Shape Characterization of the Prostate and Urethra Applicable to the Design of a Trans-Urethral Ultrasound Imaging Probe for Prostate Brachytherapy Guidance

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I. J. Radiation Oncology

● Biology ● Physics

Volume 63, Number 2, Supplement, 2005

Materials/Methods: A cohort of 45 patients undergoing radical treatment for pelvic malignancies was enrolled in this prospective study. None had gross inguinal lymph node metastases. A pre-contrast MRI scan was obtained from the mid-abdomen to below the pelvic bone. Combidex (2.6 mg Fe/kg) was infused intravenously over 30 min, and a second MRI scan was performed 24 hours after the infusion. T2W and T2* images 3 mm apart were acquired and imported to a 3D image analysis program (3D-Doctor, Able Software Corp., Lexington, MA) for nodal identification and evaluation. The number and size of the inguinal nodes, depth from the skin surface, and distance from the femoral artery were recorded. Results: The median number of right inguinal nodes was 7 (range 2–11) while on the left it was 6.5 (1–11). Median size was 0.6 (0.2–1.2) cm. The median depth from the skin surface was 2.3 (1– 6.3) cm on the right and 2.8 (1.5–7.1) cm on the left. Distance from the right femoral artery was 1.8 (0.7– 4.1) cm, and from the left 1.9 (0.7– 4.5) cm. Conclusions: MRI images enhanced with Combidex reliably identifies inguinal lymph nodes and provided a reference for adjuvant radiation treatment planning. These nodes may be deeply situated at a distance from the femoral vessels, raising concern about geographic miss with conventional planning, and reinforcing the potential value of high-resolution contrastenhanced imaging to identify nodal volume in individual patients.

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Studies on the Safety and Efficacy of an in Vivo Dosimeter During Radiation Therapy

C.W. Scarantino,1,2 M. Anscher,3 M. Aquino,2 T. Carrea,2 R. Prosnitz,3 R. Lee,4 R. Black2 Radiation Oncology, Rex Cancer Center, Raleigh, NC, 2Sicel Technologies Inc, Morrisville, NC, 3Radiation Oncology, Duke University School of Medicine, Durham, NC, 4Department of Radiation Oncology, Wake Forest University School of Medicine, Winston-Salem, NC

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Purpose/Objective: The initial results of pilot studies on an in-vivo dosimeter have been reported. The final results of these pilot studies, which included several different primary sites, confirmed the pre-clinical experience and verified the safety and lack of adverse events of the device. The purpose of this clinical pivotal study is to confirm the safety of the implantable device, as previously reported in pilot studies, only in patients with breast and prostate cancer. Materials/Methods: The implantable device is a product of Sicel Technologies Inc. as reported previously2. The pivotal study will include only patients with a diagnosis of breast and prostate cancer who require radiation therapy. The primary endpoints of this study are to a) assess safety and movement of the device (adverse events), b) to compare in vivo measured dose with the calculated dose. To date, 24 patients have been entered onto the study- 15 patients with breast cancer and 9 patients with prostate cancer. This report will present information on all patients entered onto study. Patients with breast cancer had the dosimeters implanted in the tumor bed and in normal tissue at the time of tylectomy or in the Radiation Oncology Dept. under local anesthesia. A retention device was used to secure the device. A dose point calculation for the predicted dose was determined at the location of the dosimeter end of the device. Following the initiation of radiation therapy, treatment planning CT scans were repeated every two weeks to determine movement of the dosimeters. The in vivo radiation dose was measured daily following each treatment session. Results: All the breast patients (15)and all but one prostate patient received two dosimeters. All devices were observed on CT scan or with orthovoltage X-rays. Previous experience indicates that the devices are visible on port films if the antenna of the device is composed of gold wire. In accordance with the purpose of this study, there have been no adverse events or any significant movement associated following implantation. The dosimetric readings have not been evaluated as the results are blinded and will not be available until the study is complete. Variance of ⬎ 8% between measured to expected dose required physicians to be notified. However, new treatment plans were not allowed to correct the dosimetric variance. Since the devices were visible using orthovoltage simulation, the tumor and/or tumor bed location was aided by the presence of the devices. Conclusions: The results of all the studies to date indicate the devices are safe and free of adverse events. Although the pilot studies have reported significant variance between measured and predicted dose, the results of the final pivotal study are not yet available but will be at the time of presentation. Finally, the application of the device for image guided radiotherapy will be discussed. 1. Scarantino C, Rini C, Aquino M, et al. The initial clinical results of an in vivo dosimeter during external beam radiation therapy. Int J Rad Onc Bio Phys 2005, In Press. 2. Scarantino C, Rini C, Bolick N, et al. In vivo dosimetry during external beam radiotherapy utilizing an implantable telemetric and Dosimetric device. Int J Rad Onc Bio Phys 2002, 54:123 Abst#207 Supported in part by NCI R21 CA97859.

