Postimplant Dosimetry Research Articles

Seed distribution stability in permanent breast seed implant brachytherapy

PURPOSESIn this study we aim to quantitatively evaluate the stability of implanted seeds in permanent breast seed implant (PBSI) brachytherapy and assess any impact on treatment quality. METHODS AND MATERIALSSixty-seven consecutive patients who received PBSI treatment at BC Cancer Kelowna from 2013 to 2021 with post-implant CT images available were included in this study. For each patient, two sets of post-implant CT scans were retrospectively analyzed: Day0, obtained immediately after implant, and Day30, obtained approximately one month following implant. Seed distributions were quantified using the 90% isodose contour, outlier seed maximum spread, and number of seeds located in the seroma as well as seroma quadrants. These were then compared between Day0 and Day30. Post-implant dosimetry of target volumes as well as critical structures were compared. RESULTSThe 90% isodose volume was found to decrease over time. All seeds remained in the breast region however the maximum spread of seeds increased in all directions from Day0 to Day30. All recorded target volume dosimetric parameters were, on average, lower on Day30 compared to Day0 but mean dosimetry levels still met clinical goals. Dose in critical structures was overall similar. CONCLUSIONSIn this study, we quantitatively described the changes in seed distributions as well as dosimetry from Day0 to Day30 post PBSI procedure. We addressed concerns related to seed stability in breast tissue and provided clinical evidence on dosimetric efficacy of the PBSI technique.

Dosimetric benefits and preclinical performance of steerable needles in HDR prostate brachytherapy

Prostate cancer patients with an enlarged prostate and/or excessive pubic arch interference (PAI) are generally considered non-eligible for high-dose-rate (HDR) brachytherapy (BT). Steerable needles have been developed to make these patients eligible again. This study aims to validate the dosimetric impact and performance of steerable needles within the conventional clinical setting.HDR BT treatment plans were generated, needle implantations were performed in a prostate phantom, with prostate volume > 55 cm3 and excessive PAI of 10 mm, and pre- and post-implant dosimetry were compared considering the dosimetric constraints: prostate V100 > 95 % (13.50 Gy), urethra D0.1cm3 < 115 % (15.53 Gy) and rectum D1cm3 < 75 % (10.13 Gy).The inclusion of steerable needles resulted in a notable enhancement of the dose distribution and prostate V100 compared to treatment plans exclusively employing rigid needles to address PAI. Furthermore, the steerable needle plan demonstrated better agreement between pre- and post-implant dosimetry (prostate V100: 96.24 % vs. 93.74 %) compared to the rigid needle plans (79.13 % vs. 72.86 % and 87.70 % vs. 81.76 %), with no major changes in the clinical workflow and no changes in the clinical set-up.The steerable needle approach allows for more flexibility in needle positioning, ensuring a highly conformal dose distribution, and hence, HDR BT is a feasible treatment option again for prostate cancer patients with an enlarged prostate and/or excessive PAI.

Toward a deep learning-based magnetic resonance imaging only workflow for postimplant dosimetry in I-125 seed brachytherapy for prostate cancer

Background and purposeThe current standard imaging-technique for creating postplans in seed prostate brachytherapy is computed tomography (CT), that is associated with additional radiation exposure and poor soft tissue contrast. To establish a magnetic resonance imaging (MRI) only workflow combining improved tissue contrast and high seed detectability, a deep learning-approach for automatic seed segmentation on MRI-scans was developed. Material and methodsPatients treated with I-125 seed brachytherapy received a postplan-CT and a 1.5 T MRI-scan on nominal day 30 after implantation. For MRI-based seed visualization, DIXON-sequences were acquired and deep learning-based quantitative susceptibility maps (QSM) were generated from 3D-gradient-echo-sequences from 20 patients. Seed segmentations created on CT served as ground truth. For automatic seed segmentation on MRI, a 3D nnU-net model was trained using QSM and DIXON, both solely and combined. ResultsOf the implanted seeds 94.8 ± 2.4% were detected with deep learning automatic segmentation entrained on both QSM and DIXON data. Models trained on the individual sequence data-sets performed worse with detection rates of 87.5 ± 2.6% or 88.6 ± 7.5% for QSM and DIXON respectively. The seed centers identified on CT versus QSM and DIXON were on average 1.8 ± 1.3 mm apart. Postimplant dosimetry for evaluation of positioning inaccuracies revealed only small variations of up to 0.4 ± 4.26 Gy in D90 (dose 90% of the prostate receives) between the standard CT-approach and our MRI-only workflow. ConclusionThe proposed deep learning-based MRI-only workflow provided a promisingly accurate and robust seed localization and thus has the potential to compete with current state-of-the-art CT-based postimplant dosimetry in the future.

