Plan Robustness Study of Volumetric Modulated Arc Therapy Versus Intensity Modulated Radiation Therapy for Head and Neck Cancer

Volume 93  Number 3S  Supplement 2015 and 37 mm. The PET scanner geometry approximated a commercial scanner. 35 heterogeneity metrics based on intensity histograms (IH), cooccurrence matrices (COM), neighborhood difference matrices (NDM), and size zone matrices (SZM) were evaluated within ROI surrounding the lesions. For each metric, the COV of the 50 realizations was calculated. The COV of the SDIH within each ROI was used as a benchmark. In addition, sample size was estimated for number of patients required to detect a clinical effect of 30% and 15% with Bonferroni-corrected aZ0.001 and 95% power in a theoretical clinical trial. Results: The COV of the SDIH was 0.03e0.05 for the four lesions. The majority of metrics (24/35) had greater COV than SDIH, including the outlier case of skewness with COV up to 0.76 and 1.04 for the 28 and 22 mm lesions. The minority (11/35) of metrics had COV similar to or less than the SDIH: by class, they were energy and entropy (IH), dissimilarity, energy, entropy, sum entropy, and difference entropy (COM), and small zone emphasis, large zone emphasis, zone size non-uniformity, and zone percentage (ZSM). The COV was larger for smaller lesions and as a class, NDM features exhibited greatest COV. When variability estimates were extended to estimate sample size to detect clinical effects of 30% and 15%, the median sample size across all metrics was 7 and 17 patients for the smallest lesions. However, the upper quartile of sample size was 14 and 44 patients, and the metric with the greatest variability (skewness) required sample sizes of 587 and 2337 patients in the worst case. Conclusion: The sensitivity of PET textural features to normal stochastic image variation can be large and is feature-dependent. Standards are needed to ensure that prospective trials which incorporate textural features are sufficiently powered to detect outcomes. Author Disclosure: M. Nyflot: None. S.R. Bowen: None. F. Yang: None. D. Byrd: None. G.A. Sandison: None. P.E. Kinahan: Research Grant; GE Healthcare. Co-founder; PET/X LLC.

3423 Dosimetric Analysis of 10 Fraction Stereotactic Robotic Radiation Therapy for Early-Stage Glottic Larynx Cancer C. Ding,1 S.G. Chun,2 L.A. Nedzi,2 J.S. Yordy,3 J.C. Hodges, II,4 W. Mao,5 S. Cooley,6 B. Sumer,2 D.L. Schwartz,2 S.B. Jiang,7 and R.D. Timmerman2; 1The University of Texas Southwestern Medical Center, Dallas, TX, 2University of Texas Southwestern Medical Center, Dallas, TX, 3Valley Radiation Therapy Group, Anchorage, AK, 4University of Texas Southwestern Medical School, Dallas, TX, 5University of Texas Southwestern Medical Center, Dallas, TX, 6University of Texas at Southwestern Medical Center, Dallas, TX, 7University of Texas Southwestern Medical Center, Dallas, TX Purpose/Objective(s): Laryngeal cancer is the most common non-cutaneous head and neck malignancy in the USA, for which radiation therapy is a standard treatment for early-stage tumors. However, conventional radiation therapy using opposed lateral beams delivered over 6-7 weeks is inconvenient, costly, and unnecessarily irradiates large volumes of normal tissue including the thyroid gland, carotid arteries, uninvolved vocal cord, and the laryngeal skeleton. We hypothesized that robotic radiation therapy of only the involved hemilarynx has potential to deliver therapeutic doses to tumor while providing substantial sparing of adjacent normal tissue through geometric avoidance. We present dosimetric data of patients treated for early-stage larynx cancer on a single-institutional phase 1 clinical trial with robotic radiation therapy in 10 fractions comparing with IMRT treatment. Materials/Methods: Institutional review board approval was obtained for this phase 1 clinical trial. Patients with carcinoma in situ, or cT1aT2bN0M0 carcinoma of the glottic larynx cancer were eligible. Nine patients were treated with robotic radiation therapy to a dose of 45 Gy in 10 fractions to the involved hemilarynx. Radiation dosimetric parameters of each patient treated were analyzed with respect to dose distribution and using dose volume histograms. Protocol specified normal tissue contours included the arytenoids, cord, carotid arteries, thyroid, and skin.

