Proceedings of the 51st Annual ASTRO Meeting S609

for radiotherapy have received the marker implant in different body regions (lung, liver, breast, cervix, prostate, pancreas and othersites in abdomen) using ultrasound or CT-guidance without need for anesthetics. The shape of the marker can be adjusted intoa single clump, a clump with a string, a clump-string-clump form, or a string alone. Once implanted, the marker is visible withkilo voltage imaging.

Results: An evaluation of the marker implantation procedure, safety, x-ray visualization, and positioning movements during thetherapy period will be presented.

Conclusions: Increasingly, we must be certain of exact localization of tumor targets by using markers in IGRT. The fine-needlemarker presented here allows implantation into almost any tumor with minimal risks of internal bleeding or infection and can beimplanted safely and precisely using guidance with ultrasound or CT. The Gold Anchor� is developed for visualization with kilovoltage equipment during radiotherapy.

Author Disclosure: I. Naslund, Owner of Naslund Medical, E. Ownership Interest; P. Wersall, None; E. Castellanos, None; C. Be-skow, None; S. Nyren, None.

2954 The Exactrac Snap Verification (SV), a New Tool for Ensuring the Quality Control for Stereotactic Body

Radiation Therapy (SBRT)

C. M. Udrescu1, O. Chapet1, B. de Bari1, G. Michel-Amadry2, F. Mornex1

1Department of Radiation Oncology, Centre Hospitalier Lyon Sud, Pierre Benite, France, 2Department of Medical Physics,Centre Hospitalier Lyon Sud, Pierre Benite, France

Purpose/Objective(s): The intra-fraction patient (pt) imaging and verification provided by ExacTrac SV allows tracking possibleisocenter displacement within the pt throughout treatment by SBRT and realignment during the treatment. The purpose of this studywas (1) to measure the intra-fraction variations of isocenter position, (2) to study the amplitude of variation function of the fractionduration and (3) to evaluate the impact of the table movement on pt positioning, using SV.

Materials/Methods: ExacTrac SV uses X-ray real-time images acquired at any moment during treatment delivery or betweenfields to instantly detect and visualize isocenter displacement (beam on or off). The displacement can be measured using thefusion of the bony structures or the implanted markers with the DRRs. A tolerance margin indicates if a patient setup cor-rection is needed or not. We already evaluated 5 pts who were treated with the ExacTrac X-ray 6D, for T1-T2 lung (4 pts) orliver (1 pt) tumors, using SBRT (mean treatment fields number = 9). The pts had a SV at each fraction, before each treatmentbeam. Finally, 477 beams were verified with a measure of the isocenter displacement. The time from the pt installation on thetable to each SV was systematically noted. Three ‘‘SV time’’ groups were identified: SV performed at less than 10 minutes(group 1 = 36 beams), between 11 and 20 minutes (group 2 = 255 beams) and more than 21 minutes (group 3 =186 beams),after patient setup.

Results: The mean isocenter deviation for all the beams and all the fractions was 2.37 mm. The total mean time of one fraction was22 minutes [9- 36 minutes]. The mean deviations were 1.8mm [1 - 8], 2.12mm [0 - 8] and 2.7mm [0 - 8] for group 1, 2 and 3,respectively. For the same groups, the percentages of deviation $ 3 mm were 2.6% (1/36), 20% (51/255) and 25% (46/186)and the percentage of deviation $ 5 mm were 5.2% (2/36), 4% (10/255) and 14% (26/186) respectively. In 3 pts, a table rotationwas necessary. The mean isocenter deviations increased from 1.92mm before table rotation (178 beams) to 2.88mm for the firstbeam treated after rotation (35 beams). Pain, cough, or talk may explain the observed deviations in a few situations.

Conclusions: SV allows detecting isocenter deviations, which increase in amplitude and frequency with the fraction duration. Itmakes possible and highly suitable the intra-fraction verification for the SBRT, taking in account clinical condition and technicalissues. With high-quality images at low X-ray doses, SV gives an accurate targeting from the beginning to the end of each fraction,inducing confidence to escalate the dose. SV appears to be an important tool for ensuring the quality control for SBRT. At the timeof the meeting, results for more patients treated for lung, liver and prostate cancers will be available.

Author Disclosure: C.M. Udrescu, None; O. Chapet, None; B. de Bari, None; G. Michel-Amadry, None; F. Mornex, None.