A postimplant dosimetry simulation framework for robustness evaluation in permanent breast seed implant brachytherapy

Permanent breast seed implant (PBSI) brachytherapy is a promising treatment that has the potential to be widely utilized with increased standardization, optimization, and robustness. Excellent early efficacy and very high patient acceptance were reported, however, to further evaluate and improve planning strategies, a framework to quantify plan robustness to implant uncertainties is necessary. In this study, we aim to quantify clinical seed displacement using an automated algorithm and develop and validate a PBSI post-implant dosimetry simulation framework to evaluate PBSI plan robustness to implant uncertainties. Clinical PBSI seed displacements were quantified for 63 consecutive patients. A PBSI simulator was developed in Matlab (2020) by resampling clinical seed displacements and computing a range of possible post-implant dosimetry outcomes under various seed displacement scenarios. Simulations were performed retrospectively on 63 previous clinical plans to evaluate plan robustness to seed displacement. Mean seed displacement for the whole cohort was 10 ± 6 mm. A clinical seed displacement database was established and a user interface was developed for the simulation framework. For all clinical plans, the median (range) value of simulated median ETV V90 in various seed displacement scenarios was 97.8% (87.5-100%). A PBSI postimplant dosimetry simulation framework was developed and validated. Simulation results showed that the current PTV planning margin is sufficient to provide adequate postimplant dose coverage of ETV. This simulator can be used to evaluate plan robustness to seed displacement and will facilitate future research in improving PBSI planning methods.

Sexual Function and Dosimetric Relationships to Erectile Structures among Patients Treated Definitively with Pd-103 LDR Prostate Brachytherapy

Sexual potency and erectile function are important quality of life metrics for many men and is a significant consideration when planning and treating prostate cancer. There are limited long-term data correlating post-implant dosimetry with patient-reported sexual toxicity outcomes following LDR prostate brachytherapy using Pd-103, specifically as it pertains to genital substructures. Our aim was to correlate dosimetric patterns with quality-of-life outcomes to determine if dose to the pudendal arteries and neurovascular bundles is a significant consideration when planning prostate LDR brachytherapy. We analyzed a prospectively collected IRB-approved database of men receiving LDR prostate brachytherapy between 11/2014-04/2019 at our institution. Patients received either LDR brachytherapy only, or combined with intensity-modulated radiation therapy (IMRT) to the prostate with or without pelvic lymph node coverage. Patients were given quality of life questionnaires at consultation and at subsequent follow up visits. Outcomes data related to sexual toxicity were evaluated based on the Sexual Health Inventory (SHIM) Questionnaire. The right/left pudendal arteries, and right/left neurovascular bundles (NVB) were contoured retrospectively on CT-MRI fusion sequences. Dosimetric data for each of the erectile substructures was analyzed. Statistical analyses included generalized linear mixed effects models with random intercept for patient to explore the association between dose and SHIM confidence. A total of 50 patients met criteria for inclusion in the analysis. 5 patients received combined IMRT to 45 Gy with Pd-103 (90-100 Gy) and 45 patients received Pd-103 monotherapy (125 Gy). Median follow-up was 18 months for 50 patients; 40 (80%) patients had follow-up greater than 2 years, and 15 (30%) greater than 5 years. There were 344 individual questionnaires completed. 28% of records reported low or very low confidence level (Question #1 of SHIM questionnaire). Likelihood of moderate-very high confidence increased up to 2 years after beginning of treatment (p = 0.052) and then subsequently decreased between 2- and 5 years post treatment (p = 0.042). NVB doses were not associated with SHIM confidence level. D100 greater than 150 cGy to either pudendal artery was associated with worse SHIM confidence score after treatment (p = 0.003). Sexual function is variable in the post treatment setting following definitive prostate radiation with Pd-103 LDR-brachytherapy. Avoiding pudendal artery dose may improve sexual outcomes and should be considered when planning prostate brachytherapy. Further investigation with a larger prospective, cohort may be warranted.

