Rectal sparing in prostate radiotherapy with combination-brachytherapy and hydrogel spacer.

BACKGROUND: Though some techniques that facilitate rectal sparing such as brachytherapy and intensity modulated radiotherapy (IMRT) have been examined in detail, technical aspects of hydrogel spacer (HS) have been studied less exhaustively. We examined HS quality metrics and approaches to placement for superior dosimetric outcomes. MATERIALS AND METHODS: A single site retrospective review of radiation plans was conducted for patients who received combination-brachytherapy (CBT) with 90 Gy low-dose-rate implant followed by external beam radiotherapy (45 Gy/25 fractions) with operating room (OR) placed HS (2017-2021). A randomly selected set of patients that received CBT without HS over the same time period was used for comparison. Dosimetric outcomes included D1cc and D5% rectum. Dose gradients were quantified. Student's t-test was used for statistical comparisons. RESULTS: Sixty patients (30 with and 30 without HS) who received CBT for prostate cancer were examined. Those with HS had lower mean D1cc [65.31 Gy (SD = 13.53)] and D5% [53.20 Gy (SD = 10.18)] compared to those treated without HS [91.67 Gy (SD = 8.31) and 75.00 Gy (SD = 8.45), respectively, p < 0.001]. Patients with superior HS (average thickness ≥1 cm; n = 12) had lower mean D1cc [58.49 Gy (SD = 13.25, p = 0.026)] and D5% [48.69 Gy (SD = 9.85, p = 0.049)] than those with thinner HS. When dose gradients were considered, HS spanning the interface between the prostate and perirectal tissues to a thickness ≥1 cm can reduce rectal maximum dose to 50-60 Gy. CONCLUSIONS: Through effective use of CBT and HS, extreme rectal dose restriction is possible. The goal for HS placement should be thickness ≥1 cm from base to apex.

Use and uncertainties of mutual information for computed tomography/ magnetic resonance (CT/MR) registration post permanent implant of the prostate.

Post-implant dosimetric analysis for permanent implant of the prostate benefits from the use of a computed tomography (CT) dataset for optimal identification of the radioactive source (seed) positions and a magnetic resonance (MR) dataset for optimal description of the target and normal tissue volumes. The CT/MR registration process should be fast and sufficiently accurate to yield a reliable dosimetric analysis. Since critical normal tissues typically reside in dose gradient regions, small shifts in the dose distribution could impact the prediction of complication or complication severity. Standard procedures include the use of the seed distribution as fiducial markers (seed match), a time consuming process that relies on the proper identification of signals due to the same seed on both datasets. Mutual information (MI) is more efficient because it uses image data requiring minimal preparation effort. A comparison of MI registration and seed-match registration was performed for twelve patients. MI was applied to a volume limited to the prostate and surrounding structures, excluding most of the pelvic bone structures (margins around the prostate gland were approximately 2 cm right-left, approximately 1 cm anterior-posterior, and approximately 2 cm superior-inferior). Seeds were identified on a 2 mm slice CT dataset using an automatic seed identification procedure on reconstructed three-dimensional data. Seed positions on the 3 mm slice thickness T2 MR data set were identified using a point-and-click method on each image. Seed images were identified on more than one MR slice, and the results used to determine average seed coordinates for MR images and matched seed pairs between CT and MR images. On average, 42% (19%-64%) of the seeds (19-54 seeds) were identified and matched to their CT counterparts. A least-squares method applied to the CT and MR seed coordinates was used to produce the optimum seed-match registration. MI registration and seed match registration angle differences averaged 0.5 degrees, which was not significantly different from zero. Translation differences averaged 0.6 (1.2 standard deviation) mm right-left, -0.5(1.5) mm posterior-anterior, and -1.2(2.0) mm inferior-superior. Registration error estimates were approximately 2 mm for both the MI and seed-match methods. The observed standard deviations in the offset values were consistent with propagation of error. Registration methods as applied here using mutual information and seed matching are consistent, except for a small systematic difference in the inferior-superior axis for a minority of cases (approximately 15%). Cases registered with mutual information and with bony anatomy misregistration of greater than approximately 5 mm should be evaluated for rescan or seed-match registration. The improvement in efficiency of use for the MI registration method is substantial, approximately 30 min compared to several hours using seed match registration.

The use of mutual information in registration of CT and MRI datasets post permanent implant.

PURPOSE: To determine the feasibility of registration of MRI and CT datasets post permanent prostate implant by the use of mutual information. METHODS AND MATERIALS: Five patients who underwent permanent (125)I implant for prostate carcinoma were studied. Two weeks postimplant an axial CT, T2-weighted-axial, sagittal and coronal MRI, and T1-fat-saturation MRI scans were obtained. Registrations of MRI to CT and MRI to MRI datasets were performed by mutual information, an automated process of data registration matching all information in specified dataset regions of interest. Registration quality was evaluated by visual inspection, agreement with seed- to-seed registration, and histogram analysis. RESULTS: Rapid registration (<30 minutes) of CT and MRI datasets can be accomplished through the use of mutual information. All methods of registration evaluation confirmed excellent registration quality. Although D90 and V100 for the prostate were comparable between MRI- and CT-based dosimetry, dose to critical structures/microenvironments (anterior base, posterior base, bladder outlet, lower sphincter, bulbar urethra) defined on MRI varied widely. CONCLUSIONS: Efficient and accurate registration of MRI and CT datasets following prostate implant is possible, and improves the accuracy of postimplant dosimetry by superior definition of the prostate. Definition of critical microenvironments and adjacent structures will improve dose and toxicity correlation and ultimately improve planning strategies.