Proceedings of the 51st Annual ASTRO Meeting S597

2926 Quantification of the Volume Change and Interfractional Motion and Volume of the Prostate for Prostate

Cancer Patients undergoing Concurrent Androgen Deprivation Therapy and Intensity ModulatedRadiation Therapy

S. Choi1, J. L. Johnson1, R. de Crevoisier2, L. Dong1, J. C. O’Daniel1, S. J. Frank1, A. K. Lee1, R. M. Cheung1, D. A. Kuban1

1M.D. Anderson Cancer Center, Houston, TX, 2Institut Gustave Roussy, Villejuif, France

Purpose/Objective(s): To evaluate the volume change and interfractional motion of the prostate in prostate cancer patients un-dergoing concurrent androgen deprivation therapy (ADT) and intensity modulated radiation therapy (IMRT).

Materials/Methods: 18 patients receiving concurrent ADT and IMRT underwent sequential CT scan of the pelvis during treat-ment (2 times per week) using a CT-on-rails system. The prostate, seminal vesicles (SV), bladder, rectum, and pelvic bones werecontoured for 338 CT scans. The changes in the volume and position of the prostate were analyzed using the planning CT for com-parison. The position analysis was performed using an in-house developed software (CAT) for soft tissue registration. ADT (con-sisting of Lupron with 2 weeks of Casodex) was started 2 months before the start of the radiation therapy and was continued for atleast 6 months. All of the patients were treated to a total dose of 75.6 Gy in 1.8 Gy fractions to the prostate using an 8-field IMRTtechnique. The seminal vesicles were treated when clinically indicated.

Results: The median absolute change in the volume of the prostate and SV during treatment was -6.0 cc (range -18.8 to +0.6 cc) and-1.1 cc (range -3.8 to +6.3 cc), respectively. The mean relative change in the volume of the prostate and SV during treatment was-8.0% +/- 8.3% and -0.9% +/- 3.7%, respectively. The mean variation of the position of the prostate during treatment was 1.1 mm+/- 4.9 mm in the anterior-posterior (AP) axis, 0.7 mm +/- 3.5 mm in the superior-inferior (SI) axis, and 0.1 mm +/- 3.3 mm in thelateral (RL) axis. The mean magnitude variation was 3.8 mm +/- 3.2 mm in the AP axis, 2.8 mm +/- 2.2 mm in the SI axis, and 2.5mm +/- 2.2 mm in the RL axis.

Conclusions: For patients receiving ADT during radiation therapy, there was a measurable decrease in the volume of the prostateduring treatment. Further study will be needed to determine if this change in prostate volume has any significant dosimetric con-sequences. There was no significant change in the volume of the SV. Variation in the position of the prostate was seen in all threeaxes, with the greatest variation in the AP direction. These results suggest that very wide margins (up to 10 mm) are needed whendesigning a planning target volume (PTV) to account for interfractional motion. As using such wide margins could lead to increaserisk of normal tissue toxicity, daily localization and/or immobilization of the prostate is recommended to minimize the likelihood ofdecreased dose delivery to the prostate due to interfractional motion.

Author Disclosure: S. Choi, None; J.L. Johnson, None; R. de Crevoisier, None; L. Dong, None; J.C. O’Daniel, None; S.J. Frank,None; A.K. Lee, None; R.M. Cheung, None; D.A. Kuban, Calypso Medical, F. Consultant/Advisory Board.

2927 Evaluating Intrafraction Motion of the Prostate in the Prone and Supine Positions using Electromagnetic

Tracking

A. P. Shah, S. L. Meeks, T. R. Willoughby, K. M. Langen, P. A. Kupelian

M.D. Anderson Cancer Center Orlando, Orlando, FL

Purpose/Objective(s): To evaluate differences in target motion during prostate irradiation when positioning a patient in the pronevs. supine positions using 4D real-time tracking of the prostate gland.

Materials/Methods: Twenty patients received radiotherapy in compliance with the institution’s standards for external beam pros-tate irradiation utilizing the Calypso 4D Localization System with a 3 mm threshold for position corrections in the supine position.For 10 fractions following radiation therapy, patients were set up in the prone position and the prostate was tracked for 10 minutes.A separate localization plan was created for prone positioning and no patient immobilization was used. The fraction of time that theprostate was displaced by . 3, . 5, . 7, and . 10 mm was calculated for each session and patient.

Results: In the 20 cases, clear patterns of respiratory motion were seen in the patient tracks due to the influence of abdominal breath-ing from prone positioning. Averaged over all patients, the prostate was displaced . 3 mm and . 5 mm for 28.7% and 4.2% of thetotal tracking time in the prone position, respectively. The maximum displacement time . 5 mm for any one individual was 44.2%of total tracking time for that session. Compared to previously reported supine positioning data, the fraction of time the prostate wasdisplaced . 3 mm and . 5 mm for patients in the supine position was 13.6% and 3.3% of total tracking time, respectively. Inferiorand posterior drifts of the prostate position were also seen with prone positioning. Averaged over all tracking sessions, the prostatewas displaced . 3 mm in the posterior and inferior directions for 6.5% and 7.5% of the time, respectively. For individual sessions,the maximal values for displacements . 5 mm in the posterior and inferior directions were 21.5% and 26.0%, respectively.

Conclusions: With the capability for real time tracking of the prostate, it is possible to study the effects of prostate motion in dif-ferent setup positions without additional radiation. In order to reduce rectal toxicity, radiotherapy in the prone position is a suitablealternative to supine positioning provided respiratory motion is accounted for in treatment planning. In comparison to supine po-sitioning, the percentage of time the prostate moved more than 3, 5, 7, and 10 mm while in the prone position showed similar trendsin reduction with increasing displacement. However, the time of prostate displacement was approximately twice as long in theprone position. Dosimetric impact on this motion remains to be evaluated.

Author Disclosure: A.P. Shah, Calypso Medical, C. Other Research Support; S.L. Meeks, Calypso Medical, C. Other ResearchSupport; T.R. Willoughby, Calypso Medical, C. Other Research Support; K.M. Langen, Calypso Medical, C. Other Research Sup-port; P.A. Kupelian, Calypso Medical, C. Other Research Support.

2928 A New Implantable Multi-function Transmitter for 4DRT, Including Positioning, Dose Measurement

B. Lennernas

University of Gothenburg, Goteborg, Sweden

Purpose/Objective(s): To investigate accuracy in dose and position measurement in an implantable transmitter for 4DRT.

Materials/Methods: A standard Micropos Medical implantable electromagnetic RayPilot positioning transmitter implant for realtime organ/tumor positioning was modified. A radiation sensitive diode was mounted near the positioning device in the tip of the