Postimplant Computed Tomography Research Articles

Saturday, June 18, 20229:30 AM - 10:30 AM GPP06 Presentation Time: 9:30 AM: Image-Guided Cervical Cancer Brachytherapy: Summary of Consensus Recommendations from the Society for Abdominal Radiology and American Brachytherapy Society

<h3>Purpose</h3> To summarize recommendations regarding imaging guidance for cervical cancer brachytherapy implant placement from the Society of Abdominal Radiology (SAR) and American Brachytherapy Society (ABS). <h3>Materials and Methods</h3> A comprehensive literature search was conducted on PubMed for image-guided cervical cancer brachytherapy (BT) using radiography, ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI). Original research studies were included for evaluation if they included sensitivity and specificity of tumor detection, complication rates, or additional quantitative imaging results directly related to brachytherapy, such as geometric distortion measurements. The literature review was evaluated by an expert panel of members from the Society for Abdominal Radiology (SAR) and American Brachytherapy Society (ABS). These data were analyzed to determine an overall level of evidence for each recommendation and the subsequent strength of each claim. <h3>Results</h3> Three-dimensional (3D) imaging is recommended for treatment planning whenever feasible due to the rate of toxicities ≥ Grade 3 decreasing from between 0-13.5% with 2D planning to 0-5.8% with 3D planning. US guidance is highly recommended for tandem selection and insertion to reduce uterine perforation rates from 2.3-34%/insertion without US-guidance to 0-1.4%/insertion with US. US-guided interstitial needle placement is feasible when post-implant 3D imaging is available for treatment planning. Post-implant CT is recommended over orthogonal radiography to reduce genitourinary and gastrointestinal complications and evaluate proper applicator positioning. Tumor visualization is optimal on T2-weighted (T2w) MRI, which also provides improved specificity for assessment of parametrial invasion. CT-based treatment planning should refer to prior MRI for high risk-clinical target volume (HR-CTV) contouring, when available. Treatment planning on MRI is highly recommended for HR-CTV delineation, and post-applicator imaging should include at minimum both a 2D sagittal T2w and 3D axial T2w fast spin echo-based sequence. <h3>Conclusions</h3> Imaging-guidance for cervical brachytherapy implant placement is now standard-of-care. 3D imaging with MRI is highly recommended for treatment planning purposes, although CT-based planning also results in fewer complications as compared to 2D planning techniques. Ultrasound is useful for applicator placement, but application to treatment planning is limited.

Computed tomography analysis of coronary ostia location following valve-in-valve transcatheter aortic valve replacement with the ACURATE neo valve: Implications for coronary access.

Valve-in-valve transcatheter aortic valve replacement (ViV-TAVR) is an emerging alternative to re-do surgery. However, the challenge of coronary access (CA) following ViV-TAVR is a potential limitation as TAVR expands to younger lower-risk populations. Using post-implantation computed tomography (CT) scans to evaluate the geometrical relationship between coronary ostia and valve frame in patients undergoing ViV-TAVR with the ACURATE neo valve. Post-implant CT scans of 18 out of 20 consecutive patients treated with the ACURATE neo valve were analyzed. Coronary ostia location in relation to the highest plane (HP) (highest point of the ACURATE neo or surgical valve) was determined. Ostia located below the highest plan were further subclassified according to the gap available between the transcatheter heart valve frame and ostium (transcatheter-to-coronary [TTC] distance). The impact implantation depth has on these geometrical relationships was evaluated. A total of 21 out of 36 coronary ostia (58%) were located below the level of the HP with the left coronary artery (36%) more likely to be affected than the right (22%). Further sub-classification of these ostia revealed a large (>6 mm), moderate (4-6 mm), and small (<4 mm) TTC distance in 57% (12/21), 38% (8/21), and in 6% (1/18) of cases, respectively. At an implantation depth <4 mm compared to >4 mm, all ostia were located below the HP with no difference in post-procedural mean gradients (14.5 mmHg ± 4.7 vs. 12.6 mmHg ± 5.8; p=.5, 95%CI 3.8-7.5). CA following ACURATE neo implantation for ViV-TAVR could potentially be challenging in a significant proportion of patients and specific consideration should be given to the implantation depth.

Risk analysis of hemorrhage in stereo-electroencephalography procedures.