Poster Viewing Session E567 Results: The average GTV was 1.24 cm3, CTV 2.7 cm3, and PTV 6.6 cm3 (STD 3.85, range 1.22-14.45 cm3). Dose was prescribed to the 85-88% isodose line, which provided 95-97% PTV coverage and conformity indices of 1.11-1.33. The average dose to the ipsilateral arytenoid was 49 Gy (STD 2.3, range 46.7-51.8), and contralateral arytenoid was 28.6 Gy (STD 7.9, range 22-42.2). The average maximum dose to the ipsilateral carotid artery was 20.3 Gy (STD 10, range 10.6-34.9), and average maximum dose to the contralateral carotid artery was 13.1 Gy (STD 6.9, range 6.35-22.2). The mean dose to the thyroid was 4.35 Gy (STD 2.7, range 1.1-7.8). SBRT plans delivered a 4.9 Gy lower contralateral arytenoid dose, 4.3 Gy lower ipsilateral carotid artery maximum dose, and similar mean thyroid doses, in comparison to IMRT plans developed for the same patients. Conclusion: We have analyzed the dosimetric parameters of nine patients treated with a 10 fraction robotic radiation therapy protocol for early-stage larynx cancer. Robotic radiation therapy was able to achieve reductions in high doses of radiation delivered to normal structures such as the carotid arteries and contralateral arytenoid compared to IMRT plans. These dosimetric parameters will be used to guide future prospective protocols using robotic radiation therapy for larynx cancer. Author Disclosure: C. Ding: None. S.G. Chun: None. L.A. Nedzi: None. J.S. Yordy: None. J.C. Hodges: None. W. Mao: None. S. Cooley: None. B. Sumer: None. D. Schwartz: None. S.B. Jiang: None. R.D. Timmerman: None.

3424 Plan Robustness Study of Volumetric Modulated Arc Therapy Versus Intensity Modulated Radiation Therapy for Head and Neck Cancer W. Liu,1 S.H. Patel,2 J. Shen,1 J. Stoker,1 X. Ding,1 Y. Hu,3 W.W. Wong,2 M.Y. Halyard,2 G.A. Ezzell,1 S.E. Schild,2 and M. Bues1; 1Mayo Clinic Arizona, Phoenix, AZ, 2Mayo Clinic, Phoenix, AZ, 3Washington University School of Medicine, St. Louis, MO Purpose/Objective(s): Studies have shown that plan robustness may be an important factor for local failure in volumetric-modulated arc therapy (VMAT) to treat head and neck (H&N) cancer. Therefore, we investigated the plan robustness of VMAT compared to the more commonly used static field intensity modulated radiation therapy (IMRT) using three robustness quantification tools. Although developed for proton therapy, it is useful to benchmark the effectiveness of the tools in photon external beam therapy since photons external beam therapy is considered to be significantly more robust than proton beam therapy. Materials/Methods: VMAT and IMRT plans were created for 9 H&N cancer patients. For each plan, six new perturbed dose distributions were also computed - one each for simulated  3mm setup deviations along the S-I, A-P and L-R directions. We used three robustness quantification tools: (1) worst-case analysis (WCA); (2) dose-volume histograms (DVHs) band (DVHB); and (3) root-mean-square-dose volume histogram (RVH) to compare plan robustness of VMAT and IMRT. In WCA we extracted the highest and the lowest dose per voxel from the 7 doses. A shaded area in the DVH plot bounded by the DVHs from the lowest and highest dose per voxel was then displayed. In DVHB we displayed the envelope of all DVHs in band graphs of all the 7 dose distributions. The width from these two methods at different target DVH indices (such as D95 for cold spot and D5 for hot spot) was used to indicate plan robustness. In RVH the root-mean-square dose deviation (RMSD) per voxel was calculated from the 7 dose distributions and was then used to construct the RVH, analogous to computing a DVH. The area under the RVH curves (AUC) for the target was used as quantitative measures of plan robustness. In all three robustness quantification tools, the smaller the value, the more robust the plan is. The plan robustness evaluation metrics of the VMAT and IMRT groups were compared using Wilcoxon test. A p-value less than 0.05 was considered statistically significant. Results: DVHB showed the width at D95 from IMRT to be larger than from VMAT (unit Gy; IMRT vs VMAT) [1.59 vs 1.18 (pZ0.65)], while