Uncertainty in magnetic resonance imaging-based prostate postimplant dosimetry: Results of a 10-person human observer study, and comparisons with automatic postimplant dosimetry

Uncertainties in postimplant quality assessment (QA) for low-dose-rate prostate brachytherapy (LDRPBT) are introduced at two steps: seed localization and contouring. We quantified how interobserver variability (IoV) introduced in both steps impacts dose-volume-histogram (DVH) parameters for MRI-based LDRPBT, and compared it with automatically derived DVH parameters. Twenty-five patients received MRI-based LDRPBT. Seven clinical observers contoured the prostate and four organs at risk, and 4 dosimetrists performed seed localization, on each MRI. Twenty-eight unique manual postimplant QAs were created for each patient from unique observer pairs. Reference QA and automatic QA were also performed for each patient. IoV of prostate, rectum, and external urinary sphincter (EUS) DVH parameters owing to seed localization and contouring was quantified with coefficients of variation. Automatically derived DVH parameters were compared with those of the reference plans. Coefficients of variation (CoVs) owing to contouring variability (CoVcontours) were significantly higher than those due to seed localization variability (CoVseeds) (median CoVcontours vs. median CoVseeds: prostate D90-15.12% vs. 0.65%, p < 0.001; prostate V100-5.36% vs. 0.37%, p < 0.001; rectum V100-79.23% vs. 8.69%, p < 0.001; EUS V200-107.74% vs. 21.18%, p < 0.001). CoVcontours were lower when the contouring observers were restricted to the 3 radiation oncologists, but were still markedly higher than CoVseeds. Median differences in prostate D90, prostate V100, rectum V100, and EUS V200 between automatically computed and reference dosimetry parameters were 3.16%, 1.63%, -0.00 mL, and -0.00 mL, respectively. Seed localization introduces substantially less variability in postimplant QA than does contouring for MRI-based LDRPBT. While automatic seed localization may potentially help improve workflow efficiency, it has limited potential for improving the consistency and quality of postimplant dosimetry.

A Contemporary Report of Low-Dose-Rate Brachytherapy for Prostate Cancer Using MRI for Risk Stratification: Disease Outcomes and Patient-Reported Quality of Life.

We examined a prospective consecutive cohort of low dose rate (LDR) brachytherapy for prostate cancer to evaluate the efficacy of monotherapy for unfavorable-intermediate risk (UIR) disease, and explore factors associated with toxicity and quality of life (QOL). 149 men with prostate cancer, including 114 staged with MRI, received Iodine-125 brachytherapy alone (144-145 Gy) or following external beam radiation therapy (110 Gy; EBRT). Patient-reported QOL was assessed by the Expanded Prostate Index Composite (EPIC) survey, and genitourinary (GU) and gastrointestinal (GI) toxicity were prospectively recorded (CTC v4.0). Global QOL scores were assessed for decline greater than the minimum clinically important difference (MCID). Univariate analysis (UVA) was performed, with 30-day post-implant dosimetry covariates stratified into quartiles. Median follow-up was 63 mo. Men with NCCN low (n = 42) or favorable-intermediate risk (n = 37) disease were treated with brachytherapy alone, while most with high-risk disease had combined EBRT (n = 17 of 18). Men with UIR disease (n = 52) were selected for monotherapy (n = 42) based on clinical factors and MRI findings. Freedom from biochemical failure-7 yr was 98%. Of 37 men with MRI treated with monotherapy for UIR disease, all 36 men without extraprostatic extension were controlled. Late Grade 2+/3+ toxicity occurred in 55/3% for GU and 8/2% for GI, respectively. Fifty men were sexually active at baseline and had 2 yr sexual data; 37 (74%) remained active at 2 yr. Global scores for urinary incontinence (UC), urinary irritation/obstruction (UIO), bowel function, and sexual function (SF) showed decreases greater than the MCID (p < 0.05) in UC at 2 mo, UIO at 2 and 6 mo, and SF at 2-24 mo, and >5 yr. Analysis did not reveal any significant associations with any examined rectal or urethral dosimetry for late toxicity or QOL. Disease outcomes and patient-reported QOL support LDR brachytherapy, including monotherapy for UIR disease.

