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to minimize this error. The second is machine delivery error and although quality assurance measurements are done prior to treat-ment, there is not a good method to monitor the actual delivered dose which is often complicated by highly intensity-modulated X-rays. Multi-leaf collimator sequence errors could lead to dosimetric errors which may not be detectable currently. Since direct mea-surement of the radiation dose is not feasible, exit radiation received by MV detectors when treating patients can potentially be usedto reconstruct 3D dose. In this study, detector signals from radiation exiting the patient on a TomoTherapy Hi-Art system are an-alyzed and reconstructed by a dose-verification (DV) software tool. The reconstructed dose is compared with the calculated dose.

Materials/Methods: Ten spinal patients were prescribed 24 Gy delivered in 3 fractions, which were further divided into six sub-fractions of 4 Gy. Treatment plans were generated on TomoTherapy Hi-Art Planning Software V. 3.1 using a pitch of 0.287, a jaw of2.5 cm and a nominal modulation factor of 2.5. For each patient, there was a QA plan performed on a cylindrical phantom with ionchamber measurement at the center of the phantom. All patients were treated with image guidance using bony anatomy alignment.The detector signals were first backprojected to calculate the entrance sinogram using attenuation correction based on the MVCT, andthen reprojected on the CT images to calculate dose. The reconstructed point dose from the QA plan was compared with the ionchamber measurement. Cumulative reconstructed dose to the PTV and the spinal cord were compared with the original plan.

Results: A root mean square (RMS) error of 2.0% was observed comparing the DV and ion chamber measured point dose in thephantom plans. Ion chamber measurements for two patients were 4-5% different than the predicted value and the DV doses werewithin 1.5% of the measured values. For the 10 patients, the error of the minimal dose to the PTV varied from -7% to 2.4%, witha RMS error of 2.7%. The RMS error of maximum dose to the spinal cord was 1.9% with a range of -0.1% to 5.2%.

Conclusions: DV reconstructed dose is sensitive to dose delivery errors of more than 2-3%. Good agreement between the recon-structed and planned dose was observed on most patients with the exception of 1 PTV being underdosed and 1 spinal cord beingslightly overdosed. The study shows that real time dose verification is both feasible and necessary for spinal SBRT treatment.

Author Disclosure: P.W. Read, TomoTherapy, B. Research Grant; TomoTherapy, D. Speakers Bureau/Honoraria; W. Yang, To-moTherapy, B. Research Grant; TomoTherapy, D. Speakers Bureau/Honoraria; R. Jones, None; Q. Chen, TomoTherapy, A. Em-ployment; G. Olivera, TomoTherapy Inc, A. Employment; G. Sobering, TomoTherapy, A. Employment; J. Larner, None; K.Sheng, TomoTherapy, B. Research Grant; TomoTherapy, D. Speakers Bureau/Honoraria.

3278 Improvement of Delivery Efficiency for Static Stereotactic Body Radiotherapy (SBRT) by Converting to

the Variable Dose Rate Dynamic Arc Therapy

B. Yi, K. Prado, S. Feigenberg

University of Maryland, Baltimore, MD

Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) has revolutionized the treatment of medically inoperable lung withoutcomes similar to surgery. A common technique employs 10 or more static beams with total doses between 10 and 20 Gy perfraction. The technique requires longer treatment times which can be difficult for patients to tolerate, especially poor performancestatus patients. RapidArc (RA, Varian, CA) offers dynamic arc with variable dose rate, which makes the treatment times shorter.RA, however, involves modulation of intensities, which is not needed for SBRT. This study evaluates a method to deliver sophis-ticated therapy more quickly.

Materials/Methods: Three random cases were chosen for this study. Selection of the beam directions and optimization of the beamweights are performed by experienced dosimetrists/physicists and the plans were approved by a radiation oncologist. For each plan,every static field was converted into a dynamic arc. Gantry angle ranges of 20-30 degrees were assigned to each static field. Thesame monitor units were assigned to each corresponding dynamic arc. All arc segments were then merged into an arc sequence withvariable dose rate. A program enabling this procedure was developed, which generates a treatment DICOM plan compatible withRA delivery. The dose rate of each arc was calculated based on the weight of each beam. Dose distributions from the new arc planswere then compared to the static field plans using a Pinnacle V8.0 3-D planning system (Philips, WI).

Results: Very little differences between the dynamic arc and the static field plans in regards to tumor coverage and dose to the lungswere found. Dose differences in the V95 of the targets were less than 1%. In order to deliver a single dynamic arc, it was necessaryto assign a portion of monitor units to beam directions that were not included in the design of the static plan. Monitor units assignedto those regions were small: 0.1% of the total monitor units per control point, which is less than 5% of those of the total treatmentdelivery. All leaves were completely closed for those regions. Treatment time for delivering 12 Gy of the static field plan was 7-8minutes excluding the patient setup time for three cases when with 1000 mu/min dose rate. Treatment time with the variable doserate arc was less than 3 minutes for all three cases.