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the width at D5 from IMRT was found to be slightly larger than from VMAT [0.59 vs 0.54 (pZ1.0)]. WCA showed similar results [D95: 3.28 vs 3.00 (pZ0.65); D5: 1.68 vs 1.95 (pZ0.36)]. RVH showed the AUC from IMRT to be slightly smaller than from VMAT [1.13 vs 1.15 (pZ0.57)]. RVH represents the relative volume (y) on the vertical axis and the RMSD (x) on the horizontal axis. Similar to DVH assessing the nominal dose, this means that y% of the volume of the indicated structure has the RMSD at least x Gy. Conclusion: VMAT plan robustness is comparable to IMRT plan robustness. The plan robustness conclusions from WCA and DVHB are DVH parameter and patient dependent. On the other hand RVH captures the overall effect of uncertainties on the dose to a volume of interest. Author Disclosure: W. Liu: Research Grant; NIH, The Fraternal Order of Eagles Cancer Research, The Lawrence W. and Marilun W Matteson Fund, The Kemper Marley Foundation, ASU. S.H. Patel: None. J. Shen: None. J. Stoker: None. X. Ding: None. Y. Hu: None. W.W. Wong: None. M. Halyard: None. G.A. Ezzell: None. S.E. Schild: None. M. Bues: None.

City Hospital, Okazaki, Japan, 3Department of Radiology and Radiation Oncology, Aichi Medical University, Nagakute, Japan, 4Nanbu Tokushukai Hospital, Shimajiri-gun Yaese-machi, Japan, 5Department of Radiology, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan

3425 Real Time 3D Beam Visualization for Monitoring External Beam Radiation Therapy C.H. Jenkins,1 D.J. Naczynski,2 S.J. Yu,2 and L. Xing1; 1Stanford University School of Medicine, Stanford, CA, 2Stanford University, Stanford, CA Purpose/Objective(s): The administration of radiation therapy has advanced steadily throughout its history. However, in spite of large advances in planning accuracy and motion tracking techniques, the actual administration of radiation continues to be a blind procedure. We propose that a system consisting of a flexible scintillation sheet and an array of digital cameras can be used to simultaneously visualize the patient surface and the delivered radiation beam in three dimensions and in real time. Materials/Methods: For the proof of concept system, scintillating sheets were made by mixing Gd2O2S:Tb (GOS) with silicone and casting the mixture into a thin sheet. The sheet was placed on a solid water phantom and irradiated with therapeutic photon beams from a medical linear accelerator (LINAC). The light emitted from the sheet was collected using a pair of cameras calibrated as a stereo pair. Custom designed image processing software was used to reconstruct the 3D scene and extract the beam profile. The shape and location of the extracted profile was compared to known collimator settings for validation. Results: The system was capable of acquiring high quality images of both the phantom and the beam under various levels of ambient room lighting including those most commonly used during therapy. Images were obtained at a rate of 20 fps. The system demonstrated sub-millimeter resolution and accuracy in identifying both the shape and location of radiation beams. The intensity of the beam profile was found to be linear with dose rate and correlated to the expected surface dose as beam energy changes. Conclusion: A system consisting of digital cameras and a flexible scintillation sheet is capable of three dimensional visualization of external beam radiation therapy in real-time. The accuracy of the data is sufficient that further development of the system may provide the ability to verify the accuracy of treatment as it is occurring and provide a record for posttreatment analysis. Author Disclosure: C.H. Jenkins: None. D.J. Naczynski: None. S. Yu: None. L. Xing: None.