To examine the true incidence of hemorrhage related to stereo-electroencephalography (SEEG) procedures. To analyze risk factors associated with the presence of different types of hemorrhage in SEEG procedures. This was a retrospective, single-center observational study examining every SEEG implantation performed at our center from 2009 to 2017. This consisted of 549 consecutive SEEG implantations using a variety of stereotactic and imaging techniques. A hemorrhage grading system was applied by a blinded neuroradiologist to every postimplant and postexplant computed tomography (CT) scan. Hemorrhages were classified as asymptomatic or symptomatic based on neurologic deficit seen on examination. Statistical analysis included multivariate regression using relevant preoperative variables to predict the presence of hemorrhage. One hundred five implantations (19.1%) had any type of hemorrhage seen on postimplant CT. Of these, 93 (16.9%) were asymptomatic and 12 (2.2%) were symptomatic, with 3 implantations (0.6%) resulting in either a permanent deficit (2, 0.4%) or death (1, 0.2%). Male sex, increased number of electrodes, and increasing age were associated with increased risk of postimplant hemorrhage on multivariate analysis. Increasing score in the grading system was related to a statistically significant increase in the likelihood of a symptomatic hemorrhage. Detailed examination of every postimplant CT reveals that the total hemorrhage rate appears higher than previously reported. Most of these hemorrhages are small and asymptomatic. Our grading system may be useful to risk stratify these hemorrhages and awaits prospective validation.

Dose-volume analysis of target volume and critical structures in computed tomography image-based multicatheter high-dose-rate interstitial brachytherapy for head and neck cancer.

PurposeTo evaluate dose-volume relationships of target volume and critical structures in computed tomography (CT) image-based brachytherapy for head and neck cancer.Material and methodsThirty-seven patients with mobile tongue, floor of mouth, and base of tongue cancer treated with brachytherapy (post-operative alone and as a boost after external beam radiotherapy [EBRT], or definitive alone or as a boost after EBRT) were selected. Treatment plans were made using post-implant CT images. The fractionation schedule was 7-15 × 3-5 Gy for post-operative (with or without EBRT), 14-15 × 3 Gy for definitive alone, and 5-10 × 3 Gy for boost treatments. For the target volume, V100, D90, and dose non-uniformity ratio (DNR) were calculated. For the mandible, spinal cord and salivary glands doses to specified volumes were reported.ResultsThe median values of V100 and D90 were 89.9% and 99.9%, respectively; the median values of DNR was 0.46. The median D2cm 3 of the mandible and spinal cord were 48.3% and 5.8%, respectively. The ipsilateral median D2cm 3 of parotid and submandibular glands were 6.4% and 12.5%, whereas on the contralateral side, the corresponding values were 5.3% and 7.0%, respectively.ConclusionsUsing conformal treatment planning, it was desirable to keep the dose to the mandible, spinal cord, and salivary glands at an acceptable level. The quantitative plan evaluation may help us find correlations between dosimetric parameters and clinical outcome, which may lead to improve the quality of the treatment, but it requires longer follow-up and results from other studies.

Treatment planning for multicatheter interstitial brachytherapy of breast cancer - from Paris system to anatomy-based inverse planning.

In the last decades, treatment planning for multicatheter interstitial breast brachytherapy has evolved considerably from fluoroscopy-based 2D to anatomy-based 3D planning. To plan the right positions of the catheters, ultrasound or computed tomography (CT) imaging can be used, but the treatment plan is always based on postimplant CT images. With CT imaging, the 3D target volume can be defined more precisely and delineation of the organs at risk volumes is also possible. Consequently, parameters calculated from dose-volume histogram can be used for quantitative plan evaluation. The catheter reconstruction is also easier and faster on CT images compared to X-ray films. In high dose rate brachytherapy, using a stepping source, a number of forward dose optimization methods (manual, geometrical, on dose points, graphical) are available to shape the dose distribution to the target volume, and these influence dose homogeneities to different extent. Currently, inverse optimization algorithms offer new possibilities to improve dose distributions further considering the requirements for dose coverage, dose homogeneity, and dose to organs at risk simultaneously and automatically. In this article, the evolvement of treatment planning for interstitial breast implants is reviewed, different forward optimization methods are discussed, and dose-volume parameters used for quantitative plan evaluation are described. Finally, some questions of the inverse optimization method are investigated and initial experiences of the authors are presented.

