stock; RefleXion Medical. C.A. Wages: None. P. Balter: None. J.Y. Chang: None. D.R. Gomez: None. R.U. Komaki: None. J.W. Welsh: Consultant; RefleXion Medical. Stock; RefleXion Medical.

Therapeutic Advantage of Intrabeam IORT for Treating Shallowly Populated Breast Cancer Lesions M. Schwid,1 H. Zhang,1 and E.D. Donnelly2; 1Department of Radiation Oncology, Northwestern University Feinberg School of Medicine, Northwestern Memorial Hospital, Chicago, IL, 2Northwestern University Robert H. Lurie Comprehensive Cancer Center, Chicago, IL Purpose/Objective(s): The purpose of this study was to use radiobiological models to predict the therapeutic impact of Intrabeam intra-operative radiation therapy (IB-IORT) for treating breast cancer after lumpectomy compared to external beam IORT (EB-IORT) such as Mebetron-based IORT, for different cancer cell distributions within the tumor bed. Materials/Methods: Based on the commissioning data, the three dimensional (3D) radiation doses of IORT using a 4-cm diameter spherical applicator at the energy of 50 keV were calculated. The cancer cells were assumed to have different depths of infiltration and populated with different density distributions after surgery. The modified linear quadratic model (MLQ) was used to estimate the radiobiological response of the tumor and interspersed normal cells with different radio-sensitivities. Equivalent uniform doses (EUD) were calculated for two types of breast cancer cells (acutely responding: a/bZ10 and slow responding: a/bZ3.8). 20 Gy dose was prescribed at the applicator surface. Cancer cell distributions in the postsurgical tissue field were assumed to be the Gaussians with the standard deviations of 0.5, 1.0 and 2.0 mm respectively, which also correspond to the cancer cell infiltration of 1.5, 3.0 and 6 mm respectively. The maximum cancer cell percentages of 10%, 1%, 0.1%, and 0.01% at the surface were respectively tested. By comparing the average survival fraction of normal cells in IB-IORT and at the EUD the therapeutic ratios (TRs) were calculated. Results: The EUDs were found to be dependent on the distributions of cancer cells, but independent of the cancer cell radio-sensitivities and the density of cancer cells at the surface. For a prescription dose of 20 Gy, EUDs are 17.51, 16.11 and 12.96 Gy respectively for 0.5, 1.0 and 2.0 mm for the standard deviation of Gaussian distributions. The TR was found to be strongly dependent upon cancer distributions and cancer cell radiosensitivities and weakly dependent on the cancer cell density. At the above standard deviations of Gaussians and 1% cancer cell density at the surface, TRs were 2.59, 25.68 and 57.44 for the acutely responding breast cancer, and 2.52, 21.94 and 45.26 for the slow responding breast cancer. When the surface cancer cell density decreased from 10% to 0.01%, TR only decreased by 3%. The IB-IORT favors the acutely responding breast cancer cells inside radiosensitive normal tissues. Conclusion: IB-IORT provided a pronounced therapeutic advantage in maximally killing cancer cells and sparing normal cells compared with that seen with single fraction, open uniform EB-IORT. This would decreases radiation related complications of the treatment and even cosmetic impacts. IB-IORT should be considered as the first choice among all types of IORTs for post breast conserving surgery treatment. Author Disclosure: M. Schwid: None. H. Zhang: None. E.D. Donnelly: None.

3535 Use of Emission Guided Radiation Therapy Can Better Spare Critical Structures Compared With Intensity Modulated Radiation Therapy, Volumetric Modulated Arc Therapy, or Proton Therapy S.N. Seyedin,1 O.R. Mawlawi,2 L.M. Turner,1 S. Mazin,3 Y. Voronenko,3 P.D. Olcott,3 C.A. Wages,4 P. Balter,1 J.Y. Chang,1 D.R. Gomez,1 R.U. Komaki,1 and J.W. Welsh1; 1MD Anderson Cancer Center, Houston, TX, 2Department of Imaging Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 3RefleXion Medical, Hayward, CA, 4 Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX Purpose/Objective(s): Emission-guided radiation therapy (EGRT) combines advances in medical imaging (positron emission) with real-time conformal RT to minimize uncertainties associated with tumor motion and