PP11 Presentation Time: 11:10 AM: Assessment of Brachytherapy-Induced Prostate Edema on Post-Implant Dosimetric Analysis in MRI-Assisted Radiosurgery

<h3>Purpose</h3> Prior studies have shown that brachytherapy-induced prostate edema (PE) results in altered target dose coverage on post-implant CT dosimetry in prostate cancer patients treated with low dose rate brachytherapy (LDR). The pre-plan target volume (PTV) accounts for PE by adding 1-5mm margins to the prostate for calculation of the goal dose coverage on the day of the implant (day zero, D0). However, visualization of PE on CT is limited, diminishing PE measurement accuracy. MRI-based LDR (MRI-assisted radiosurgery, MARS) improves accuracy through better delineation of target, normal structures, and seeds compared to CT. Thus, MARS may improve the accuracy of PE measurements. We investigated PE using MARS to analyze its effect on D0 dosimetry and determine if current pre-plan PTV margins are adequate in all isotopes. <h3>Materials and Methods</h3> We identified 346 patients with low to intermediate-risk prostate cancer treated with Cesium-131 (Cs-131, 15 [4.3%] patients, prescription dose (PD)=115 Gy), Iodine-125 (I-125, 83 [23.9%] patients, PD=144 Gy), or Palladium-103 (Pd-103, 248 [71.7%] patients, PD=125 Gy) MARS as definitive LDR from 2016-2021. Patients treated with hormone therapy, as a boost with external-beam RT, or for salvage were excluded. Implants were pre-planned using MRI (MIM Symphony treatment planning system) and our published nomograms verified the activity per volume within 5%. PE was measured as the change in prostate volume from the pre-plan to D0 based on physician contours. Post-implant dosimetry was performed using MRI on D0. Treatment characteristics, including pre-plan and D0 prostate volumes, prostate volumes receiving 100%, 150%, and 200% of the PD (V₁₀₀, V_150, V₂₀₀), percent of the PD going to 90% of the prostate volume (D₉₀), rectum volume receiving 100% of the PD (V₁₀₀), and external urethral sphincter (EUS) volume receiving 200% of the PD (V₂₀₀), were extracted from MIM. Simple linear regression with Pearson's correlation analysis was used to assess the effect of PE on post-implant D0 prostate V₁₀₀, V₁₅₀, V₂₀₀, and D₉₀, rectum V₁₀₀, and EUS V₂₀₀. Ordinary one-way ANOVA analysis was used to compare PE between categorical variables. <h3>Results</h3> Median age at diagnosis was 63 years. Median PSA at diagnosis was 6. 24 (6.9%) patients had GS 6 and 322 (93.1%) had GS 7. On the pre-plan, 55 (15.9%) patients had a prostate size of 0-20 ccs, 222 (64.2%) had 20-40 ccs, 63 (18.2%) had 40-60 ccs, 6 (1.7%) had 60+ ccs. On D0 analysis, the median [IQR] change in prostate volume was +28% [15-42%] as compared to the pre-plan. There was no significant difference in magnitude of PE by isotope (median change in prostate volume: +22.6% for Cs-131, +27.3% for I-125, +29.5% for Pd-103, p=0.33). There was no difference in PE by increasing number of needles (R²=0, p=0.70). PE decreased with increasing pre-plan prostate size (R²=0.05, p<0.001). Greater PE was associated with decreased post-implant D0 V₁₀₀ (<b>Graphic</b>, R²=0.08, p<0.001), V₁₅₀ (R²=0.03, p<0.001), and D₉₀(R²=0.12, p<0.001); there was no association with D0 V_{200 (}R²=0.01, p=0.06). Greater PE was associated with decreased D0 rectum V₁₀₀ (R²=0.02, p=0.02) and trended toward decreased EUS V₂₀₀ (R²=0.01, p=0.05). <h3>Conclusions</h3> MARS confirmed that patients undergoing LDR experienced PE. PE was uniform between isotopes, supporting the use of the same pre-plan PTV margins among isotopes. There was decreased D0 prostate V₁₀₀, V₁₅₀, and D₉₀ in patients with greater PE. We suggest pre-plan PTV margins as follows: 3mm laterally, 2mm anteriorly/cranially/caudally, and 0mm posteriorly. These PTV margins can help ensure adequate D0 dosimetry. <b>Graphic:</b> Correlation of post-implant D0 prostate V₁₀₀ with the magnitude of prostate edema (PE). PE was calculated as the percent change in prostate volume from the pre-plan to post-implant D0.