Single-energy metal artifact reduction in postimplant computed tomography for I-125 prostate brachytherapy: Impact on seed identification

PurposeTo evaluate the effectiveness of the single-energy metal artifact reduction (SEMAR) technique for improving the accuracy of I-125 seed identification in postimplant computed tomography (CT) after prostate brachytherapy. Methods and MaterialsPostimplant CT images of 40 patients treated with I-125 prostate brachytherapy were acquired. For all patients, 2 data sets were reconstructed, 1 with SEMAR algorithms (SEMAR image), and the other without SEMAR algorithms (non-SEMAR image). Seed locations are automatically detected by the automatic seed finder tool, and their locations were compared between the SEMAR and non-SEMAR images. Dosimetric parameters using seed locations as detected were compared. ResultsThe true-positive fraction of properly detected seeds on the SEMAR image as determined from a reference seed distribution defined by one investigator was significantly higher than the true-positive fraction on the non-SEMAR image (p = 0.011). The variabilities in D90 (p = 0.001), V100 (p = 0.007), and V150 (p = 0.007) were significantly reduced for seed location on the SEMAR image as compared with non-SEMAR image. ConclusionsProstate postimplant CT with SEMAR improved the accuracy of seed localization and postimplant dosimetric parameters.

Prostate Postimplant CT With Metal Artifacts Reduction for Improvement of Seed Localization

Low dose-rate brachytherapy involves the implantation of many (50-100) permanent radioactive seeds in and around the prostate volume. The standard dosimetry technique is based on computed tomography (CT) imaging, however, the CT images themselves suffer from metal artifacts due to the I-125 seeds, resulting in insufficient visualization of the I-125 seeds and prostate. Recently, some CT manufacturers unveiled the metal artifact reduction technique, which removes metal artifacts in images due to metallic implants in the body.

Dosimetric effects of prone and supine positions on post-implant assessments for prostate brachytherapy

PurposePost-implant dosimetric assessment is essential for optimal care of patients receiving prostate brachytherapy. In most institutions, post-implant computed tomography (CT) is performed in the supine position. This study aimed to assess variability in dosimetric parameters with postural changes during acquisition of post-implant CT scans.Material and methodsIn total, 85 consecutive patients were enrolled in this study. Fifty-three patients underwent seed implantation alone, and the remaining 32 received a combination of seed implantation and external beam radiotherapy. For post-implant analyses, CT scans were obtained in two patient positions, supine and prone. To evaluate differences in dosimetric parameters associated with postural change, the dosimetric data obtained in the supine position were defined as the standard.ResultsThe median prostate volume was 22.4 ml in the supine and 22.5 ml in the prone position (p = 0.51). The median prostate D90 was 120.1% in the supine and 120.3% in the prone position, not significantly different. The mean prostate V100 was 97.1% in the supine and 97.0% in the prone position, again not significantly different. Median rectal V100 in supine and prone positions were 0.42 ml and 0.33 ml, respectively (p < 0.01). Rectal D2cc was also significantly decreased in the prone as compared with the supine position (median, 59.1% vs. 63.6%; p < 0.01). A larger post-implant prostate volume was associated with decreased rectal doses in the prone position.ConclusionsThough there were no significant differences among prostate D90 assessments according to postural changes, our results suggest that post-implant rectal doses decreased in the prone position.

Impact of MRI-based postimplant dosimetric assessment in prostate brachytherapy using contrast-enhanced T1-weighted images

PurposeTo compare contrast-enhanced T1-weighted (CE-T1WI) magnetic resonance imaging (MRI) with computed tomography (CT) for postimplant dosimetry and seed recognition in prostate brachytherapy. Methods and MaterialsA total of 245 patients who received 125I prostate brachytherapy with or without external beam radiotherapy were enrolled. For postimplant analysis, CT and MRI scans were obtained at 1 month after seed implantation. For MRI-based dosimetry, T2-weighted images were fused with the CE-T1WI; the prostate was delineated on the T2-weighted images, and the seed detection was performed manually on the CE-T1WI. In CT-based dosimetry, the seed detection was essentially performed automatically. The dosimetric results obtained by MRI-based and CT-based dosimetry were compared. ResultsThe mean prostate D90 (the minimum dose received by 90% of the prostate volume) estimated by MRI-based and CT-based dosimetry were 113% and 115%, respectively, with no significant difference. The mean prostate V100 (the percent volume of the postimplant prostate receiving 100% of the prescribed dose) estimated by MRI-based and CT-based dosimetry were 95.2% and 95.8%, respectively, again with no significant difference. The mean prostate V150 (the percent volume of the postimplant prostate receiving 150% of the prescribed dose) estimated by MRI-based and CT-based dosimetry were 52.8% and 57.0%, respectively (p<0.01). In all of the 35 patients (14%) in whom the MRI-based V150 were at least 10% lower than the CT-based results, the seed detection by CT-based dosimetry was overestimated in highly seed-clustered areas or in the areas close to calcifications because of reconstruction artifacts in CT images. ConclusionsMRI-based dosimetry using CE-T1WI appears to be acceptable. Our results suggest that MRI-based dosimetry is a practical method for estimation of the higher dose distribution, especially if seeds are clustered together or when they are close to calcifications.