3536 Evaluating the Utility and Accuracy of CT Imaging for Decision Making in Laryngectomy Cases Z. Nicholas, G. Krempl, T.S. Herman, C.R. Bogardus, Jr, and C.L. Matthiesen; University of Oklahoma Health Sciences Center, Oklahoma City, OK Purpose/Objective(s): Twenty-three years ago the VA larynx trial demonstrated that T4 versus T1-3 lesions had a 59% vs 29% PZ0.001 probability of requiring a laryngectomy. Invasion of the thyroid cartilage was another factor that was indicative of needing surgery. These criteria have been used to make clinical decisions about larynx preservation using CT scan information. This study aims to evaluate the utility of a CT scan as a decision tool for determining accurate pathologic stages. Materials/Methods: This study was conducted in a single institution retrospectively evaluating 194 laryngectomy cases done between 2004 and 2014. We evaluated all laryngectomy cases done by 4 oncologic otolaryngologists. Sixty-six (34%) of the cases were performed based on clinical decisions after initial presentation, 103 (53%) received prior radiation therapy and recurred or developed a second primary, 19(10%) had insufficient information to evaluate and 6(3%) had benign or nonsquamous

Volume 93  Number 3S  Supplement 2015 histology. Pathology, imaging and clinical reports were reviewed by a single physician and the data was analyzed using a chi squared test. Thirtyseven separate variables were collected on each patient such as: pathologic T stage, invasion of the thyroid cartilage and preoperative imaging reports. Four separate groups were evaluated: pT3-T4 patients, pT4 alone, patients with radiologic reports of thyroid cartilage invasion and finally patients treated with laryngectomy at initial presentation. Results: In pT3-4 disease the sensitivity, specificity, positive and negative predictive values for thyroid cartilage invasion were calculated (54%, 86%, 92% and 55% p<0.0001) respectively. Similarly in pT4 disease alone the results were (80%, 81%, 71% and 87%, p<0.0001). In the group of thyroid cartilage invasion noted on imaging reports the results were (78%, 91%, 86% and 85% p<0.0001). Finally evaluating the primary laryngectomy cases (58%, 46%, 46% and 58% pZ0.81). Twenty-five of the 54 initial laryngectomy cases were postoperatively staged as T3 or below. Conclusion: This study suggests that CT based imaging has the capability of being highly specific and sensitive for T4 disease as a method for determining pathologic extent of disease prior to surgery. Interestingly though if the same evaluation is applied to only patients taken to surgery based on clinical decisions, the CT scan information no longer bears significance suggesting confounding information. This finding is also suggested by the 46% of the patients that went to surgery with a T stage of one to three. Physician and patient preference play a role along with multiple other clinical factors in the decision to pursue laryngectomy at diagnosis. Overall CT imaging represents an important tool to consider when deciding patient treatment plans in a multidisciplinary setting. Author Disclosure: Z. Nicholas: None. G. Krempl: None. T.S. Herman: None. C.R. Bogardus: None. C.L. Matthiesen: None.

3537 Effects on Scattered Radiation Through Flattening Filter Free (FFF) IMRT at 6 and 10 MV Beam Energy B. Gauter,1 S. Scho¨nig,1 L. Jahnke,2 F. Lohr,2 and F. Wenz3; 1Department of Radiation Oncology, University Medicine Mannheim, Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany, 2Heidelberg University, Mannheim, Germany, 3Universita¨tsmedizin Mannheim, Ruprecht-Karls University Heidelberg, Mannheim, Germany Purpose/Objective(s): Removal of the flattening-filter (FFF) in IMRT decreases matter in the therapeutic beam that contributes to formation of scattered radiation (SR). By this means, it is possible to reduce contamination with ionizing radiation distant from the treated volume. It is yet unclear, what effect various beam energies have on SR in fully modulated FFF-IMRT plans with identical dose distributions. Materials/Methods: We used a LINAC with 160-leaf MLC to irradiate IMRT plans with almost identical dose distributions and for each beam energy identical therapeutic efficacy for 6 and 10MV with and without flattening filter. Plan a) Emulation of large volume pelvic RT for lymphatic metastasized prostate cancer, single dose 2 Gy. Plan b) small volume plan to prostate/seminal vesicles, single dose 2 Gy. Plan c) Intensely modulated small volume plan to prostate/seminal vesicles with integrated prostateboost, single doses 2.25/3 Gy. All plans were applied to a RW3-slate phantom. SR was measured with semiflex chamber and a referencedosimeter in 40 cm distance cranial to the isocenter. Results: With both energies, 6 and 10MV respectively, FFF-IMRT resulted in significantly reduced SR in comparison to conventional IMRT (FFIMRT). The reduction was more prominent with 10MV FFF-IMRT than with 6MV FFF-IMRT. Comparing energies, FF-IMRT with 10MV led to more SR in comparison to 6MV FF-IMRT. In contrast, with FFF-IMRT less SR was measured with plans A) and B) when 10MV was used in comparison to 6MV, but not with plan C). Conclusion: FFF-IMRT removes effectively a relevant scatter body from the therapeutic beam. Small target volumes (with integrated boost volumes) require more intensive modulation, therefore leading to increased interaction of photons with the MLC. This omits the SR-saving effect of