2955 Favorable IMRT Experience Treating Obese Prostate Cancer Patients in the Prone Position using

Electromagnetic Tracking and a ‘‘Belly Board’’

M. D. Logsdon, J. K. Bareng, L. Olson, A. Ryan, S. W. Lee

Radiological Associates of Sacramento, Sacramento, CA

Purpose/Objective(s): We sought to extend the use of electromagnetic localization and tracking technology (Calypso MedicalTechnologies, Seattle, WA) to obese men with large anterior-posterior (AP) separations who would otherwise not be eligible tohave their prostate localized and/or tracked if they were treated in a supine position.

Materials/Methods: Four patients of large girth (weights 189-326 lbs) were treated. The A-P dimensions of these patients pre-cluded electromagnetic localization and tracking when they were in a supine position. This is due to the specifications of the track-ing system, which limits the maximum distance from the array to the transponders to prevent gantry collision. These patients wereplaced in the prone position for both the treatment planning CT as well as daily radiation treatments. The first patient was treated ona solid slab of Styrofoam. In an attempt to minimize prostate motion caused by respiration, the remaining three patients were treatedon a conventional ‘‘belly board.’’ PTV prescription doses ranged from 77.4-79.2 Gy. Tracking limits were set to 5 mm for all di-mensions except for 4 mm posterior.

Results: Localization and tracking were accomplished successfully for all 173 fractions. The tracking logs for all patients demon-strated patterns of prostate motion attributable to breathing, predominantly in the AP axis with lesser effect along the superior-in-ferior (SI) axis and no effect on the lateral axis. For the first patient, the excursions due to respiratory motion were typically ± 1 mmin the SI axis and ± 2 mm in the AP axis. When a ‘‘belly board’’ was used for the subsequent 3 patients, the excursions were de-creased and were typically ± 0.5 mm in the SI axis and ± 1 mm in the AP axis. Beam pauses and/or interruptions for patient re-positioning were no more frequent than for more slender patients treated in the supine position.

S610 I. J. Radiation Oncology d Biology d Physics Volume 75, Number 3, Supplement, 2009

Conclusions: In our experience, prone positioning on a ‘‘belly board’’ allows electromagnetic localization and tracking technologyto be used on all obese patients. Prostate motion from breathing is small and acceptable. As a result, high dose IMRT for prostatecancer can be delivered accurately and reliably to obese men.

Author Disclosure: M.D. Logsdon, None; J.K. Bareng, None; L. Olson, None; A. Ryan, None; S.W. Lee, None.

2956 Determining a Dosimetric Correlate for Acute Esophagitis in Patients with Advanced Non-small Cell Lung

Cancer Treated with Intensity Modulated Radiation Therapy (IMRT)

G. N. Gan1, D. E. Heron2,1, E. Brandner2, J. Flickinger2,1, R. Smith2, S. Huq2, S. Bahri2

1University of Pittsburgh School of Medicine, Pittsburgh, PA, 2University of Pittsburgh Cancer Institute, Department ofRadiation Oncology, Pittsburgh, PA

Purpose/Objective(s): Acute radiation esophagitis is a common complication in the treatment of lung malignancies which canlead to treatment interruption and undesirable morbidity. We report our experience treating locally-advanced non-small celllung cancers using chemoradiotherapy via IMRT and the associated acute treatment-related toxicities, and we determined potentialdosimetric parameters predictive for acute esophagitis.

Materials/Methods: This retrospective, multi-site study assessed 37 Stage IA-IV lung cancer patients who received IMRT. Allpatients were scanned during free breathing using coached retrospective 4D-CT and 21 had phase-based gated treatment delivery.A median dose of 70.0 Gy (range: 50 - 81.9 Gy) over 35 fractions (range 25 - 39 fractions) was delivered to the tumor using in-homogeneity correction. The esophagus was contoured and the mean dose and the volume of esophagus receiving 30, 50, 60 and 70Gy (V30, V50, V60, V70, respectively) was calculated for each patient. Patients were graded for acute and chronic toxicities using theCTCAE 3.0 and logistic regression was performed to test associations between dosimetric parameters and esophageal toxicity.