Evaluation of interobserver differences in postimplant dosimetry following prostate brachytherapy and the efficacy of CT/MRI fusion imaging

Interobserver differences in postimplant dosimetry based on computed tomography (CT) and CT/magnetic resonance imaging (MRI) fusion images were assessed to evaluate the efficacy of the fusion image. In addition, the part of the prostate contour responsible for the interobserver differences in CT was identified. In June 2008, 1-month postimplant CT data from two patients who underwent (125)I prostate brachytherapy were sent to 90 institutions for postimplant dosimetry. Subsequently, MRI data from the same patients were sent for fusion-based postimplant dosimetry. The variance of the difference between MRIbased D90 and CT-based or fusion-based D90 was compared. Prostate volume on CT was plotted on the y-axis against the position of the most cranial and caudal slices in the prostate contour delineated at each institution to analyze interobserver differences. The prostate volume from CT was significantly greater than from the CT/MRI fusion image (P = 0.0014). Fusion-based variance was significantly greater than CT-based variance (P < 0.01). CT-based postimplant dosimetry showed that 88%-96% of the institutions had an apical and basal position within a range of 5 mm. There were marked interobserver differences in CT/MRI fusion-based postimplant dosimetry.

Interactive-plan technique conquers the disadvantages of volume-reducing hormone therapy in 125I permanent implantation for localized prostate cancer

The purpose of this study was to assess the impact of hormone therapy on post-implant dosimetry in patients in whom pre-plan and interactive-plan techniques were used for transperineal brachytherapy against prostatic cancer. The subjects comprised 244 patients treated using (125)I seed implantation as monotherapy. The prescribed dose to the periphery of the prostate was 145 Gy. The pre-plan technique was used for 116 patients, and the interactive-plan technique for 128 patients. Hormone therapy was used in 71 patients (29.1%). The D90 (dose to 90% of prostate volume) of post-implant computed tomography (CT) analysis was assessed in both groups. In addition, the ratio of post-implant CT volume to preoperative ultrasonography (US) volume was assessed. In the pre-plan group, D90 was significantly lower for patients who received hormone therapy than for those who did not (P = 0.035). However, in the interactive-plan group, D90 did not differ between patients with and without hormone therapy (P = 0.467). The CT-to-US prostate volume ratio was 1.022 for patients who received hormone therapy and 0.960 for patients who did not (P = 0.021). Post-traumatic swelling following implantation is increased by cessation of hormone therapy and may reduce D90. However, the present results suggest that the interactive-plan technique overcomes this disadvantage of hormone therapy.

Difference in rectal dosimetry between pre-plan and post-implant analysis in transperineal interstitial brachytherapy for prostate cancer

Background and Purpose To investigate differences in rectal dosimetry between pre-plan ultrasonography (US) and post-implant computed tomography (CT). Patients and Methods Subjects comprised 49 patients who underwent prostate brachytherapy using 125I seed implants. Prescribed dose was 145 Gy to the periphery of the prostate. Differences in rectal dosimetry between pre-plan US and post-implant CT analysis were evaluated. In addition, patients were divided into two groups according to timing of pre-planning (pre-plan group, n=28; intraoperative pre-plan group, n=21), and differences in rectal dosimetry between groups were assessed. Results The average of volume differences between pre-plan and post-implant analysis (pre-plan minus post-implant analysis) for all patients were follows: −0.08 cm 3 in V60 (volume of rectal wall receiving 60% of prescribed dose); −0.05 cm 3 in V70; −0.16 cm 3 in V80; −0.38 cm 3 in V90; −0.40 cm 3 in V100; −0.32 cm 3 in V110; −0.22 cm 3 in V120; −0.15 cm 3 in V130; −0.10 cm 3 in V140; −0.07 cm 3 in V150; and −0.05 cm 3 in V160. Apparent differences between pre-plan US and post-implant CT in rectal dosimetry were small. However, considering the steep curve of the relationship between tolerable volume and dose, a large actual difference should be assumed. No advantage was identified for the intraoperative pre-plan group. Safe volume to avoid proctitis tended to be smaller on ultrasonography than on CT at 1 month. Conclusions The present work shows that direct comparison of CT analysis and pre-plan US is unfavorable due to large differences in these modalities and overestimation of tolerable volume. However, by comprehending the degree of difference, comparison of data from CT analysis with a US pre-plan may be feasible and useful for providing feedback between these modalities.