Poster Viewing Session E613 FFF-IMRT. Because of different depth dose curves, more monitor units (MU) are used for comparable efficacy between 6 and 10MV. This leads to more SR with 6MV in comparison to 10MV in lesser modulated FFFIMRT. Again, this effect is obscured in small target volumes with several doses levels due to more MLC-modulation. Author Disclosure: B. Gauter: Employee; University Medical Center Mannheim, University of Heidelberg. S. Scho¨nig: None. L. Jahnke: Partner; University Medical Center Mannheim, University of Heidelberg. Honoraria; Elekta. Consultant; Elekta. F. Lohr: Research Grant; Elekta. Honoraria; Elekta. Advisory Board; C RAD. Travel Expenses; Elekta. Stock; IMUC S. F. Wenz: Research Grant; Zeiss, Elekta. Honoraria; Ipsen, Lilly, Roche, Celgene, Zeiss, Elekta. Advisory Board; Celgene, Elekta. Travel Expenses; Ipsen, Lilly, Roche, Celgene, Zeiss. Patent/License Fees/ Copyright; Zeiss. Chairman; University Medical Center Mannheim, University of Heidelberg.

3538 Feasibility Study of Real Time Ultrasound Intrafractional Monitoring in Liver Stereotactic Body Radiation Therapy Underactive Breathing Control S.K. Ng,1 L. Su,1 Y. Zhang,1 I. Iordachita,2 J.W. Wong,3 J.M. Herman,4 H.T. Sen,5 P. Kazanzides,5 M. A. Lediju Bell,5 and K. Ding1; 1Department of Radiation Oncology, Johns Hopkins University, Baltimore, MD, 2 Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD, 3Johns Hopkins University, Baltimore, MD, 4Johns Hopkins University School of Medicine, Baltimore, MD, 5Department of Computer Science, Johns Hopkins University, Baltimore, MD Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) allows delivery of high doses to liver tumor with steep dose gradients to surrounding normal tissue and thus limits risk of liver toxicity. Intra-fraction target motion remains one of the major sources of uncertainty in liver SBRT. It blurs the planned dose distribution and thus undermines the purpose of SBRT. Ultrasound is a cost effective and non-ionizing approach for motion monitoring. In this study, we investigated the feasibility of an existing ultrasound prostate monitoring system for liver SBRT under active breathing control (ABC) and its monitoring accuracy in simulated phantom experiment. Materials/Methods: An abdominal ultrasound phantom with six tumors inside the liver was used. The phantom was secured on a programmable respiratory motion platform. A 4D probe was modified for transabdominal imaging with flexible infrared tracking angle. A medical grade arm and bridge secure system (ABS) was developed to hold ultrasound probe. The motion platform was programmed to perform repeated mechanical movement to simulate ABC patient motion. Platform was programmed to move in superior-inferior direction, which is the major tumor motion direction observed in our patients. The movement step was set to be 0 mm with 20 seconds followed by 10 mm in inferior direction with 20 seconds in one cycle to simulate the expiration and inspiration breath hold phases of the ABC. In total of 10 cycles of movement were repeated and the expiration ultrasound was taken as the reference image. 4D ultrasound images were then acquired continuously for monitoring. Results: For the 10 cycles of movements of 10 mm in inferior direction, the average real-time movement distance reported by 4D motion monitoring system was 9.920.13 mm and with maximum deviation less than 0.3 mm compared with programmed step. No probe fixation alert was detected during the whole experiment by the infrared tracking system. Conclusion: Our 4D motion monitoring system was able to accurately recover the liver tumor motion in our phantom study. Our ABS system is able to hold the probe steady during the whole motion simulation. This suggests with modification, current 4D prostate ultrasound system can be applied to real-time intra-fractional motion monitoring in liver SBRT with ABC. Author Disclosure: S. Ng: None. L. Su: None. Y. Zhang: None. I. Iordachita: None. J.W. Wong: None. J.M. Herman: None. H. Sen: None. P. Kazanzides: None. M. Lediju Bell: None. K. Ding: None.