Proceedings of the 52nd Annual ASTRO Meeting S837

reduces tongue and groove effect during gantry rotation. All plans were generated with 6MV X-rays for CLINAC 2100 linear ac-celerator. Calculations were done in the Eclipse treatment planning system (version 8.6) using the AAA algorithm.

Results: Double arc plans show superior dose homogeneity in PTV compared to single arc and IMRT 9 field technique. Targetcoverage was almost similar in all the techniques. The sparing of spinal cord in terms of the maximum dose was better in Doublearc technique by 4.5% when compared to IMRT (9 Field) and Single arc techniques. For healthy tissue no significant changes wereobserved between the plans in terms of the mean dose and integral dose. But RapidArc plans showed a reduction in the volume ofthe healthy tissue irradiated at V15Gy (5.81% for Single arc and 4.69% for double arc) and V20Gy (7.55% for Single arc and 5.89%for double arc) dose levels when compared to 9-Field IMRT technique. For brain stem, maximum dose was similar in all the tech-niques. The average MU (±SD) needed to deliver the dose of 200 cGy per fraction was 474 ± 80 MU and 447 ± 45 MU for doublearc and single arc as against 948 ± 162 MU for 9-Field IMRT plan. A considerable reduction in maximum dose to the mandible by6.05% was observed with double arc plan. Double arc plans shows a reduction in the parotid mean dose when compared with singleArc and IMRT plans.

Conclusions: Rapid Arc using double arc proved a significant sparing of OARs and healthy tissue without compromising targetcoverage compared to IMRT. The inconvenience with IMRT observed was higher monitor units and longer treatment time.

Author Disclosure: S. Kumar, None; P. Sriram, None; K. Saranya, None; N. Bhuvaneswari, None; N. Vivekanandan, None.

3416 Hypofractionated Stereotactic Radiotherapy for Brain Metastases: A Dosimetric and Treatment Efficiency

Comparison between Volumetric Modulated Arc Therapy and Intensity Modulated Radiotherapy

Y. Ma, J. Yu, M. Li, Y. Yin, L. Kong, X. Sun, X. Lin, Y. Yang

Department of Radiation Oncology, Shandong Tumor Hospital and Institute, Jinan, China

Purpose/Objective(s): A treatment planning comparison study was performed to evaluate the dosimetric characteristic and treat-ment efficiency of volumetric modulated arc therapy (VMAT) with step-and-shoot intensity modulated radiotherapy (IMRT) forthe hypofractionated stereotactic radiotherapy (HFSRT) in patients with oligo brain metastases.

Materials/Methods: CT datasets of 10 patients with two to four brain metastases were selected for the comparison. Three planswere generated for each case: step-and-shoot IMRT, single (RA1) and double (RA2) arcs with RapidArc technique (RA, VarianMedical System). The prescribed dose was 50Gy in 10 fractions and all plans were normalized to the mean dose to the PTV. ForPTV, plans aim to achieve at least 95% of PTV was encompassed by the prescription dose. Dosimetric parameters including targetcoverage, conformity index (CI), homogeneity index (HI), maximum dose to critical structures and the volume of the healthy tissuereceiving low dose were analyzed from dose-volume histogram. Additionally, we compared the monitor units and the pure beam-on time to evaluate the treatment efficiency.

Results: The plans generated using three techniques were acceptable clinically. The Paddick CI was 0.868 (IMRT), 0.863(RA1)and 0.895 (RA2), the HI was 7.7 (IMRT), 7.5 (RA1) and 6.5(RA2), respectively. The target conformity and homogeneity wereimproved slightly with RA2 compared to IMRT and RA1.Compared with IMRT, the maximum dose in RA2 plans to the brainstem,left and right optic nerves, left and right lens was reduced by 1.6Gy, 6Gy, 3Gy, 1.5Gy, 1.3Gy, respectively. The percentage ofhealthy tissue volume receiving 5Gy was larger with RA1 (56.7%) and RA2 (57.1%) than with IMRT (52.9%), while the percent-age of volume receiving 15Gy and 20Gy were smaller with RA1 (27.1%, 18.7%) and RA2 (25%, 16.3%) than with IMRT (28.8%,19.1%). No significantly difference was observed between RA1 and RA2. The number of MU per fraction was 1944 ± 374 (IMRT),1199 ± 173 (RA1) and 1387 ± 186 (RA2), respectively. The pure beam-on time needed per fraction was 6.5 ± 1.2min (IMRT),1.25 min (RA1) and 2.5 min (RA2), respectively.

Conclusions: RA, single or double arcs, is a feasible technique with highly conformal dose distribution for the HFSRT in patientswith oligo brain metastases. Compared with IMRT, RA1 provides similar plan quality, while RA2 provide additional improvementin sparing of OARs with slightly better PTV coverage. The treatment efficiency, using less monitor units and shorter delivery time,is the most obvious advantage.