3426 Efficacy of Stereotactic Radiation Therapy for Brain Metastases Using Dynamic Jaws Technology in the Helical Tomotherapy System: Planning and Clinical Studies T. Murai,1 A. Hayashi,2 Y. Mori,3 M. Yoshihiko,4 T. Yanagi,1 M. Matsuo,1 T. Takaoka,1 Y. Ogawa,5 T. Kondo,5 and Y. Shibamoto1; 1Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan, 2Okazaki

Purpose/Objective(s): Stereotactic radiation therapy (SRT) for brain metastases (BMs) can be performed using helical tomotherapy. Recently, a new dynamic jaws (DJs) technology has been introduced into tomotherapy. In the new system, jaw sizes are optimized during sweeping across the target, and reduction of treatment time and improvement in dose distribution may be expected using this system. To evaluate the DJs system in SRT for brain metastases, we first conducted a planning study to compare 2 treatment plans with and without the DJs system. As a second step, the efficacy was evaluated in actual patients. Materials/Methods: In the planning study, imaginary spherical PTVs with a diameter of 10-22 mm were contoured on 5 patients’ head CT images. Treatment plans were calculated for the following 2 conditions: 1-cm field width with fixed jaws (FJs) and 2.5-cm field width with DJs. Prescribed dose was 30 Gy/3 fr. Evaluated parameters were the treatment time, homogeneity index and conformity index (CI) for PTV, and D20Gy and D10Gy (cc) for the brain. To evaluate clinical efficacy, we conducted a study of tomotherapy SRT using DJs or FJs for BM patients. Demographic, treatment and outcome data were analyzed. Results: In the planning study, irrespective of the PTV diameter, 2.5-cm field width DJs plans could shorten treatment time and had almost the same dose distribution as in 1.0-cm field-width FJs plans. Treatment time decreased to 73% in 2.5-cm field-width DJs plans compared to 1cm field-width plans (p< 0.0001). Homogeneity index and CI in 2.5-cm field-width DJs plans were comparable to 1-cm field-width FJs plans. V20Gy and V10Gy did not differ significantly between the two plans. In total, 27 patients (performance status; 0-2) were treated with tomotherapy as the first line therapy. Among them, 7 were treated with whole brain radiation (No of BMs > 4) and excluded from this analysis. Twenty patients with 31 BMs underwent SRT (30 Gy/3 fr or 35 Gy/5 fr) (No of BMs  4). All patients were RPA class II. Primary tumor site was the lung in 13 patients, breast in 2, and others in 5. Three fractions were applied to 5 BMs and 5 fractions to 26 BMs. The local control (LC) rate was 87% at 12 months. Median overall survival was 11 months. Ten patients with 15 BMs underwent SRT using 2.5-cm field-width DJs and others using 1.0-cm field-width FJs. PTV (cc) did not differ significantly between the DJs and FJs groups (11.4 cc vs. 13.9 cc). In patients treated with the DJs system, the treatment time was 75% shorter and the LC rate was 100% at 12 months. Brain necrosis (Grade 2) was observed in 2 patients; one had been treated with DJs and the other with FJs. Conclusion: The DJs system offers treatment time reduction and dose distribution comparable to that obtained by the FJs system using a smaller jaw width. Clinical results are favorable and further studies are warranted. Author Disclosure: T. Murai: None. A. Hayashi: None. Y. Mori: None. M. Yoshihiko: None. T. Yanagi: None. M. Matsuo: None. T. Takaoka: None. Y. Ogawa: None. T. Kondo: None. Y. Shibamoto: None.

3427 Whole-Brain Radiation Therapy With Hypothalamic-Pituitary Axis and Hippocampus Avoiding: A Planning Study X. Fan; Fudan University Shanghai Cancer Center, Shanghai, China Purpose/Objective(s): Hypothalamus-pituitary axis, the vital organs for endocrine regulation, is sensitive for irradiation and associated with cognitive impairments, somnolence syndrome, and fatigue after brain irradiation. The purpose was to evaluate the feasibility of delivering wholebrain radiation therapy (WBRT) with hypothalamic-pituitary axis and hippocampus avoiding (HPA-HA-WBRT). Materials/Methods: Ten lung cancer patients with brain metastases underwent planning using linear accelerator based intensity modulated