Perirectal seeds as a risk factor for prostate brachytherapy-related rectal bleeding

Purpose: To correlate rectal wall doses and perirectal seed numbers with late rectal bleeding after prostate brachytherapy. Methods and Materials: We studied 148 patients randomized to implantation with I-125 vs. Pd-103 at the VA Puget Sound HCS from 1998 through 2001 and for whom postimplant dosimetry was available. Implants were performed by standard techniques, using a modified peripheral loading pattern. A postimplant computed tomography (CT) scan (3 mm slice thickness) was obtained 1–4 h after implantation. Rectal doses were expressed as the R100, R200, and R300, defined as the rectal volume (cc) that received more than 100%, 200%, or 300% of the prescription dose, respectively. The rectum was considered to be a solid structure defined by the outer wall, without attempting to differentiate the inner wall or contents. In addition to conventional dose parameters, each patient's postimplant CT scan was reviewed for the number of seeds within 0, 0.1–2, and 2.1–4 mm of the outer rectal wall. The proximal edge of the seed was used for distance determinations from the outer rectal wall. Patients who reported Grade 1 or higher Radiation Therapy Oncology Group morbidity were contacted by telephone to obtain more details regarding their rectal bleeding. Those who reported persistent bleeding lasting for more than 1 month were categorized as Grade 2. Results: Patients had a wide range of rectal wall doses, with R100 values ranging from 0.0 to 10.4 cc (median, 0.95 cc). Similarly, the number of perirectal seeds within 0.0 to 2.0 mm of the rectum varied widely, ranging from 0 to 12 seeds (median: 1 seed). Seven patients (7 of 144 = 5%) developed persistent rectal bleeding, one of whom required a colostomy. Both rectal radiation doses and the number of perirectal seeds were higher in patients with persistent rectal bleeding. The number of perirectal seeds ≤2.0 mm of the rectal wall was higher in patients with rectal bleeding ( p = 0.037), but the number of seeds 2–4 mm from the wall were not related ( p = 0.72). In multivariate regression analysis including prostatic D90 (the dose that covers 90% of the postimplant prostate), preimplant transrectal ultrasound volume, R300, and the number of seeds ≤2 mm from the rectal wall as independent variables, only the R300 was statistically significantly associated with the likelihood of persistent rectal bleeding ( p = 0.025). Conclusion: A limited number of errant perirectal sources in itself does not appear to place patients at increased risk of rectal bleeding, providing that the overall rectal wall doses are within acceptable values.

Automatic localization of implanted seeds from post-implant CT images

An automatic localization method of implanted seeds from a series of post-implant computed tomography (CT) images is described in this paper. Post-implant CT studies were obtained for patients who underwent prostate brachytherapy. Bright areas were segmented using binary thresholding in each CT slice, and geometrical information on these areas was collected. Large areas (possibly containing two connected seeds) were split into smaller ones by geometry-based filtering in each slice. The area connectivity along the longitudinal direction was analysed using a geometry-based connection search algorithm executed on every area slice by slice, so that the connected areas were combined into one object. The weighted centroid of each object was taken as the seed position. This method was tested on a seed-containing prostate phantom as well as using CT studies from patients. Statistical analysis demonstrates that it can achieve above 99% detection rate with reliable localization accuracy and high speed. It is reliable and convenient for localizing implanted seeds on CT and can be used to assist post-implant dosimetry for prostate brachytherapy.