Author Disclosure: Y. Ma, None; J. Yu, None; M. Li, None; Y. Yin, None; L. Kong, None; X. Sun, None; X. Lin, None; Y. Yang,None.

3417 A Comparison of Treatment Planning Techniques for Lung Stereotactic Body Radiation Therapy

M. Michaletz-Lorenz, C. G. Robinson, D. A. Low, C. Bloch, R. J. Bertrand, A. P. Apte, D. F. Mullen, S. A. Fergus, S. M. Goddu

Washington University School of Medicine, St. Louis, MO

Purpose/Objective(s): To evaluate stereotactic body radiation therapy (SBRT) treatment planning techniques for early stage non-small cell lung carcinoma (NSCLC) patients, using fixed beam conformal radiotherapy (FBCRT), helical tomotherapy (HT), andnext generation of axial tomotherapy (AT) on a research platform.

Materials/Methods: CT images and structures from 7 clinical patients previously planned and treated by FBCRT, with maximumplanning target volume (PTV) dimensions from 3.0 cm-3.9 cm, were retrospectively planned with HT and AT. Prescription dosewas 54 Gy in 3 fractions with 95% PTV coverage using 6 MV photons. Dosimetric constraints were largely per RTOG 0236.FBCRT was planned using 8-12 non-coplanar beams with multileaf collimation (120-MLC) on Philips Pinnacle3 RTP System.HT and AT were planned on TomoTherapy Hi-Art Planning Station using constant couch speed (HT) or fixed couch (AT), a fixedjaw setting of 1.0 cm (HT) or 5.0 cm (AT), and intensity modulating MLCs. Directional blocking was applied to avoid beamletentry to the heart and contralateral lung. Several methods were explored with AT to best obtain PTV homogeneity and conformal-ity, leading to an optimized pseudo-PTV with S/I extensions of 3-6 mm (original PTV was used for DVH and isodose evaluation).Of the 7 patients, 2 were excluded from AT due to tumor length exceeding axial planning software specifications.

Results: The value of each result is listed in the following technique sequence: FBCRT, HT, AT. Values are reported as means ±SD. High dose PTV conformality (VRx/Vptv) was 1.17 ± .11, 1.03 ± .02, 1.23 ± .10; low dose conformality R50% (V50%Rx/Vptv) was 5.83 ± 1.53, 4.91 ± .60. 7.06 ± .77; total lung-ITV mean dose was 4.03 ± 1.07 Gy, 3.31 ± .95 Gy; 4.44 ± 1.27 Gy

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and V5 was 20.85 ± 4.53 Gy, 15.80 ± 3.07 Gy, 20.08 ± 4.32 Gy. Total maximum dose to organs at risk for FBCRT, HT, and ATrespectively, were heart: 18.01 ± 17.43 Gy, 15.76 ± 17.57 Gy, 21.80 ± 18.92 Gy; esophagus: 8.25 ± 2.89 Gy, 8.46 ± 2.81 Gy, 8.13± 3.31 Gy; spinal cord: 8.33 ± 4.87 Gy, 10.16 ± 2.64 Gy, 11.33 ± 4.29 Gy; proximal trachea and bronchial tree: 11.79 ± 5.0 Gy,8.50 ± 5.82 Gy, 9.71 ± 7.68 Gy. Treatment times in minutes for FBCRT, HT, AT respectively were 15.2 ± 1.15 min, 37.94 ± 5.58min, 8.05 ± 1.26min.

Conclusions: HT appears to provide excellent high and low dose conformality, as well as improved lung (mean dose, V5) and heartdosimetry, at the cost of increased treatment time. Standard FBCRT had superior cord sparing, and meets all dosimetric constraintswhile simultaneously providing an acceptable overall treatment time. In properly selected patients, AT is an interesting new de-livery technique which appears comparable to FBCRT, while potentially decreasing treatment time by up to 50%. Future compar-isons of these disparate SBRT techniques are warranted.

Author Disclosure: M. Michaletz-Lorenz, Tomotherapy, B. Research Grant; C.G. Robinson, None; D.A. Low, Tomotherapy, B.Research Grant; C. Bloch, None; R.J. Bertrand, None; A.P. Apte, None; D.F. Mullen, None; F.A. Sandra, None; S.M. Goddu,Tomotherapy, B. Research Grant.

