Proceedings of the 52nd Annual ASTRO Meeting S699

with the camera. The tracking signal was used to direct real-time couch motion such that the marker was stationary with respect tothe treatment room in the superior-inferior (SI) direction. The motion of the marker without couch compensation and the residualmotion of the marker after couch compensation were analyzed for 5 min of free breathing from each subject.

Results: The localization latency associated with the camera was approximately 0.02 sec. The free-breathing SI end-exhale posi-tion varied by 2.4 mm to 4.2 mm in the four subjects. Peak-to-peak SI marker motion ranged from 6.1 mm to 15.2 mm beforecorrection, with 32.1% of points more than 2 mm from the setup position. The mean ± SD of marker displacements with respectto the treatment room after couch-based motion compensation was 0.0 ± 0.5 mm, with 0.1% of points greater than 2 mm from thesetup position.

Conclusions: The tracking system compensated for the respiratory motion of the subjects’ abdominal surfaces, as measured by thetracking camera. The distribution of marker displacements was centered about the setup position, and the SD of marker displace-ments was less than 1 mm.

Author Disclosure: K.T. Malinowski, None; M.A. Yousuf, None; T.J. Mc Avoy, None; W.D. D’Souza, None.

3106 Effect of Imaging Frequency on Offline Determination of Setup Correction for Head and Neck

(HN) Radiotherapy

S. K. Martin, L. H. Kim, D. Yan

Beaumont Hospitals, Royal Oak, MI

Purpose/Objective(s): To evaluate the effect of cone-beam CT (CBCT) imaging frequency and the predictive value of the firstweek of setup errors towards the determination of off-line setup corrections for patients receiving radiation treatment to thehead and neck (HN).

Materials/Methods: Fifty-four HN patients received daily CBCT imaging. CBCT images were registered automatically to theplanning image based on bony anatomy. Clinically, a setup correction was implemented if the five most recent CBCT imagesshowed an average setup error in any direction greater than 3 mm. In this study, these corrections are compared with those obtainedfrom daily imaging for the first week only followed by 1) weekly or 2) bi-weekly imaging with setup errors evaluated using a two-week rolling average (our current clinical practice). A sensitivity/specificity analysis was performed to compare the setup correc-tions obtained using these imaging schedules to those obtained clinically based on daily imaging.

Results: Based on daily imaging, 42 patients required 76 corrections. 21of the 76 corrections were based on the first week of im-aging, although 9 of those 21 required further corrections in the same direction as the treatment progressed. Of the remaining 55corrections, 39 (71%) were correctly identified within ±1 week using weekly imaging, and 36 (65%) were correctly identifiedwithin ±1 week using bi-weekly imaging. Weekly imaging produced 10 false positives (i.e., corrections not identified by dailyimaging), and bi-weekly imaging produced 4 false positives. Sensitivity/specificity values were 77.5%/89.6% for weekly imagingand 74.3%/95.6% for bi-weekly imaging.

Conclusions: Weekly and bi-weekly imaging to determine offline setup correction for HN patients identify approximately 30%fewer corrections compared with daily imaging. This is due to the combination of reduced imaging frequency and a longer aver-aging period. The first week of imaging is generally not predictive of HN patient setup error over the course of treatment. If weeklyor bi-weekly imaging is used to determine offline setup correction, daily verification images should be acquired afterwards to re-duce the possibility of false positives.

Author Disclosure: S.K. Martin, None; L.H. Kim, None; D. Yan, None.

3107 Prostate Volume Changes Monitored during a Course of External Beam Radiation Therapy and for the

Next Six Months using Electromagnetic Transponders

J. Park, W. M. Butler, G. S. Merrick, B. S. Kurko, J. L. Reed, B. C. Murray

Schiffler Cancer Center/Wheeling Jesuit University, Wheeling, WV

Purpose/Objective(s): This study measured daily changes in prostate volume using tracking of implanted electromagnetic tran-sponders during a course of intensity modulated radiation therapy (IMRT) and monthly changes in transponder positions for sixmonths following treatment.

Materials/Methods: Twenty-four men with biopsy-proven T1c-T3a prostate cancer each had three electromagnetic transpondersimplanted transperineally. Their coordinates were recorded by the Calypso system, and the perimeter of the triangle formed by thetransponders was used to calculate prostate volumes during the course of IMRT to a dose of 81 Gy in 1.8 Gy fractions. Followingthe treatment course, the transponder positions were determined once each month for six months.

Results: There was a significant decrease in mean prostate volume of 14.0% from the first to the final day of radiation therapy. Thevolume loss did not occur monotonically, but increased in 3̌̌/4 of the patients during the first several weeks, achieving a mean max-imum volume increase of 3.6% on day 3. From the point of maximum volume, the prostate volume of each decreased by a meanmaximum difference of 18.4% to nadir during the last week of IMRT (p\0.001 for both increase and decrease). Glandular shrink-age was asymmetric with the apex to right base dimension varying more than twice that of the lateral dimension. The mean changein any dimension was\0.5 cm. Over the ensuing six months following the course of IMRT, there was no further significant changein transponder positions. The relative prostate volume at the end of IMRT was 0.860 and this reduced to 0.834 after 6 months.

Conclusions: Real-time transponder positions indicated a slight volume increase during the initial days of IMRT and then significantand asymmetric shrinkage by the final day. After the final day, the transponder positions remained nearly unchanged for six months.

Author Disclosure: J. Park, None; W.M. Butler, None; G.S. Merrick, None; B.S. Kurko, None; J.L. Reed, None; B.C. Murray,None.