Radiography-based treatment planning compared with computed tomography (CT)-based treatment planning for intracavitary brachytherapy in cancer of the cervix: Analysis of dose-volume histograms

Purpose To analyze the dose-volume histograms (DVHs) of the tumor volume and organs at risk by CT-based treatment planning compared with conventional radiography-based treatment planning for intracavitary brachytherapy in cancer of the cervix. Methods and materials Fifteen consecutive patients with cancer of the cervix (1 IB 1, 3 IB 2, 7 IIB, 4 IIIB) were treated with plastic CT-compatible HDR intracavitary applicators and underwent postimplant pelvic CT scans with applicators in place. CT-images were transferred to the PLATO treatment planning system. The gross tumor volume (GTV) and organs at risk were digitized. Dwell positions in the uterine tandem and colpostats were identified and registered for each patient. All patients were treated with 6 Gy per fraction to Point A using radiography-based planning. For the CT-based planning, DVHs were performed for the GTV, bladder, rectum, sigmoid colon, and small bowel in the pelvis. The dose delivered to 3% volume of the organs at risk (D3%) was compared with the respective ICRU reference doses. Results For stage IB I, IB 2, IIB, and IIIB disease the mean GTV was 20.5 cc, 56.6 cc (54.2–57.2), 63.7 cc (55.4–118.9), and 77.6 cc (49.4–102.9), respectively. The 6 Gy pear-shaped volume (PSV) encompassed an average GTV of 98.5%, 89.5%, 79.5%, and 59.5% for stages IBI, IB2, IIB, and IIIB, respectively. The mean dose for the ICRU bladder point and D3% was 3.72 Gy (1.51–5.53) and 4.74 Gy (1.70–10.10), respectively. The mean dose for the ICRU rectal point and D3% was 3.97 Gy (2.09–5.37) and 3.52 Gy (2.05–4.08), respectively. The D3% for the sigmoid colon was highest (3.88 Gy), followed by the rectum (3.52 Gy), and the small bowel (3.36 Gy). Conclusion Radiography-based conventional treatment planning overestimates tumor dose, especially those with more advanced tumors. To correlate DVHs for tumor control, improved tumor imaging is necessary.

Penile bulb imaging

Purpose: Because of the apparent relationship between potency loss and radiation doses to the erectile bodies, there is increasing rationale for incorporating penile bulb dosimetry into treatment planning and posttreatment evaluation. Because the location and shape of the penile bulb have not been described in detail on various imaging modalities, we herein describe the anatomic boundaries of the penile bulb on computed tomography (CT), magnetic resonance imaging (MR), and transrectal ultrasound (TRUS), before and after brachytherapy.Methods and Materials: Nonenhanced axial CT images were taken on a CTi CT Scanner (General Electric Medical Systems, Milwaukee, WI) with the patient in the supine position. Settings were at 300 ma, 140 kvp, 4-s scan time per slice, and collimation of 3 mm with data obtained at 3-mm intervals. Nonenhanced MR images were obtained with a 1.5 Tesla Signa Horizon LX Scanner using fast spin-echo T1-weighted (TR/TE, 466/20) and T2-weighted (TR/TE, 8000/90) images, with a slice thickness of 2 mm and an interslice gap of 0.5 mm. TRUS images were obtained with a Siemens SONOLINE Prima ultrasound machine at 6.0 MHz and a Winston-Barzell stepper unit.Results: The penile bulb is best visualized on T2-weighted MR images in the axial, sagittal, and coronal planes, appearing as an oval-shaped, hyperintense midline structure. On axial CT imaging, the bulb of the penis is typically readily identifiable, bounded by the paired crura laterally, the corpora spongiosum anteriorly, and the levator ani posteriorly. The penile bulb is typically well visualized on transverse TRUS, but usually only faintly seen on sagittal TRUS. The bulb is partially obscured on postimplant CT and MR images, presumably because of implant-related edema. Bulb volumes vary markedly from patient to patient, ranging from 5.6 to 12.4 cc (median: 8.1 cc).Conclusion: Closer attention to penile erectile tissue doses should lead to improved external beam radiation and brachytherapy delivery. It will benefit the radiation oncology community to become familiar with these imaging findings, so that penile bulb dosimetry can be incorporated into our daily practice.

An algorithm for automatic, computed-tomography-based source localization after prostate implant.