3418 Dosimetric Characteristics of a New Helical Applicator for APBI Delivery

R. Rice D. Scanderbeg

Moores UCSD Cancer Center, La Jolla, CA

Purpose/Objective(s): There are currently several different single-entry, intracavitary, breast brachytherapy applicators includingthe MammoSite, MammoSite Multi-Lumen, Contura MLB, and SAVI. The multi-lumen devices provide a multitude of dwell po-sitions which have improved normal tissue sparring by allowing the user to modulate dose away from the skin, chestwall, or lung. Anew prototype of a single-entry intracavitary applicator has emerged from Cianna Medical, the manufacturer of SAVI. This newdevice is a revolution in the design of an intracavitary applicator taking the shape of a double helix. This helical design allowsmultiple, peripheral dwell positions without adding several additional lumens. In this study, we assess the dose distribution ofthis new device compared with the MammoSite, MammoSite ML, Contura, MLB and the SAVI 6-1 applicators.

Materials/Methods: A breast phantom was CT scanned with each of the devices in the cavity. Each CT data set was fused to theother and contours were drawn on one data set and copied to each of the other devices, ensuring an exact comparison betweenstructures. The cavity, PTV, and PTV-EVAL were all contoured and then each of the devices was digitized and planned. A pre-scription dose of 3.4 Gy/fx was assigned and the dose was optimized. Dose volume histograms (DVHs) were used for evaluation ofthe plans.

Results: The single channel MammoSite had the lowest coverage, followed by the Contura and MammoSite ML. The helical SAVIand the SAVI 6-1 provided identical target coverage. The volume covered by 90% of the dose (V90) for each was 80.3%, 95.0%,95.5%, 100%, and 100%, respectively. The V150 and V200 for each of the devices, respectively, were: 29.8 cc and 11.9 cc, 30.1 ccand 8.0 cc, 29.8 cc and 7.9 cc, 32.3 cc and 11.2 cc, and finally, 32.4 cc and 10.2 cc.

Conclusions: The new helical SAVI has three lumens, making it less time consuming to digitize each of the catheters, and no moredifficult to plan than any of the other treatment devices. Additionally, this design provides compatibility to 3-channel afterloaderswith no loss in coverage and no need to split a treatment plan. Dosimetric results demonstrate the feasibility of this new device asa potential delivery system for accelerated partial breast irradiation. The hotspots (V150 and V200) for each of the devices weresimilar; however, their PTV-EVAL coverage differed substantially. The single channel MammoSite would not have been a viableoption for treatment due to its coverage of\90% of the PTV_EVAL by 90% of the dose. The Contura and MammoSite ML wouldhave been appropriate for treatment; however, the new helical SAVI and SAVI 6-1 both had better coverage of the target withsimilar hotspots. Further study is warranted to evaluate the feasibility and ease of use of the new helical design, but preliminaryresults are positive.

Author Disclosure: R. Rice, None; D. Scanderbeg, Cianna Medical, F. Consultant/Advisory Board.

3419 Analyses of the Mechanical Accuracy of RapidArc Treatment using Log File Data

K. Sasaki1, S. Sato2, Y. Miyabe2, T. Takakura3, E. Tsubota3, M. Nakata3, A. Sawada2, T. Mizowaki2, A. Itoh1, M. Hiraoka2

1Graduate School of Engineering Department of Nuclear Engineering, Kyoto University, Kyoto, Japan, 2Department ofRadiation Oncology and Image-applied Therapy Kyoto University Graduate School of Medicine, Kyoto, Japan, 3Division ofClinical Radiology Service, Kyoto University Hospital, Kyoto, Japan

Purpose/Objective(s): RapidArc technology is a novel delivery method that enables high-dose conformality by optimizing thedose rate, gantry speed, and multi-leaf collimator (MLC) shapes. In evaluating RapidArc using radiographic film, it has been re-ported that the MLC movement, variable dose rate, and gantry speed can be controlled precisely during RapidArc. However, thegravity effect due to the gantry angle could not be taken into account because the radiographic film rotated with the gantry. There-fore, this study evaluated the accuracy of the MLC leaf position, gantry angle, and monitor units (MUs) using log files. Further-more, the effects of variable gantry speed and dose rate on the accuracy of these parameters were evaluated.

Materials/Methods: The accuracy of the MLC leaf position, the control of the variable dose rate and gantry speed, and the com-bined use of different leaf speeds and dose rates to obtain a designed dose pattern were examined when performing RapidArc. TheMLC leaf positions and gantry angles were stored in a Dynalog file every 50 ms. In addition, the gantry angles and cumulative MUswere recorded in a Dynamic beam delivery log file at every control point for operating of these parameters. An error was defined asa difference between the actual and planned values for the MLC leaf positions, gantry angles, and cumulative MUs. The accuracy ofthe MLC leaf position and gantry rotation could be estimated every 50 ms using a combination of the Dynalog file and Dynamicbeam delivery log file.

Results: The maximum MLC leaf positional error at the isocenter plane was 0.21 mm. The MLC carriage tended to be delayed inthe leaf traveling direction. When the MLC was retracted in the direction opposite to gantry rotation, leaf positional errors were