CT Prostate Volume Research Articles

Integration of Intraoperative Ultrasound Volume Into CT-Based Treatment Planning in Prostate HDR Brachytherapy: A Pilot Study

Purpose/Objective(s): The main challenge for CT-based prostate HDR treatment planning is to accurately define target (prostate) volume in CT images due to the poor soft-tissue contrast. To overcome this limitation, we propose a novel approach that integrates an intra-operative ultrasound (US)-based prostate volume into treatment planning through US-CT coregistration based on the catheters (needle tracks). Materials/Methods: In CT-based prostate HDR brachytherapy, needle placement is commonly performed under the guidance of intra-operative transrectal ultrasound, while treatment planning is based on post-operative prostate CT images. However, prostate delineation using CT images is difficult. We have developed a technology that integrates an intraoperative US-based prostate volume into HDR treatment planning. Our approach requires an additional 3D US images acquired in the operating room right after the catheters are inserted, which takes less than 5 minutes. These US images are then used to create prostate contours. The catheters are reconstructed on intra-operative US and post-operative CT images, and subsequently used as landmarks for the US-CT image registration using a fuzzy-to-deterministic algorithm. The intra-operative US-based prostate volume is then deformed using B-splines to the CT image to obtain the accurate CT prostate volume, which is used for treatment planning. Three fiducial (gold) markers are implanted into the prostate and the displacements between their positions on the postregistration US and CT images are used to evaluate the accuracy of our technique. This technique was first tested using a prostate phantom, and further investigated in a prospective study of 10 consecutive patients (age: 59 10) who had received HDR monotherapy or boost brachytherapy for prostate cancer. Results: For the prostate phantom study, 14 catheters and 3 gold markers were implanted. The mean displacement of the 3 gold markers between the post-registration US and CT images was 0.41 0.11 mm, which demonstrated the precision of our intra-operative US-based prostate volume approach. In the pilot study of 10 patients, 14-18 catheters were implanted and 9 out of 10 patients had 3 gold markers implanted. US-CT registration was successfully performed for all 10 patients. The mean displacement of 27 gold markers of the 9 patients was 1.09 0.25 mm. This further demonstrated the feasibility and accuracy of our approach. Conclusions:We have introduced a novel approach that integrates intraoperative ultrasound-based prostate volume into CT-based prostate HDR treatment planning, and demonstrated it clinical feasibility. This technique will improve prostate delineation, enable accurate dose planning and delivery, and potentially enhance prostate HDR treatment outcome. Author Disclosure: X. Yang: None. P. Rossi: None. T. Ogunleye: None. W.J. Curran: None. T. Liu: None.

SU‐E‐T‐697: Correlation Between Day 0 Post‐Implant Dosimetry Parameters and Differences in Pre‐and Post‐Implant Prostate Volume‐How Much Can Post‐Plan Quality Be Improved Through Better Assessment of Volume for Permanent Interstitial I‐125 Prostate Implants?

Purpose: To evaluate the relationship between the difference in pre‐and post‐implant prostate volume and post‐implant dosimetric parameters for pre‐planned permanent interstitial prostate implants. Methods: Between January 2007 and December 2011, 964 patients underwent TRUS guided interstitial prostate I‐125 brachytherapy either as monotherapy (MPD 144Gy) or in combination with external irradiation (MPD 108Gy). All procedures and contouring for pre‐and post‐planning was performed by a single physician. Post‐implant dosimetric analysis was performed on CTs taken on the day of implant. Differences in prostate volume as determined by TRUS and CT were evaluated by computing a parameter, δvol, defined as the TRUS prostate volume (VTrus) minus the CT prostate volume (VCT) divided by VTrus. Implants were grouped into the following categories based on their post‐implant dosimetry: 1.Hot (D90>120% and V150>70%); 2. Cold (D90<80%) and; 3.) Standard. Differences in δvol across groups were evaluated. D90, V100 and V150 were plotted against their corresponding δvol values. Results: Overall, the average and standard deviation of δvol, D90, V100 and V150 was 0.0%+/−17.6, 101%+/−12.9, 89.3%+/− 7.0, and 47%+/−12.8, respectively. For the standard, hot, and cold groups, the average and standard deviation of δvol was 0.3%+/−15.5, − 18.0%+/−9.3 and 26.6%+/−24.0, respectively. Plots of the dosimetric parameters versus δvol show wide dispersion, for example R^2 = 0.422 for the D90 plot, but they strongly suggest a downward slope, highlighting the fact that cases with a large difference between VTrus and VCT are more likely to have nonstandard dosimetry than cases with a small difference in these volumes. It is noted that there are cases with δvol close to 0 that have nonstandard post‐plan dosimetry. Conclusion: Our work demonstrates that limiting variation in δvol may reduce but not eliminate the occurrence of nonstandard post‐plans.

