S800 I. J. Radiation Oncology d Biology d Physics Volume 78, Number 3, Supplement, 2010

associated with such treatments. In this study, we evaluated the intrafractional motion of mask immobilization using orthogonal kVimage guidance data.

Materials/Methods: Seventy-one intrafractional shifts (213 x, y, z data points) were analyzed for eleven multi-lesion patients treatedon our Novalis machine (median = 3 lesions, range, 2-6). Nine patients received a single fraction SRS treatment, and two patientswere treated with 10 SRT fractions each. The three-piece BrainLab mask system was used for immobilization (BrainLAB AG, Heim-stetten, Germany). Following initial setup to the first lesion isocenter, a pair of orthogonal stereoscopic x-ray images were acquiredand initial 6DOF localization shifts were determined. These shifts were then performed by the BrainLab 6DOF robotic couch. Aftertreating the first lesion, the ExacTrac system was used to move the couch to the second lesion’s isocenter while maintaining the 6DOFimage guidance shifts from the first isocenter. A pair of ExacTrac x-ray images was acquired for the second isocenter, and imageguidance shifts were calculated. These image guidance shifts were interpreted as the intrafractional motion that occurred duringthe treatment of the first lesion. The same workflow was repeated to determine the intrafractional motion between subsequent lesions.

Results: Intrafractional motion was analyzed for both single- and multiple-fraction patients. The mean total treatment times perlesion were 18 min (range, 15-28 min) and 10 min (range, 7-12 min) for the single- and multiple-fraction patients, respectively.These times include imaging and setup time. For the single-fraction patients, the intrafractional motion had a median value of0.41 mm (range, 0.02-2.13 mm). Of the 25 intrafractional motion shifts for the single fraction patients only 15% were greaterthan 1 mm. For the two patients with 10 fractions, the intrafractional motion had a median value of 0.24 mm (range, 0.00-1.65mm). Of the 46 intrafractional shifts for the multiple fraction patients only 4% were greater than 1 mm. Not surprisingly, thevast majority occurred toward the end of the total treatment time.

Conclusions: The vast majority of individual-lesion intrafractional motions observed here were less than 1 mm. Larger intrafrac-tional motions (up to 2.13 mm) were seen to be more likely towards the end of treatment, thus emphasizing the importance of main-taining patient compliance through efficient delivery times.

Author Disclosure: B. Wang, None; P. Rassiah-Szegedi, None; H. Zhao, None; Y.J. Huang, None; V. Sarkar, None; M. Szegedi,None; R.L. Jensen, None; D.C. Shrieve, None; B.J. Salter, None.

3333 Dosimetric Comparison of Radiosurgical Modalities using EDR Film: Gamma Knife vs. CyberKnife

S. Lee1,2, S. Jang1,2, T. T. Sio2, J. Segala1, D. Allard1, B. H. Curran1,2, E. S. Sternick1,2, D. E. Wazer1,2

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

Purpose/Objective(s): There are several techniques available to the clinician for the delivery of radiosurgical treatments, but fewside-by-side comparisons of different treatment modalities. In this study, dosimetric differences between the Gamma Knife (GK)and the CyberKnife (CK) are examined for a set of hypothetical brain metastases using EDR films.

Materials/Methods: In our institution, both the GK (modified model C) and the CK (model G4) are available for radiosurgery. CTimages of an anthropomorphic head phantom were obtained with and without a GK head frame. Using these images, three hypothet-ical, slightly ellipsoidal tumors were contoured in the supra-tentorial as well as the infra-tentorial regions for the GK and CK treatmentplanning systems. Lesion one was created in the right frontal region with diameters of approximately 2 cm and a volume of 4.5 cm3.Lesions two and three were created in the left frontal and occipital areas. Both tumors were equal in size, approximately 1 cm indiameter, and with a volume of 0.45 cm3. The center of the left frontal tumor was located 3 mm above the central plane of theleft occipital tumor. Using the 6 cm diameter cone of the CK unit, an optical density curve was established for EDR2 film. A GKplan was prescribed to deliver 180 cGy to the 50% isodose line (IDL) for lesion one. Lesions two and three were prescribed to deliver200 cGy to the 50% IDL. For the CK plan, 180 cGy was prescribed to the 64% IDL for lesion one. For the left occipital and frontaltumors, 200 cGy was prescribed using the 64% and 67% IDLs respectively. All tumors were fully covered by the prescription dose.For each plan, three deliveries were created; to lesion one alone, to lesions 2 and 3, and to all lesions. The film was placed such that thefilm plane encompassed the prescription IDL for lesion one, the left occipital tumor, and a portion of the left frontal tumor. In all sixplans (3 each for GK and CK) were created, approved, and delivered in the same manner as would be done for a patient.

Results: The irradiated films were processed, scanned using a Vidar Dosimetry Pro film scanner, and analyzed. Dose fall-off for theGK plans was superior to that of the CK plans in the peripheral area. For lesion one, GK doses were higher than CK, ranging from8.4% (3 tumors) to 44.8% (single tumor). Peripheral doses for CK were generally higher than GK by as much as 65.8% dependingon the beam path, increasing with multiple target deliveries.

Conclusions: Dosimetric differences between GK and CK were compared for single and multi-target cases. The tumor dose to GKwas higher than CK due to lower IDL prescription but this effect diminished as the number of treatment targets increased. Periph-eral doses were higher for CK than GK for most cases.

Author Disclosure: S. Lee, None; S. Jang, None; T.T. Sio, None; J. Segala, None; D. Allard, None; B.H. Curran, None; E.S. Ster-nick, None; D.E. Wazer, None.

3334 A Database of Online Registry for Dose Tolerance Limits in Hypofractionated SBRT

J. Xue1, T. LaCouture1, M. Chew2, N. Pahlajani1, L. Hughes1, N. Kramer1, S. Asbell1, J. Grimm1

1Cooper University Hospital, Camden, NJ, 2Lehigh University, Bethlehem, PA

Purpose/Objective(s): To compile an online database of up-to-date published information on dose tolerance limits for SBRT clin-ical use and research.

Materials/Methods: Based on an extensive literature review and our clinical experience with Gamma Knife and CyberKnife, a da-tabase of the dose tolerance limits in hypofractionated SBRT has been developed in our institution. It includes many anatomicalstructures from the head to the body, and is applicable to SBRT treatments in one to five fractionated sessions. The goal is to makethe database available online, to enable other SBRT clinics to contribute to the work and to gain quick access to the information.Dose tolerance limits in hypofractionated SBRT are still evolving and far from consensus. The proposed online database can pro-vide a useful resource for easy access to and better understanding of SBRT dose tolerance limits. The information is organized byeach critical structure, followed with the dose limits of specified format (volumes, percentage and maximum, etc.) as well as com-plication probability and reference source. The newly published QUANTEC report refines the dose tolerance limits for most