Permanent implant of the prostate using I-125 and Pd-103 seeds is a popular choice of treatment for early-stage prostate cancer in the United States. Evaluation of the quality of the implant is best based on the calculated dose distribution from postimplant computed tomography (CT) images. This task, however, has been time-consuming and inaccurate. We have developed an algorithm for automatic source localization from postimplant CT images. The only requirement of this algorithm is knowledge of the number of seeds present in the prostate, thus minimizing the need for human intervention. The algorithm processes volumetric CT data from the patient, and pixels of higher CT numbers are categorized into classes of definite and potential source pixels. A multithresholding technique is used to further determine the number of seeds and their precise locations in the CT volume data. A graphic user interface was developed to facilitate operator review of and intervention in the calculation and the results of the algorithm. This algorithm was tested on two phantoms containing nonradioactive seeds, one with 20 seeds in discrete locations and another with 100 seeds with small distances between seeds. The tests showed that the algorithm was able to identify the seed locations to within 1 mm of their physical locations for discrete seed locations. It was further able to separate seeds at close proximity to each other while maintaining an average seed localization error of less than 2 mm, with no operator intervention required.

Postimplant analysis of transperineal interstitial permanent prostate brachytherapy: evidence for a learning curve in the first year at a single institution

Purpose: The utilization of transperineal interstitial permanent prostate brachytherapy (TIPPB) is increasing in the United States. Quality assessment of TIPPB is in its infancy, and to date, dosimetric analyses have only been reported from centers with a large experience in prostate brachytherapy. The purpose of this report is to critically analyze the dosimetric coverage achieved following TIPPB in the first 63 cases performed by a multidisciplinary group of investigators with no prior experience with TIPPB. Methods and Materials: The information in this report concerns the first 63 men treated with TIPPB alone at our institution between September 1997 and September 1998. All men were treated similarly, adapting the methods described by Blasko and Grimm. All men were treated with 125I. The prescription dose was 144 Gy according to the TG43 formalism. TIPPB was performed jointly by a radiation oncologist and a urologist. One month following TIPPB, all men underwent a computed tomography (CT) scan of the pelvis according to a protocol using 3-mm abutting slices. CT images were transferred by a local area network to a commercially available treatment planning system and dose-volume histograms were calculated with 0.5-mm pixel spacing. A variety of dosimetric endpoints were examined. A single measure of dose homogeneity, the dose-homogeneity index (DHI), is defined as the volume within the prostate that receives 100–150% of the prescription dose (144–216 Gy) divided by the volume within the prostate that receives 100% of the prescription dose (144 Gy). Three measures of target (prostate) dosimetric coverage are provided. C100 is defined as the percentage of the prostate volume defined on postimplant CT that receives at least 100% of the prescription dose. C90 and C80 are similar but represent the percentage of the prostate volume that receive 90% and 80% of the prescription dose, respectively. Statistical analyses were performed using commercially available computer software. To investigate any changes with time the first 30 cases (group 1) are compared to cases 31–63 (group 2). All p-values are two-sided. Results: The mean C100, C90, and C80 for all 63 patients were 80.7% (SD 10.1), 85.1% (SD 10.2), and 89.3% (SD 9.5). The quantifiers of implant adequacy were all improved in the most recent 33 patients compared to the first 30 patients, (group 1: C100, 75.8% [SD 12.2], C90 79.9% [SD 11.4], C80 84.3% [SD 11.1]; group 2: C100, 85.2 [SD 7.0], C90 89.9% [SD 5.8], C80 93.8% [SD 4.2]; p < 0.001). The mean DHI was 0.538 SD (0.124). A multivariate model incorporating a number of variables (ultrasound volume, CT volume, total activity, activity/seed, implant number) with C100 as the dependent variable found that the implant number was the only statistically significant predictor of C100 ( p = 0.0001). Using C90 and C80 as the dependent variable produced similar results (C90, p = 0.0001; C80, p = 0.0001). Conclusion: In this single institution experience with the first 63 men receiving TIPPB by a multidisciplinary group of investigators, there is evidence for a learning curve. All quantifiers of implant adequacy improved as clinicians gained experience. In the most recent group of patients, quantifiers of implant adequacy are similar to those reported from other groups with significantly more experience with TIPPB.