Implications of CT imaging for postplan quality assessment in prostate brachytherapy

Postplan quality assurance using CT shows considerable interobserver contour variability. We examined CT postplans of four experienced brachytherapists for comparison with MR-determined prostate volumes. Seventy-five patients had CT and MR scans 1 month post-(125)I prostate brachytherapy. CT scans were contoured by the treating physician and dosimetry calculated. The prostate was contoured independently on MR by one observer with extensive MR experience, the scans were fused and dosimetric parameters compared. The mean prostate volume on CT was 38.3 cc (17.5-78.6 cc), on MR 33.3 cc (16.3-66.1 cc). On average, the volume on CT was 16.1% larger than on MR (range, 8% smaller to 64% larger). Craniocaudal discordance of the CT vs. MR prostate contours ranged from 4 mm cranial to 10 mm caudal to MR base and from 6 mm cranial to 14 mm caudal to MR apex. The CT prostate volume not only included an average of 90% of the MR prostate (range, 75-99%) but also included normal tissue (mean, 8.3 cc; range, 2.9-17.1 cc). The average difference between the calculated D(90) from CT contours vs. MR contours was 10.0 Gy (standard deviation, 8.8; range, -37.6 to +41.6 Gy). On average, only 90% of the MR-defined prostate is included in CT contours, while a volume of normal tissue is erroneously designated as prostate. Lack of awareness of this deficiency in planning and/or operative technique gives a false sense of appreciation of the true conformality, delays implementation of corrective measures, and risks unnecessary side effects.

Can the Number of Leftover Seeds Be Reduced? A Study of Prostate Volume, Nomogram, and Delivered Activity for Ultrasound-Guided 125I Prostate Seed Implant

Purpose: Using a current nomogram system, based on pre-implant CT prostate volume and source activity, 10% extra seeds were ordered at our institution, but an average of 28% excess seeds were leftover after implant. In this analysis, we compare total activity of 125I seeds for TRUS-guided prostate brachytherapy calculated from 1) pre-implant CT volume and nomogram, 2) intra-op US volume and nomogram, and 3) implanted activity based on intraoperative planning. We also investigate the main factors contributing to excess seeds ordered (versus seeds implanted) and then derive a customized nomogram for a specific seed activity that is used in our institution.

WE-B-BRA-02: Deformable Registration Based Tool to Obtain Cumulative Doses from External Beam IMRT and HDR Interstitial Brachytherapy

Purpose: To obtain cumulative doses from external beam (EB) IMRT and HDR interstitial brachytherapy (BT) for prostate cancer. Methods and Materials: A tool was developed for multimodality image manipulation and registration within our current treatment planning framework to obtain cumulative doses from BT and EB-IMRT. The HDR BT boost performed under transrectal ultrasound guidance is planned using Nucletron Oncentra prostate TPS. A total implant dose of 21 Gy is administered in two 10.5 Gy fractions. The daily external beam dose to the PTV was 2 Gy given in 23 fractions and planned with IMRT using Pinnacle TPS. US DICOM images, structures and 3-D dose matrix were pre-processed for import with in Pinnacle TPS. US and CT images were rigidly registered as a preliminiary step inside Pinnacle based on prostate contours on two imagesets and implanted visicoils markers. A linear-quadratic model is applied to calculate the total equivalent dose in 2Gy fractions (EQD2) with αβ=4 for prostate. Tools were developed to perform deformable registration based on biomechanical model of human organs and the finite element analysis methods. Result: A biomechanical model of US and CT prostate volume was generated on a clinical case using 5544 nodes (5240 subvolumes). The deformation vector field generated after deforming prostate contours was used to deform 3D US dose matrix on to CT grid. Using EQD2 of 50.4 Gy from brachytherapy and 46 Gy from IMRT, the cumulative D99 and D95 to CT prostate volume was 98.2 Gy and 100.4Gy. Conclusion: There is a lack of availability of commercial TPS in which EBRT and BT plans can be fully integrated. The tool provides a robust and accurate means of obtaining cumulative doses from brachytherapy and EB- IMRT. It also has the potential to incorporate doses from brachytherapy during inverse planning adaptive optimization of external beam IMRT.