PP09 Presentation Time: 10:50 AM: Interobserver Variability of Prostate MRI-Assisted Radiosurgery Postimplant Dosimetry: Comparison with Automatic Postimplant Dosimetry

<h3>Purpose</h3> To quantify the 2 sources of human interobserver variability (IoV) introduced during prostate MRI-assisted radiosurgery postimplant dosimetry, and compare this human variability to automatically generated postimplant dosimetry. <h3>Materials and Methods</h3> Twenty-five patients underwent LDR prostate brachytherapy (LDRPBT) and were imaged with fully balanced SSFP for postimplant dosimetry. Four certified medical dosimetrists (CMDs) identified the radioactive seeds in each of the 25 postimplant MRIs. Seven additional clinical observers, including 3 radiation oncologists (ROs), contoured the prostate and 4 organs at risk (OARs) on the MRIs. Consensus segmentation masks from the 3 ROs' contours were computed for each patient using the simultaneous truth and performance level estimation (STAPLE) algorithm. Thirty unique postimplant dosimetry plans were computed for each patient. Twenty-eight of the dosimetry plans were computed from each unique pair of the CMDs' seed plans and observer contours (4 CMDs × 7 contourers). One dosimetry plan was performed automatically for each patient using custom autosegmentation and automatic seed identification software developed previously. A reference dosimetry plan for each patient was computed using the consensus STAPLE segmentations and the seeds identified by the most experienced CMD. IoV of 5 dosimetry parameters was quantified separately for each IoV source: IoV introduced by seed localization, and IoV introduced by anatomy contouring. The coefficient of variation (CoV) of each dosimetry parameter was quantified for each patient across CMDs using the consensus STAPLE segmentations. Similarly, the CoV of each dosimetry parameter was quantified for each patient across contouring observers using the seed locations determined by the most experienced CMD. Additionally, the differences in dosimetry parameters between the reference and automatic dosimetry plans were quantified. <h3>Results/Discussion</h3> CoVs due to variability of anatomy contouring (CoV_contours) were significantly higher than those due to variability of seed localization (CoV_seeds) for all dosimetry parameters (median CoV_contours vs. median CoV_seeds: prostate D90 - 15.116% vs. 0.651%, p<0.001; prostate V100 - 5.359% vs. 0.365%, p<0.001; prostate V150 - 7.017% vs. 1.252%, p<0.001; rectum V100 - 79.230% vs. 8.688%, p<0.001; EUS V200 - 107.740% vs. 21.176%, p<0.001). CoV_contours were lower when the contouring observers were restricted to only the 3 ROs, but they were still significantly higher than CoV_seeds for all dosimetry parameters. Median differences in prostate D90, prostate V100, prostate V150, rectum V100, and EUS V200 between automatically computed and reference dosimetry parameters were 3.156%, 1.630%, -1.071%, -0.001%, and -0.002%, respectively. Most automatically computed dosimetry parameters were within the expected variability of those produced by human observers (Figure 1). <h3>Conclusion</h3> IoV of contouring constitutes the largest source of IoV in postimplant dosimetry plans performed by clinical observers. When using automatic contouring and seed localization tools, review and refinement of computer predictions will be necessary for some patients. However, automatic dosimetry tools can estimate postimplant dosimetry that is close to, or within, the expected variability of dosimetry plans produced by clinical observers.

Patient-specific dose correction for prostate postimplant evaluation with flexible timing of postimplant imaging.

The dosimetric effect of edema on prostate implants have long been realized, but large uncertainties still exist in the estimation of dose actually received by the prostate. This study attempted to develop a new method to accurately estimate dose delivered to the prostate accounting for the variation of prostate volume and seed distribution, edema half-lives, and times for postimplant evaluation. A series of prostate seed implants for Cs-131, Pd-103, and I-125 with various prostate volumes were simulated in a water phantom using the TG-43 algorithm on the Varian Eclipse treatment planning system. Dose analysis was performed to derive a quantitative relationship between the prostate peripheral dose and the prostate radius with the variation of prostate volume and seed distribution. Using this relationship to calculate dynamically, the total dose accumulated in the prostate (DT ) accounting for the variation of prostate volume and seed distribution and edema half-lives. Moreover, the total dose can be estimated statically based on the prostate volume that can be determined in a computerized tomography (CT) image taken at a time after implantation. The statically estimated total dose (DCT ) was compared with DT to determine optimal imaging times as well as dose correction factors for other imaging times. An inverse power law was established between the prostate peripheral dose and prostate radius. The value of the power was 1.3 for Cs-131 and I-125, and 1.5 for Pd-103, respectively. DT was derived dynamically using the inverse power law. Given the edema half-lives, TE , of 4, 9.3, and 25 days and the volume expansion of 1.1 and 2.0 times of the prostate without edema, the optimal times for postimplant imaging were: 7, 9, and 16 days for TE = 4 days; 10, 14, and 28days for TE = 9.3 days; and 12, 19, 45 days TE = 25 days, for Cs-131, Pd-103, and I-125, respectively. DCT calculated using the prostate volume determined on the optimal days agreed with DT to 0.0%-1.8% and within 0.3% for most cases. For various prostate volumes, edema half-lives, and nonoptimal times, DCT was able to achieve a 1% accuracy. The postimplant dose calculation based on the proposed inverse power law for prostate seed implants with edema has improved the accuracy of postimplant dosimetry with accurate and patient-specific dose corrections accounting for prostate size, edema half-life, and postimplant imaging times. Optimal times for postimplant imaging have been accurately determined, and the high accuracy of postimplant dose calculation can be achieved for both optimal imaging times and nonoptimal imaging times.