Improving the dosimetric coverage of interstitial high-dose-rate breast implants

Purpose/Objective: We performed a retrospective computed tomography (CT)-based three-dimensional (3D) dose-volume analysis of high-dose-rate (HDR) interstitial breast implants to evaluate the adequacy of lumpectomy cavity coverage, and then designed a simple, reproducible algorithm for dwell-time adjustment to correct for underdosage of the lumpectomy cavity. Methods and Materials: Since March 1993, brachytherapy has been used as the sole radiation modality after lumpectomy in selected protocol patients with early-stage breast cancer treated with breast-conserving therapy. In this protocol, all patients received 32 Gy in 8 fractions of 4 Gy over 4 days. Eleven patients treated with HDR brachytherapy who underwent CT scanning after implant placement were included in this analysis. For each patient, the postimplant CT dataset was transferred to a 3D treatment planning system, and the relevant tissue volumes were outlined on each axial slice. The implant dataset, including the dwell positions and dwell times, were imported into the 3D planning system and then registered to the visible implant template in the CT dataset. The calculated dose distribution was analyzed with respect to defined volumes via dose-volume histograms. Due to the variability of lumpectomy cavity coverage discovered in this 3D quality assurance analysis, dwell times at selected positions were adjusted in an attempt to improve dosimetric coverage of the lumpectomy cavity. Using implant data from 5 cases, a dwell-time adjustment algorithm was designed and was then tested on 11 cases. In this algorithm, a point P was identified using axial CT images, which was representative of the underdosed region within the cavity. The distance ( d) from point P to the nearest dwell position was measured. A number of dwell positions (N) nearest to point P were selected for dwell time adjustment. The algorithm was tested by increasing the dwell times of a variable number of positions (N = 1, 3, 5, 7, 10, and 20) by a weighting factor (α), where α = f( d) and α > 1, and subsequently performing 3D dose-volume analysis to evaluate the improvement in lumpectomy cavity coverage. Results: Before adjustment in the 11 implants, the median proportion of the lumpectomy cavity and target volume that received at least the prescription dose was 85% and 68%, respectively. After dwell-time adjustment, lumpectomy cavity coverage was significantly improved in all 11 cases. The median distance from point P to the nearest dwell position ( d) was 1.4 cm (range 0.9–1.9). The median volume of the lumpectomy cavity receiving 32 Gy increased from 85.3% in the actual implant to 97.0% (range 74–100%) by increasing the dwell time of a single dwell position by a median factor (α) of 12.2 according to the above algorithm. With N = 3, the median proportion of the cavity volume receiving 32 Gy was improved to 97.5% (range 77–100%), with a median α of 5.7. Further improvement in lumpectomy cavity coverage was relatively small by increasing additional dwell times. In addition, with N = 20, the median absolute volume of breast tissue receiving 150% of the prescription dose was 70.3 cm 3 compared to 26.3 cm 3 in the actual implant; whereas with N = 1 or N = 3, this median volume was only 35.9 and 42.0 cm 3, respectively. Conclusion: Lumpectomy cavity coverage sometimes appears suboptimal with interstitial HDR breast brachytherapy using our current technique. A simple dwell-time increase at only 1–3 dwell positions can compensate for some underdosage without creating significant regions of overdosage. Using simple methodology, a single reference point representing the underdosed region can be utilized for initial selection of the dwell positions to be increased.

Effect of post-implant edema on the rectal dose in prostate brachytherapy

Purpose: To characterize the effect of prostate edema on the determination of the dose delivered to the rectum following the implantation of 125I or 103Pd seeds into the prostate. Methods and Materials: From 3 to 5 post-implant computed tomography (CT) scans were obtained on 9 patients who received either 125I or 103Pd seed implants. None of the patients received hormone therapy. The outer surface of the rectum was outlined on each axial CT image from the base to the apex of the prostate. The D 10 rectal surface dose, defined as the dose which encompasses only 10% of the surface area of the rectum, was determined from each CT scan by compiling a dose-surface histogram (DSH) of the rectal surface. The magnitude and half-life of the post-implant edema in each of these implants is known from the results of a previously published study based on the analysis of the serial CT scans. Results: As the prostate edema resolved, the distance between the most posterior implanted seeds and the anterior surface of the rectum decreased. As a result, the D 10 rectal surface dose increased with each successive post-implant CT scan until the edema resolved. The dose increased exponentially at approximately the same rate the prostate volume decreased. The D 10 rectal surface dose at 30 days post-implant ranged from 16% to 190% (mean 68 ± 50%) greater than on day 0. The dose on day 30 was at least 50% greater in 6 of 9 cases. Conclusion: The rectal surface dose determined by analysis of a post-implant CT scan of an 125I or 103Pd prostate seed implant depends upon the timing of the CT scan. The dose indicated by the CT scan on day 30 is typically at least 50% greater than that indicated by the CT scan on day 0. Because of this difference, it is important to keep the timing of the post-implant CT in mind when specifying dose thresholds for rectal morbidity.