S888 I. J. Radiation Oncology d Biology d Physics Volume 81, Number 2, Supplement, 2011

Materials/Methods: A 100 ml volume of 50 nm polymer coated GNPs was separately injected into 0.5 cm x 0.5 cm x 0.5 cmclinical bolus material (tissue equivalent). This material provided a sealed cavity with no air bubbles, leakage or edge effects.Solutions of the following concentrations were considered: 0, 2.5, 3, 4, 5 and 10 mg/ml. Each of the individual GNP-loadedmini-phantoms was surgically implanted post euthanasia into the same mouse between the liver and the stomach. Cone-beamCT images were acquired at 65 kV, 0.8 mA and reconstructed using 360 projections acquired in 100 seconds with the SARRP(Small Animal Radiation Research Platform). The ratio of the image intensity within the volume of GNPs versus within the ab-domen was measured using Image J for axial, sagittal and coronal views. For each concentration, 5 measurements of intensityratio were obtained.

Results: For the concentrations of 0, 2.5, 3, 4, 5 and 10 mg/ml of AuNPs, the corresponding average intensity ratios were: 1.00 ±0.03, 1.02 ± 0.01, 1.02 ± 0.01, 1.06 ± 0.01, 1.08 ± 0.02 and 1.14 ± 0.02, respectively. Statistically significant (p\0.001) imagecontrast was observed as the concentration was increased beyond 3 mg/ml.

Conclusions: Significant image contrast was observed for concentrations 4 mg/ml and higher. Previous pre-clinical work hasshown that average concentrations of 7 mg/ml may be delivered in vivo. Based on the results of this study, it may be possibleto use gold nanoparticles as a contrast agent for image-guided therapy targeting or for image-based kinetic studies of tumoruptake. Future work will include in vivo targeting studies with gold nanoparticles in tumor-bearing mice.

Author Disclosure: H. Korideck: None. W. Ngwa: None. M. Makrigiorgos: None. R.I. Barbeco: None.

3419 WITHDRAWN

3420 Dosimetric Impact of Patient Rotational Setup Errors with Frameless Single-Isocenter, Multi-Target

Volumetric Modulated Arc Radiosurgery for Multiple Brain Metastases

G. M. Clark1, J. B. Fiveash1, B. M. Prendergast1, C. W. Willey1, S. A. Spencer1, E. M. Thomas1, B. L. Guthrie2, R. A. Popple1

1University of Alabama at Birmingham Radiation Oncology, Birmingham, AL, 2University of Alabama at BirminghamNeurosurgery, Birmingham, AL

Purpose/Objective(s): Single-isocenter, multi-target volumetric modulated arc cranial radiosurgery (VMAT radiosurgery) de-livers conformal dose with favorable gradient and extreme efficiency. If invasive fixation is not utilized, rotational setup errorscan range from 1 - 2 degrees (common) to $3 degrees (infrequent) to with orthogonal kV and cone-beam CT verification. Weaim to investigate the dosimetric impact of these rotations.

Materials/Methods: Ten patients with$ 2 brain metastases were planned using a single-isocenter, 4-arc (non-coplanar) VMATtechnique (Varian Eclipse, RapidArc). All plans were normalized to deliver 100% of the prescription to 99% of the total targetvolume (GTV = PTV). Per target conformity indices (CI = 100% isodose volume/ PTV) and per plan gradient indices (GI = 50%isodose volume / 100% isodose volume) were calculated. Plans were exported into custom dose calculation/distribution software(validated to within ± 1% of Eclipse dose distribution). Within the software utility, simultaneous rotations of 1o, 2o, and 3o eachfor roll, pitch, and yawwere applied about the isocenter while keeping dose distribution constant and resultant dose statistics wereexamined (see Table 1).

Results:Mean (and range) number of metastases per plan = 3.4 (2 - 9, total = 34), per target volume (TV) = 2.1 cc (0.1 - 18),distance from PTV to isocenter = 4.2 cm (1.0 -7.1). Mean CI per target = 1.1 (0.9 -1.7) and mean GI per plan = 4.3 (3.3 - 6.4).Univariate analysis of V95% revealed the following significant associations/trends: 1� rotations - distance from isocenter(p = 0.001). 2� rotations - TV (p = 0.07); CI (p = 0.02) and distance from isocenter (p = 0.001). 3� rotations - distancefrom isocenter (p = 0.008), TV (p = 0.008), and CI (p\ 0.001). For PTVs # 2 cc (n = 17) vs. . 2 cc, mean V95% was lowerwhen rotations of 2� (85.4 vs. 94.0, p = 0.01) and 3� (68.3 vs. 88.2, p\0.01) were applied, but not for 1� rotations (97.5 vs. 96.1,p = 0.24).

Mean, range, and standard deviation dosimetric values for rotational error scenarios

1� roll + 1� pitch+ 2� roll + 2� pitch+ 3� roll + 3� pitch+

No rotation 1� yaw 2� yaw 3� yaw

% PTV receiving

100 (98.6 - 100) 98.0 (85.1 - 100) 89.4 (57.0 - 100) 77.6 (25.4 - 99.1)

95% of prescription (V95%)

SD 0.25 SD 2.92 SD 10.62 SD 19.07

Min dose to target

97.9 (91.0 - 103.3) 89.6 (76.0 - 98.6) 78.4 (59.3 - 95.1) 68.6 (46.7 - 92.3)

SD 3.07

SD 5.98 SD 10.47 SD 13.72

Conclusions: Target coverage for single-isocenter, multi-target VMAT radiosurgery has a heightened sensitivity to rotationalsetup error. Minimizing rotational error to #1� is important for adequate coverage and is especially crucial for small lesionsand those further from the isocenter. Given the dose-delivery efficiency of this technique, prolonged patient setup time to ensureaccurate positioning will likely not diminish clinical efficiency.

Author Disclosure: G.M. Clark: None. J.B. Fiveash: D. Speakers Bureau/Honoraria; Received honoraria from Varian MedicalSystems for educational and scientific presentations. B.M. Prendergast: None. C.W. Willey: None. S.A. Spencer: None. E.M.Thomas: None. B.L. Guthrie: None. R.A. Popple: C. Other Research Support; University of Alabama at Birmingham has anagreement with Varian Medical Systems for the evaluation of RapidArc, for which R. A. Popple is the principal evaluator. D.Speakers Bureau/Honoraria; received honoraria from Varian Medical Systems for educational and scientific presentations.