Evaluation of dosimetric effects of metallic artifact reduction and tissue assignment on Monte Carlo dose calculations for 125 I prostate implants.

Monte Carlo (MC) simulation studies, aimed at evaluating the magnitude of tissue heterogeneity in 125 I prostate permanent seed implant brachytherapy (BT), customarily use clinical post-implant CT images to generate a virtual representation of a realistic patient model (virtual patient model). Metallic artifact reduction (MAR) techniques and tissue assignment schemes (TAS) are implemented on the post-implant CT images to mollify metallic artifacts due to BT seeds and to assign tissue types to the voxels corresponding to the bright seed spots and streaking artifacts, respectively. The objective of this study is to assess the combined influence of MAR and TAS on MC absorbed dose calculations in post-implant CT-based phantoms. The virtual patient models used for 125 I prostate implant MC absorbed dose calculations in this study are derived from the CT images of an external radiotherapy prostate patient without BT seeds and prostatic calcifications, thus averting the need to implement MAR and TAS. The geometry of the IsoSeed I25.S17plus source is validated by comparing the MC calculated results of the TG-43 parameters for the line source approximation with the TG-43U1S2 consensus data. Four MC absorbed dose calculations are performed in two virtual patient models using the egs_brachy MC code: (1) TG-43-based Dw,w-TG 43 , (2) Dw,w-MBDC that accounts for interseed scattering and attenuation (ISA), (3) Dm,m that examines ISA and tissue heterogeneity by scoring absorbed dose in tissue, and (4) Dw,m that unlike Dm,m scores absorbed dose in water. The MC absorbed doses (1) and (2) are simulated in a TG-43 patient phantom derived by assigning the densities of every voxel to 1.00gcm-3 (water), whereas MC absorbed doses (3) and (4) are scored in the TG-186 patient phantom generated by mapping the mass density of each voxel to tissue according to a CT calibration curve. The MC absorbed doses calculated in this study are compared with VariSeed v8.0 calculated absorbed doses. To evaluate the dosimetric effect of MAR and TAS, the MC absorbed doses of this work (independent of MAR and TAS) are compared to the MC absorbed doses of different 125 I source models from previous studies that were calculated with different MC codes using post-implant CT-based phantoms generated by implementing MAR and TAS on post-implant CT images. The very good agreement of TG-43 parameters of this study and the published consensus data within 3% validates the geometry of the IsoSeed I25.S17plus source. For the clinical studies, the TG-43-based calculations show a D90 overestimation of more than 4% compared to the more realistic MC methods due to ISA and tissue composition. The results of this work generally show few discrepancies with the post-implant CT-based dosimetry studies with respect to the D90 absorbed dose metric parameter. These discrepancies are mainly Type B uncertainties due to the different 125 I source models and MC codes. The implementation of MAR and TAS on post-implant CT images have no dosimetric effect on the 125 I prostate MC absorbed dose calculation in post-implant CT-based phantoms.

CT-MR Image Fusion for Post-Implant Dosimetry Analysis in Brain Tumor Seed Implantation- a Preliminary Study.

Purpose To calculate and evaluate postimplant dosimetry (PID) with CT-MR fusion technique after brain tumor brachytherapy and compare the result with CT-based PID. Methods and Materials 16 brain tumor patients received MR-guided intervention with Iodine-125 (125I) seed implantation entered this preliminary study for PID evaluation. Registration and fusion of CT and MR images of the same patients were performed one day after operation. Seeds identification and targets delineation were carried out on CT, MR, and CT-MR fusion images, each. The number and location of seeds on MR or CT- MR fusion images were compared with those of actually implanted seeds. Clinical target volume (CTV) and dosimetric parameters such as %D90, %V100 and external V100 were measured and calculated. In addition, the correlation of the fusion to CT CTV ratio and other factors were analyzed. Results The numbers of fusion seeds were not significantly different compared with reference seeds (t =1.76, p >0.05). The difference between reference seeds numbers and truly extracted MR seeds numbers was statistically significant (t =3.91, p <0.05). All dosimetric parameters showed significant differences between the two techniques (p <0.05). The mean CTV delineated on fusion images was 34.3 ± 33.6, smaller than that on CT images. The mean values of external V100, %V100 and %D90 on fusion images were larger than those on CT images. Correlation analysis showed that the fusion-CT V100 ratio was positively and significantly correlated with the fusion-CT volume ratio. Conclusions This preliminary study indicated that CT-MR fusion-based PID exhibited good accuracy for 125I brain tumor brachytherapy dosimetry when compared to CT-based PID and merits further research to establish best-outcome protocols.