Early urinary morbidity after I125 transperineal interstitial permanent prostate brachytherapy. A multivariate analysis of 600 patients treated by the Institut Curie/Hopital Cochin/Hopital Necker Paris Group

To assess the early urinary morbidity experienced by the patients undergoing prostate brachytherapy by I125 permanent implants, and to identify the relevant clinical and dosimetric parameters. From May, 1998 and June, 2003, 600 patients were treated by the Institut Curie/Hopital Cochin/Hopital Necker group, by I125 transperineal interstitial permanent brachytherapy (TPIPB) for localized prostate cancer. All patients had no or moderate urinary symptoms (I-PSS < 15). In all cases, a systematic preventive treatment of the urinary symptoms was given (steroid or non steroid anti-inflammatory drugs and two different alpha-blockers). For the early urinary morbidity was analyzed at 2 and 6 months, and scored according to the RTOG classification. Multivariate analysis included clinical factors (age, T stage, PSA), hormonal therapy and its duration, ultrasound (US) dosimetric criteria (prostate volume, D90, D95, V100, rectal volume receiving more than 160 Gy), CT-scan criteria (prostate volume, D90, D95, V100, V150, rectal volume receiving more than 30 Gy, 50 Gy, 160 Gy and 215 Gy), and preventive treatments. Different cutoffs were used for each linear criteria. At 2 months, 19% of the patients were scored grade 0, 40% grade 1, 30% grade 2 and 9% grade 3, only 2% patients presented with a grade 4-toxicity (catheter). At 6 months, 54% of the patients were scored score 0, 36% grade 1, 8% grade 2, 2% grade 3 and 0% grade 4. Multivariate analysis shows that a total implanted activity >30 U (p < 0.0002), the first 200 patients (p < 0.002), a CT prostate volume >50 cc (p < 0.017) and a D90>180 Gy (p < 0.046) were independent factors for increased grade 2/3/4 urinary morbidity at 2 months. At 6 months, only a total implanted activity >40 U (p < 0.001) and T2 stage (p < 0.029) were found to be factors predicting for grade 2/3 urinary morbidity. Our results confirm that a significant urinary morbidity (RTOG ≥ 2) is observed in almost half of our patients at two months after TPIPB. The symptoms decrease thereafter; at 6 months only 10% of the patients reach a RTOG score of 2 or more.

A comparison of CT scan to transrectal ultrasound-measured prostate volume in untreated prostate cancer

Purpose To compare CT and transrectal ultrasound (TRUS)-measured prostate volumes in patients with untreated prostate cancer. Methods and materials Between 1995 and 1999, 48 consecutive patients at the Portland Veterans Affairs Medical Center were treated with external beam radiotherapy. In 36 of these patients, TRUS and CT measurements of the prostate volume were obtained before treatment and <6 months apart. The TRUS volume was calculated using the prolate ellipsoid formula. The CT volume was calculated from the contours of the prostate drawn by one physician, who was unaware of the TRUS volume calculation, on axial CT images. Results The TRUS and CT prostate volume measurements correlated strongly (Pearson’s correlation coefficient = 0.925, 95% confidence interval 0.856–0.961, p < 0.0001). The CT volume was consistently larger than the TRUS volume by a factor of approximately 1.5. In men with a TRUS prostate volume less than the median (<28 cm 3), the CT/TRUS volume ratio was 1.7, and it was 1.4 for men whose volume was greater than the median. The CT volumes were correlated similarly with the TRUS volumes regardless of the CT slice interval. Conclusion A strong correlation was found between CT scan and TRUS measurement of the prostate volume; however, CT consistently overestimated the prostate volume by approximately 50% compared with TRUS.

