S650 I. J. Radiation Oncology d Biology d Physics Volume 69, Number 3, Supplement, 2007

2810 Comparison of Transabdominal Ultrasound and Electromagnetic Transponders for Prostate Localization:

Clinical Experience in a Large Patient Population

R. D. Foster1, I. J. Chetty1, H. Li1, C. A. Enke1, T. R. Willoughby2, P. Kupelian2, T. D. Solberg1

1University of Nebraska Medical Center, Omaha, NE, 2M.D. Anderson Cancer Center Orlando, Orlando, FL

Purpose/Objective(s): The aim of this study is to compare two methodologies of prostate localization in a large cohort of patients.

Materials/Methods: Daily prostate localization using a transabdominal ultrasound system (BAT) has been performed at our in-stitution since 2000. More recently, a technology using electromagnetic transponders implanted within the prostate was introducedinto our clinic. The Calypso system uses an array of AC magnetic coils to generate a resonant response in the transponders, which issubsequently detected using a separate array of receiver coils. With each technology, patients were localized initially using threeskin marks. Localization error distributions were determined from offsets between the initial setup positions and those determinedby BAT or Calypso. BAT localization data was summarized from 16236 imaging sessions spanning over 6 years; Calypso local-ization was summarized from 1527 fractions in 41 prostate patients treated in the course of a clinical trial at five institutions.Calypso alignments were frequently verified using a kV x-ray system.

Results: The mean Calypso couch shifts in the lateral, vertical, and longitudinal directions were �1.3 ± 5.6 mm, �4.3 ± 6.4 mm,and�0.7 ± 3.8 mm, respectively. The mean lateral, vertical, and longitudinal US shifts were�0.1 ± 11.4 mm, 1.3 ± 11.6 mm, and�0.5 ± 12.8 mm, respectively. Both sets of data show that the largest initial localization error occurs in the anteroposterior direc-tion. However, the mean vertical offsets observed are in opposite directions, with ultrasound-based localization, patients are treatedon average with isocenters 5.6 mm anterior to that determined by Calypso. Ultrasound localization distributions are 2–3 times widerthan those of Calypso which is likely due to increased sources of uncertainty associated with BAT; for example, the inability tovisualize the prostate accurately as well as inter-user variations. Based on a Kurtosis analysis, the offset distributions are not Gauss-ian for either localization method. Figure 1 is a histogram showing the frequency of couch shifts in the lateral, vertical, andlongitudinal directions for Calypso and BAT.

Conclusions: The Calypso system provides an objective, user- and institution-independent method of localizing prostate patients.Relative to distributions with the Calypso system, ultrasound localization distributions suggest substantial inter-user variability. Incontrast, the Calypso system does not depend on the user’s training or technique and provides objective information, potentiallyresulting in more accurate localization.

Author Disclosure: R.D. Foster, None; I.J. Chetty, None; H. Li, None; C.A. Enke, None; T.R. Willoughby, None; P. Kupelian,None; T.D. Solberg, None.

2811 PET/CT Guided Adaptive Radiotherapy of Locally Advanced Non-Small Cell Lung Cancer

A. Sethi, J. Dombrowski, R. Hong, Y. Soni, B. Emami

Loyola University Medical Center, Maywood, IL

Purpose/Objective(s): Target size is often the dose-limiting factor in radiotherapy of lung cancer, forcing one to accept a sub-optimal plan that balances competing risk factors of tumor recurrence and organ toxicity. In this study we investigate a noveladaptive radiotherapy of lung cancer that monitors and takes advantage of tumor regression during the course of radiation usingPET/CT imaging.

Materials/Methods: Twelve patients with locally advanced or inoperable non-small-cell lung cancer were selected for this retro-spective planning study. Prior to start of treatment, each patient underwent CT and PET scanning. CT and PET data were used tooutline initial target volumes (GTVini/PTVini) and organs at risk (OARs). Between 40 and 50 Gy target dose, patients were re-scanned to evaluate therapeutic changes in target volumes. Boost volumes (GTVbst/PTVbst) were identified on the new fusedCT/PET images. Each patient underwent conformal treatment planning with optimal beam energies, orientations, and heterogene-ity corrections. Two treatment plans were considered: 66.6 Gy to PTVini (conventional plan) versus 46.8 Gy to PTVini followed by19.8 Gy to PTVbst (modified or adaptive radiotherapy plan). Treatment plans were evaluated based on target coverage and dose tothe OARs. Comparative dose indices were: Dmean and Dmax for lung, esophagus, heart and spinal cord, and V20 for lung. Normaltissue complication probabilities (NTCP) for all OARs were compared.

Results: Rescan data showed significant reductions in target volumes compared to intial volumes: 79.2 (20.4)% [average(s.d.)] forGTV and 70.0 (25.2)% for PTV. Modified treatment plans with boost PTV provided uniformly improved target coverage and re-duced OAR doses. For the lung, average Dmean was improved by 18.9 (13.7)%, and V20 decreased by 15.5 (19)% in modifiedplans. Dmean for other OARs showed comparable reductions from conventional plans; esophagus: 11.7 (8.5)%, heart: 19.4(11.7)%, and spinal cord: 12.1 (16.8)%. On average, the NTCP was decreased for each OAR; lung: 17.9 to 7.6 (�57.8% reduction),esophagus: 18.2 to 5.9 (�67.3%), heart: 11.5 to 5.6 (�51.5%), and spinal cord: 6.1 to 1.3 (�79.1%). In conventional plans, it wasnot possible to treat PTV to the prescription dose while maintaining all OARs below their threshold doses. However, with adaptiveradiotherapy planning, we were able to both meet prescription target dose and maintain OAR doses below tolerance. Moreover, thisOAR dose reduction allowed us additional dose delivery to the PTV if necessary.