ACR-ABS-ASTRO Practice Parameter for Transperineal Permanent Brachytherapy of Prostate Cancer.

The American College of Radiology (ACR), American Brachytherapy Society (ABS), and American Society for Radiation Oncology (ASTRO) have jointly developed the following practice parameter for transperineal permanent brachytherapy of prostate cancer. Transperineal permanent brachytherapy of prostate cancer is the interstitial implantation of low-dose rate radioactive seeds into the prostate gland for the purpose of treating localized prostate cancer. This practice parameter was developed according to the process described under the heading The Process for Developing ACR Practice Parameters and Technical Standards on the ACR website (https://www.acr.org/Clinical-Resources/Practice-Parameters-and-Technical-Standards) by the Committee on Practice Parameters-Radiation Oncology of the Commission on Radiation Oncology, in collaboration with ABS and ASTRO. This practice parameter provides a framework for the appropriate use of low-dose rate brachytherapy in the treatment of prostate cancer either as monotherapy or as part of a treatment regimen combined with external-beam radiation therapy. The practice parameter defines the qualifications and responsibilities of all involved radiation oncology personnel, including the radiation oncologist, medical physicist, dosimetrist, radiation therapist, and nursing staff. Patient selection criteria and the utilization of supplemental therapies such as external-beam radiation therapy and androgen deprivation therapy are discussed. The logistics of the implant procedure, postimplant dosimetry assessment, and best practices with regard to safety and quality control are presented. Adherence to established standards can help to ensure that permanent prostate brachytherapy is delivered in a safe and efficacious manner.

Erectile dysfunction and sexual quality of life in patients who underwent low-dose-rate brachytherapy alone for prostate cancer.

This study evaluated erectile function and sexual quality of life (QoL), and predictive factors for erectile dysfunction (ED) and the deterioration of sexual QoL in 70 patients who underwent low-dose-rate brachytherapy (LDR-BT) alone for prostate cancer without androgen deprivation therapy. Erectile function and sexual QoL were evaluated before and 1, 3, 6, 12, 24, 36, 48 and 60months after LDR-BT. Binary logistic regression analysis was used to determine whether age, prostate volume, hypertension, diabetes, Brinkman's index, testosterone, baseline Sexual Health Inventory for Men (SHIM) score and post-implant dosimetry parameters could predict ED and deterioration of sexual QoL at 24 and 60months after LDR-BT. After 24 and 60months, ED was noted in 39 of 70 patients and 42 of 64 patients respectively. Furthermore, sexual QoL worsened in 42 of 70 and 43 of 64 patients respectively. Baseline SHIM score was identified as a significant predictor of ED (24months: odds ratio [OR]: 0.83, p=0.02; 60months: OR: 0.83, p=0.03) and the deterioration of sexual QoL (24months: OR: 0.84, p=0.03). LDR-BT for prostate cancer promoted decreased erectile function and sexual QoL, with high preimplant potency being a significant predictor of ED and the deterioration of sexual QoL.

Detection of Failure Patterns Using Advanced Imaging in Patients With Biochemical Recurrence Following Low Dose Rate Brachytherapy for Prostate Cancer