Results: All simulated patients completed their course of chemoradiotherapy treatment. Twenty-eight patients (76%) received ra-diation treatment with concurrent chemotherapy. The most common acute toxicities were nausea, esophagitis, and fatigue observedin 11, 26, and 18 patients respectively. Only 3 patients (8%) developed Grade 3 radiation pneumonitis confirmed clinical and ra-diographically and required treatment with oral steroids. Acute Grade 1, 2 and 3 nausea was observed in 5, 4 and 2 patients; acuteGrade 1, 2 and 3 esophagitis was observed in 8, 17 and 1 patients and acute Grade 1, 2 and 3 fatigue was observed in 9, 7 and 2patients, respectively. Patients who experienced any esophagitis had a mean dose in excess of 28.7 Gy to the esophagus (Odds ratio:1.09, 95% confidence interval: 1.02 - 1.18, p = 0.01). Univariate logistic regression correlated acute Grade $2 esophagitis with V60

(p = 0.045) and mean esophageal dose (p = 0.049). We did not identify significant differences in acute esophagitis between patientsreceiving concurrent chemotherapy and radiation versus those receiving radiation alone (p = 0.058). We were not able to correlateany parameters with late/chronic esophagitis.

Conclusions: The mean dose to the esophagus and the volume of esophagus receiving higher doses of irradiation, but not chemo-therapy, are associated with the development of esophagitis. Maintaining a mean doses to the esophagus below 28.7 Gy and re-stricting the volume of esophagus receiving 60 Gy or more can help to minimize treatment associated esophagitis.

Author Disclosure: G.N. Gan, None; D.E. Heron, None; E. Brandner, None; J. Flickinger, None; R. Smith, None; S. Huq, None; S.Bahri, None.

2957 The a/b Ratio: Dose-range Dependent or Model Limitation?

C. Zhang, N. A. Mayr, S. S. Lo, L. Lu, K. Li, J. Z. Wang

The Ohio State University, Columbus, OH

Purpose/Objective(s): The Linear-quadratic (LQ) model has been widely used in modeling the cell-killing by radiation therapy.However, its application for high dose irradiation has been challenged by many investigators. Recently a published study has dem-onstrated that the a/b ratio of the LQ model is dose-range dependent, especially when single-dose irradiation exceeds 10 Gy. Wehave previously proposed a generalized LQ model (gLQ) to address the high dose dilemma of the LQ model. In the current study,we employed both the standard LQ model and our gLQ model to analyze the in vitro cell irradiation data of large dose ranges andinvestigate the dose-range dependence of the a/b ratio.

Materials/Methods: In vitro data of three cell-lines (U373MG, CP3 and DU145) published by Garcia et al. (IJROBP 2007;67:587) were used. The cell-lines were irradiated with single doses up to 14 Gy in steps of 0.5 Gy. The gLQ model accountedfor the reduction of cellular sublethal damage due to re-irradiation, an important component of cell-killing in the high-dose domain.Both, the LQ and the gLQ were used to analyze the three datasets. The minimum c2 method was adopted to fit the data and toevaluate the figure of merit for the two models. Errors of the model parameters were determined by propagating the c2

min. Thea/b ratios from both models were investigated across different dose ranges from 0 Gy to varying final doses.

Results: The a/b ratio derived from the LQ model was dose-range dependent: when the final dose varied from 5 Gy up to 14 Gy, thea/b ratio increased from 2.8 to 9.3 Gy for U373MG, from 0.67 to 3.3 Gy for CP3, and from 3.8 to 19.6 Gy for DU145 cell-line.However, the a/b ratios obtained from the gLQ model were almost constant within the data uncertainties across the dose ranges withfinal dose $5 Gy: 1.0 Gy, 0.027 Gy, and 1.8 Gy for the three cell-lines, respectively. The variances of the a/b ratio generated fordifferent dose ranges with gLQ vs. LQ fitting were: 0.049 vs. 4.7 Gy2 for U373MG, 0.0015 vs. 0.67 Gy2 for CP3, and 0.30 vs. 22Gy2 for DU145. The gLQ model provided an improved fit over the LQ model across the entire dose range: the reduced c2

min of gLQvs. LQ model were 1.5 vs. 3.2 for U373MG, 1.5 vs. 2.7 for CP3, 4.4 vs. 7.2 for DU145. The standard error bars of a/b ratios obtainedfrom the gLQ fitting were much smaller than those of the LQ fitting, and they decreased more rapidly with increasing final dose.

Conclusions: Our results indicate that the dose-range dependence of the a/b ratio is due to the inconsistency of the standard LQmodel in high-dose domain. The gLQ model provides a more consistent interpretation of the cell irradiation data for large doseranges with much more stable model parameters. Therefore, the gLQ model is useful to extend our clinical experience accumulatedfrom conventional low-dose fractionation to high dose irradiation schedules, including SRS, SBRT, and HDR.

Author Disclosure: C. Zhang, None; N.A. Mayr, None; S.S. Lo, None; L. Lu, None; K. Li, None; J.Z. Wang, None.