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Shape Characterization of the Prostate and Urethra Applicable to the Design of a Trans-Urethral Ultrasound Imaging Probe for Prostate Brachytherapy Guidance

D.R. Holmes,1 B.J. Davis,2 C.C. Goulet,2 T.M. Wilson,3 L.A. Mynderse,3 M.G. Herman,2 R.A. Robb1 Biomedical Imaging Resource, Mayo Clinic College of Medicine, Rochester, MN, 2Division of Radiation Oncology, Mayo Clinic College of Medicine, Rochester, MN, 3Department of Urology, Mayo Clinic College of Medicine, Rochester, MN

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Purpose/Objective: Recent clinical studies of trans-urethral ultrasound (TUUS) during permanent prostate brachytherapy (PPB) demonstrate that approximately 82% of seeds were visible using this modality as compared to 40 – 60% of seeds by trans-rectal ultrasound. Consequently, the potential of this modality to facilitate intra-operative radiation dosimetry is evident. The purpose of this study is to provide data relevant to the design of a custom TUUS device. (Supported by NIH Grant R33CA107933-01.) Materials/Methods: A total of 190 post-implant PPB CT datasets from our institution were used to characterize the prostate and urethra. Clinical features of these patients included: clinical stage (T1c: 77%; T2a/b: 22%; T3b: 0.5%), pre-therapy PSA (mean: 6.6, range: 0.7–34.1), and Gleason score (5: 3%; 6: 83%; 7: 15%) and CT-based post-implant prostate volume (PV)

Proceedings of the 47th Annual ASTRO Meeting

(mean: 49cc; range: 22–106cc). The prostate, rectum, urethra, and bladder were all contoured. PV shells were calculated at uniformly spaced distances from the urethra and rectum. Seed-to-urethra (SU) and seed-to-rectum (SR) measurements were also determined. The base-to-apex length of the prostate and curvilinear prostatic urethral length was calculated. Results: Prostate mean length is 4.5 cm with 5th and 95th percentile at 3.0cm and 5.7cm. The mean curvilinear length of the prostatic urethra is at 4.4cm. The mean prostatic urethral bend is 30.3 ⫹ 12.2 degrees. Mean prostate-urethra and prostate-rectal surface distances are 2.7cm and 4.5cm (p⬍0.001,one-sided paired t-test) respectively. Mean SU and SR distances are 1.6cm and 2.3cm (p⬍0.001). The smallest and largest prostates were analyzed to determine the extent of anatomical variation. For the smallest prostate, the maximum prostate-urethra and prostate-rectum surface distance is 2.4cm and 3.7cm, respectively, whereas for the largest prostate, these distances are 3.9cm and 6.0cm. Figure 1(a) shows a plot of the normalized PV as a function of these distances. Figure 1(b) shows the cumulative percentage of seeds as a function of SU and SR distances, where the maxima for SU and SR are 3.3cm and 5.2cm, respectively. Conclusions: Based on the data analyzed in this study, an optimal TUUS imaging device would have a 4 – 6cm imaging field of view (FOV). In order to capture the entire prostate during one sweep, a custom TUUS device should have a 2.5–3cm radial imaging depth. Applications envisioned for a custom TUUS device include prostate brachytherapy, cryotherapy, thermal therapy, biopsy guidance and others.

Figure 1. Cumulative distance plots for 190 patient CT datasets. On the left, the plot shows the normalized PV as a function of radial distance for the smallest and largest prostate case. On the right, the total number of seeds within a given radial distance to the urethra and rectum is shown. Both plots confirm that a trans-urethral ultrasound imaging device requires a 3cm radial FOV versus a trans-rectal device which requires at least a 6cm FOV.

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Image Guided Radiation Therapy for Prostate IMRT: Rectum Volume Changes and Dosimetric Considerations

L. Chen, K. Paskalev, J. Zhu, X. Xu, L. Wang, R.A. Price, JR, E. Horwitz, S. Feigenberg, C.C. Ma, A. Pollack Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA Purpose/Objective: To investigate the change in rectal dose due to the inter-fractional rectum volume variation for patients with prostate cancer treated with intensity modulated radiation therapy (IMRT). Materials/Methods: Twenty prostate cancer patients treated with IMRT were included in this study. An enema was administered within 2 hours of simulation to empty the bowel for each patient. Each patient underwent sequential CT- and MRI simulations with a minimal rectal volume for treatment planning. MR and CT data sets were fused for target delineation. IMRT treatment planning was performed on the CT image. Inter-fractional prostate motion was corrected using a CT-on-Rails system prior to treatment. CT images were also taken after the treatment to compare target variation and isocenter shift during a treatment. In this study, rectal contours were generated on both simulation CT images and subsequent treatment CT images. IMRT plans were generated based on our clinical acceptance criterion. The subsequent treatment CT images for each patient from the CT-on-Rails system were used to recompute the patient dose distributions with the same leaf sequences used for treatment. The isocenter was shifted relative to the simulation CT, as required by the protocol, to ensure appropriate target coverage. The rectal doses based on the subsequent treatment CT were compared with the original doses planned on the simulation CT scans using our clinical acceptance criteria. Results: The results show that patient rectal volume varies significantly between fractions, and as a response to the radiation dose, decreases during the course of treatment for some patients. Figure 1 shows the dose-volume histograms for a patient, whose rectal volume changed between 50.2 and 161.7 cc during the treatment course. For an IMRT plan based on an empty rectum, all subsequent rectal DVH values satisfy our clinical acceptance criteria (V40 ⬍ 35%; V65 ⬍17%). If the IMRT plan was based on one of the subsequent CT scans without controlling the rectal volume some of the rectal DVH values might not meet the requirements of our clinical protocol. Conclusions: Due to the large inter-fractional variation of the rectal volume it is more favorable to plan prostate IMRT based on an empty rectum. Smaller rectal volumes are generally more difficult to plan for the same target volume and acceptance criteria because they represent the worst-case scenario. Accurate prostate localization is also needed to ensure that the actual doses received by the rectum will not fail the clinical treatment criteria during a treatment course. An enema

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