Melissa Joyner, Presenter: Nothing to Disclose

Patricia Tynan, Abstract Co-Author: Nothing to Disclose

Alonso Gutierrez PhD, Abstract Co-Author: Nothing to Disclose

Niko Papanikolaou PhD, Abstract Co-Author: Nothing to Disclose

Gregory Swanson MD, Abstract Co-Author: Nothing to Disclose

Pre-and post-planning for prostate brachytherapy involves determination of the shape and volume of the gland. Ultrasound (US) is typically utilized to develop the preplan while a CT scan is performed for the postplan because of better seed identification. Since a discrepancy between the measured volumes could impact planning, we investigated whether measured volumes between CT and US were inherently different.

Between August 28, 2007 and March 12, 2008, 33 consecutive patients underwent preplanning prostate volume measurement in the dorsal lithotomy position via trans-rectal US with a B&K Leopard US scanner using 2.5 mm slices, 1 to 2 weeks prior to implant. Images were imported real-time to a Nucletron SpotPro 3.1 brachytherapy treatment planning system (TPS). Following US, a CT scan (GE Medical, Milwaukee WI) was performed with the patient in the supine position using a slice thickness of 2.5 mm. The prostate and urethra were delineated by the same physician on the US and CT scans using the SpotPro TPS. Patients were scanned with US immediately upon completion of the implant, with post-implant CT scans performed within 1 hour.

Preimplantation data showed that approximately 70% (23/33) of patients demonstrated a smaller prostate volume when using CT as compared to US. Preimplant prostate volumes on CT were an average of 23.3% smaller, indicating a discrepancy between modalities. Using subset analysis of pre-implant scans, it was observed that the volumes of smaller glands (25cc), the volume defined by CT were almost consistently 20% smaller than the US. As expected, comparison of preplan and postplan CT scans showed larger gland volumes after implantation, with an average volume increase of 66.9%. Volumes measured with post implant CT were an average of 78% larger than post implant US, representing an average volume difference of 18.1 cc.

Due to difficulty in visualizing the prostate on CT, it is likely that CT-based prostate volumes differ from US-based. Preliminary trends indicate that CT may underestimate the prostate volume by up to 50% in small glands, making this modality poorly suited to pre-implant treatment planning. For small volumes, part of the difference in measurements may be attributed to partial volume effects stemming from the finite slice thickness which dramatically impact the absolute volume. Discrepancies in post-implant measurements indicate that more study is needed, including the effect of leg positioning and seed distortion artifacts on US-based volumes. Ultimately, such volume differences between modalities must be accounted for to ensure adequate dosimetric coverage of the prostate.

Joyner, M, Tynan, P, Gutierrez, A, Papanikolaou, N, Swanson, G, Target Volume Analysis Using Ultrasound and Computed Tomography in Pre- and Post-implant Prostate Brachytherapy Planning.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/7002114.html