Conclusions: Quality of the plans of static fields and dynamic arc fields were identical. Delivery efficiency for fractions of 12 Gy,however, was improved by 60%: from 7-8 minutes for static fields, to less than 3 minutes for arcs. Dynamic arc therapy with vari-able dose rate appears to be both an effective and efficient way of delivering SBRT.

Author Disclosure: B. Yi, None; K. Prado, None; S. Feigenberg, None.

3279 Dosimetry of CyberKnife and Gamma Knife in Meningioma and Vestibular Schwannoma

S. Jang1,2, T. T. Sio2, S. Lee1,2, B. Curran1,2, G. Noren1,2, E. S. Sternick1,2, D. E. Wazer1,2

1Rhode Island Hospital, Providence, RI, 2Alpert Medical School of Brown University, Providence, RI

Purpose/Objective(s): The purpose of this study is to compare the dosimetric characteristics of CyberKnife (CK) and Gamma-Knife (GK) plans for treatment of meningiomas and vestibular schwannomas (VSs). Radiosurgical techniques are commonly ap-plied in the treatments of meningioma and VS using various modalities.

Materials/Methods: Twelve patients with 13 intracranial meningiomas and VSs were selected from the Leksell GK’s patient da-tabase. The two diseases were ideal for this unbiased comparison study, since no systematic or irregular margins were added to thetumors in the clinical practice of GK radiosurgery. Stereotactic MR (1.0� 2.0 mm slice thickness) and CT images (0.63 mm thick-ness) used in GK were transferred and re-registered in the CK planning system. After reproducing the contours of tumor and ad-jacent critical organs, algorithm-driven (i.e., sequential planning) CK treatment plans were prescribed with identical prescriptiondose (PD) as used in GK. Both CK and GK plans were calculated without margins surrounding the target volume. From the dose-

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volume histogram, the conformality index (CI: PTVPD/tumor isodose volume),% coverage, dose gradient index (GI), volume dif-ference between PTV50%PD and PTVPD, and modified homogeneity index (mHI: [D5-D95]*100/PD) were obtained. Paired t-testswere used for statistical analysis.

Results: The tumor volumes ranged from 0.04 to 5.30 cm3. Mean doses of 12.3 ± 0.5 Gy and 12.7 ± 0.5 Gy were prescribed to themeningioma (n = 7) and VS (n = 6) cases, respectively. Mean prescription isodose lines (IDLs) were 79.3 ± 7.6% for CK, and 50%for all GK plans. Mean target coverages were 96.5 ± 4.1% for CK, and 98.3 ± 2.5% for their respective GK plans. Mean GIs were7.37 ± 6.2 for CK, and 3.01 ± 0.3 for GK. Particularly, cases with tumor volume less than 1 cm3 resulted in GIs greater than 4.0 forCK, whereas the respective GIs for GK ranged between 2.0 and 4.0. The difference in GIs for CK and GK was statistically sig-nificant (p \ 0.021). Mean volume differences (PTV50%PD-PTVPD) were 8.5 ± 7.4 cm3 for CK, and 4.4 ± 4.8 cm3 for GK.Mean mHIs were 18.8 ± 11.4 for CK vs. 68.4 ± 7.1 for GK (p \ 0.001).

Conclusions: Both CK and GK planning systems produced comparable target coverages for all meningioma and VS cases. WhileCK provided more homogeneous dose distributions within the target, GK irradiated less exposed volume in normal tissues at lowradiation doses (for example, irradiated volume $ 50% of PD). Dosimetric differences in small size tumors (\0.6 cm3) noticeablyfavored the Gamma Knife system; however, careful correlations with clinical outcome should be investigated. Both systems can beoptimized more efficiently, as we apply each system’s unique dosimetric characteristics in the daily planning and treatment deliveryof radiosurgical procedures.

Author Disclosure: S. Jang, None; T.T. Sio, None; S. Lee, None; B. Curran, None; G. Noren, None; E.S. Sternick, None; D.E.Wazer, None.

3280 Whole Brain Radiotherapy with Simultaneous Integrated Boost (WBRT+SIB) for Multiple Brain

Metastases (BM) using Volumetric Modulated Arc Therapy

F. J. Lagerwaard, W. S. C. Eppinga, C. J. A. Haasbeek, P. F. de Haan, B. J. Slotman

VU University Medical Center, Amsterdam, Netherlands

Purpose/Objective(s): Treatment planning with a combination of WBRT and SIB to multiple BM has been reported by severalauthors (Bauman ‘07; Lagerwaard ‘09; Hsu ’10). Because of the superior dosimetry of this technique compared to WBRT withconventional stereotactic radiosurgery, we have adopted this approach as standard treatment in pts with multiple BM and ptswho received postoperative WBRT with a boost to residual BM. We report on the clinical outcomes in a cohort of pts treatedwith WBRT+SIB.