Comparison of MRI pulse sequences in defining prostate volume after permanent implantation

Purpose : To determine the relative value of three MRI pulse sequences in defining the prostate volume after permanent implantation. Methods and Materials : A total of 45 patients who received a permanent 125I implant were studied. Two weeks after implantation, an axial CT scan (2 mm thickness) and T 1-weighted, T 1-weighted fat saturation, and T 2-weighted axial MRI (3-mm) studies were obtained. The prostate volumes were compared with the initial ultrasound planning volumes, and subsequently the CT, T 1-weighted, and T 1-weighted fat saturation MRI volumes were compared with the T 2-weighted volumes. Discrepancies in volume were evaluated by visual inspection of the registered axial images and the registration of axial volumes on the sagittal T 2-weighted volumes. In a limited set of patients, pre- and postimplant CT and T 2-weighted MRI studies were available for comparison to determine whether prostate volume changes after implant were dependent on the imaging modality. Results : T 1-weighted and T 1-weighted fat saturation MRI and CT prostate volumes were consistently larger than the T 2-weighted MRI prostate volumes, with a volume on average 1.33 (SD 0.24) times the T 2-weighted volume. This discrepancy was due to the superiority of T 2-weighted MRI for prostate definition at the following critical interfaces: membranous urethra, apex, and anterior base-bladder and posterior base-seminal vesicle interfaces. The differences in prostate definition in the anterior base region suggest that the commonly reported underdose may be due to overestimation of the prostate in this region by CT. The consistent difference in volumes suggests that the degree of swelling observed after implantation is in part a function of the imaging modality. In patients with pre- and postimplant CT and T 2-weighted MRI images, swelling on the T 2-weighted images was 1.1 times baseline and on CT was 1.3 times baseline, confirming the imaging modality dependence of prostate swelling. Conclusion : Postimplant T 2-weighted MRI images provided superior prostate definition in all critical regions of the prostate compared with CT and the other MRI sequences tested. In addition to defining an optimal technique, these findings call two prior observations into question. Under dosing at the anterior base region may be overestimated because of poor definition of the prostate-bladder muscle interface. The swelling observed after implantation was lower on T 2-weighted images as well, suggesting that a fraction of postimplant swelling is a function of the imaging modality. These findings have implications for preimplant planning and postimplant evaluation. As implant planning techniques become more conformal, and registration methods become more efficient, T 2-weighted MRI after implantation will improve the accuracy of postimplant dosimetry.

Impact of ultrasound and computed tomography prostate volume registration on evaluation of permanent prostate implants

Ultrasound (US)-guided permanent prostate implants typically use US prostate volumes to plan the implant procedure and CT prostate volumes for 3D dosimetric evaluation of the implant. Such a protocol requires that CT and US prostate volumes be registered. We have studied the impact of prostate volume registration on postimplant dosimetry for patients with low-grade prostate cancer treated with combined US and fluoroscopic-guided permanent implants. A US image set was obtained with the patient in the lithotomy position to delineate the prostate volume that was subsequently used for treatment planning. Each plan was customized and optimized to ensure complete coverage of the US prostate volume. After implant, a CT scan was obtained for postimplant dosimetry with the patient lying supine. Sources were localized on CT by interactively creating orthogonal images of small cubes, whose dimensions were slightly larger than the source, to assure unique identification of each seed. Ultrasound and CT 3D surfaces were registered using either (a) the rectal surface and base of the prostate, or (b) the Foley balloon and urethra as the alignment reference. A dose distribution was assigned to the US prostate volume based on the CT source distribution, and the dose-volume histogram (DVH) was calculated. Prostate volumes drawn from US images differ from those drawn from CT images with the CT volumes being typically larger than the US volumes. Urethral registration of the prostate volume based on aligning the prostatic urethra generates a dose distribution that best follows the preimplant plan and is geometrically the preferable choice for dosimetry. The dose distribution and the DVH for the US prostate is sensitive to the mode of registration limiting the ability to determine if acceptable dose coverage has been achieved.

Source placement error for permanent implant of the prostate.

The performance of ultrasound (US) and fluoroscopic-guided permanent 125I source implant of the prostate using CT identification of the source positions has been evaluated. Marker seeds were implanted during the planning study to assist in the alignment of the US and CT prostate volumes for treatment planning and to guide the placement of needles. The relative positions of the needles and marker seeds were checked by fluoroscopy. A postimplant CT study was used to input the radioactive source positions and to register the sources relative to the preimplant CT and US prostate volumes and the planned source distribution. Source placement errors observed were categorized as: (1) source-to-source spacing differences; (2) needle placement error, both depth and position; and (3) seed splaying, particularly near the prostate periphery. Errors due to source splaying and spacing were in part attributed to prostate motion. Later refinements included fixed-spaced string sources, for which placement errors were smaller than for unattached sources. However, source placement errors due to needle placement error and prostate motion remained unchanged.