Materials/Methods: WBRT+SIB was delivered to 50 consecutive pts from June 2008 to March 2010 using RapidArc (RA; Varianmedical systems) with a fractionation scheme of 5x4 Gy WBRT and SIB to the BM of 5x4 Gy, as previously described (Lagerwaard’09). The median age of the 30 males and 20 females included was 61 years. Primary tumors were lung cancer (66%), breast cancer(12%), melanoma (12%), and others (10%). The median number of BM receiving a SIB was 3 (range, 1-6). The median cumulativevolume of the treated BM was 6 cm3 (range, 0.2-29.7 cm3). The largest BM treated was 27.5 cm3. Pts were followed-up with 3-monthly MRI scans, and GP’s were contacted to ensure full data on date and cause of death.

Results: All pts completed the WBRT+SIB, delivered using two volumetric arcs within 3 minutes beam-on time. The median over-all survival (OS) was 6.2 months, and survival rate at 1 year was 25.6%. OS correlated with RTOG RPA class, with grouped pts inRPA I (n = 3) and RPA II (n = 36) having a median survival of 6.9 months in contrast to only 2.2 months in 11 RPA III pts (p =.009). Neither the number of BM boosted (#3 BM vs. .3 BM; p = .22), nor the cumulative volume of the BM (#10 cm3 vs. .10cm3; p = .99) correlated with OS. Progressive intracranial disease (PD), defined as radiological and/or clinical PD was observed in15 pts (30%). Actuarial freedom from intracranial PD rates were 67.9% and 47.5% at 6 months and 1 year, respectively. Extracra-nial PD was the main cause of death in 19 of 30 deceased pts (63.3%). Early toxicity was mild with fatigue (28%), headache (16%)and nausea (8%) being most frequently reported; 44% of pts reported no side effects. In 3 of 15 pts who underwent follow-up MRIimaging at 6 months, white matter abnormalities consistent with radiation-induced leukoencephalopathy were seen. In all cases,these were asymptomatic. In 3 pts, MRI scans at 6 months showed a volume increase of a treated BM in combination withhypo-perfusion, consistent with tumor necrosis, for which one patient needed neurosurgery.

Conclusions: Combined WBRT+SIB using volumetric arc therapy is a effective option for pts with multiple BM, however, thisshould be reserved for pts with RPA Class I or II. For pts with expected longer survival, e.g., RPA Class I pts, a more protractedradiotherapy scheme may be preferable.

Author Disclosure: F.J. Lagerwaard, The VU University medical center has a research agreement with Varian medical systems, B.Research Grant; W.S.C. Eppinga, VU University medical center has a research agreement with Varian medical systems, B. Re-search Grant; C.J.A. Haasbeek, VU University medical center has a research agreement with Varian medical systems, B. ResearchGrant; P.F. de Haan, VU University medical center has a research agreement with Varian medical systems, B. Research Grant; B.J.Slotman, VU University medical center has a research agreement with Varian medical systems, B. Research Grant.

3281 Radiographic Toxicity Associated with Thoracic Stereotactic Radiotherapy

P. Mitra, D. Randolph, M. Parker, P. DeGroot, N. Mukhopadhyay, J. Karlin, J. Heffernan, M. Anscher, T. Chung, E. Weiss

Virginia Commonwealth University MCV, Richmond, VA

Purpose/Objective(s): Knowledge about treatment-related toxicity of extracranial stereotactic radiotherapy to the lung is still lim-ited. We conducted a retrospective review of patients treated with stereotactic radiotherapy at our institution to determine the in-cidence of radiographic toxicity and its correlation with dosimetric parameters.

Materials/Methods: We analyzed the records of 60 patients who received stereotactic radiotherapy to 75 lung lesions betweenApril 1999 and December 2008. Fractionation schemes ranged from 3 x 7-20 Gy to 6 x 5 Gy. A subset of 22 patients with 29 lesionswere identified who had a minimum of 12 months radiographic follow-up with serial CT scans of the chest to allow for develop-ment of late radiographic changes. Radiographic abnormalities on post-treatment CT scans were graded by two thoracic radiolo-gists using the LENT-SOMA scale to generate a consensus score. Radiologists had pretreatment scans for comparison todistinguish between tumor changes and new post-treatment lung changes. In addition, the radiologists were blinded to the treatment