Low dose rate (LDR) brachytherapy alone or with external beam radiation (EBRT) results in excellent rates of biochemical control in patients (pts) with prostate cancer (PCa). Understanding failure patterns in pts with biochemical recurrence (BCR) may inform implant technique or pt selection. The primary objective of this study was to describe locations of recurrence on multiparametric MRI (mpMRI) and prostate specific membrane antigen (PSMA) imaging in pts with BCR after LDR brachytherapy.Pts with BCR after definitive LDR (± EBRT, ± androgen deprivation therapy (ADT)) referred for IRB approved imaging protocols were included. All pts underwent 3T mpMRI (T2 weighted, dynamic contrast-enhanced, and diffusion-weighted imaging) prospectively reviewed by prostate-dedicated radiologists. A subset of pts underwent PSMA-PET/CT imaging. Pathologic confirmation was obtained unless contraindicated. Prostate lesions were categorized as left (L)/right (R)/midline (ML); transitional zone (TZ)/peripheral zone (PZ); apical/mid/base; and anterior/posterior/central. Locations of recurrence (prostate, seminal vesicles (SV), lymph nodes (LN) and distant metastases (DM)) were recorded and reported descriptively.Between Jan 2011 and Feb 2021, 57 pts with BCR after LDR underwent prostate mpMRI. Treatment was LDR (n = 39) or EBRT + LDR (n = 18), ± ADT (n = 12). At time of LDR, initial Gleason score was ≤6 (n = 29), 7 (n = 19), 8-9 (n = 8), unknown (n = 1) and median PSA was 6 ng/mL (3.4-49). Of dosimetry available (n = 27), 2 had D90 < 90%. Median time from LDR to mpMRI was 6.7 yrs (0.8-18.3). At time of mpMRI, median pt age was 68 yrs (51-85), median PSA was 5.64 ng/mL (0.17-37.47). Lesions (n = 67) in the prostate/SV were detected in 45/57 pts on mpMRI, with a median lesion size of 1.2 cm (0.3-3.6). SV involvement was present in 20/45 patients with mpMRI detected lesions. Locations of mpMRI prostate lesions were L (n = 19) vs R (n = 16) vs ML (n = 15); TZ (n = 22) vs PZ (n = 14); apical (n = 17) vs mid (n = 25) vs base (n = 23); and anterior (n = 13) vs posterior (n = 0) vs central (n = 8). In pts with PSMA (n = 32), 31 had uptake in the prostate (n = 16, 14 detected on mpMRI) and SV (n = 12, 10 detected on mpMRI). One pt had uptake in the prostate+SV detected only on PSMA. Extraprostatic uptake on PSMA included LN (n = 15), DM (n = 5) and penile bulb (n = 1). Of 37 pts who had prostate/SV biopsy, tumor recurrence was confirmed in 31 (prostate (n = 25) and SV (n = 12)). Three of 6 pts with negative biopsy had no lesion on mpMRI. SV recurrence (imaging ± biopsy) occurred in 18/39 pts with LDR alone and in 5/18 with LDR + EBRT.In this cohort of pts with BCR following LDR, the predominant locations of local recurrence were in the anterior/central TZ and SV. PSMA corroborated the majority of mpMRI findings and often demonstrated LN and DM. These findings may inform LDR technique and impact patient selection or pre-treatment imaging evaluation; and should be confirmed in larger subsets and correlated with post-implant dosimetry.

Patient assessment of lower urinary tract symptoms using the international prostate symptom score following low-dose-rate prostate brachytherapy

To correlate changes in urinary patient-reported outcomes including the International Prostate Symptom Score (IPSS), acute urinary retention and urethral stricture with urethral dose in those treated with low dose rate (LDR) prostate brachytherapy. Patients treated with prostate LDR between 2012 and 2019 (n=117) completed IPSS urinary symptom assessments prior to treatment and at each follow-up. CT simulation was obtained with urinary catheter 1-month post-implant for dosimetric analysis. 113 patients with pre- and ≥1 post-LDR IPSS score available were analyzed. Urethral dosimetric parameters including U75, U100, U125, U150 and U200 were abstracted from post-implant dosimetry and assessed for association with urinary toxicity using bivariate logistic regression and Spearman correlation. Outcomes included clinically significant change (CSC, defined as 4 or more points or 25% rise above baseline) in IPSS score at 6 and 12 months, acute urinary retention (AUR), and urethral stricture (US). 89 (79%) patients were treated with LDR monotherapy (145 Gy) and 24 (21%) with LDR boost (110 Gy)with external beam radiation therapy. Twenty(18%) had baseline IPSS ≥15. Median IPSS scores were: baseline 6 (3-12; n=113), 1-month 17 (10-25; n=110), 6 months 12 (7-18; n=77), 1 year 8 (5-14; n=52). CSC-6 was observed in 59 (77%), CSC-12 in 26 (50%), AUR in 12 (11%), and US in 4 (4%). No association was identified between urethral dose parameters and CSC-6, CSC-12, AUR, or US. No correlation between urethral dose and IPSS at 6- and 12-months was identified. The IPSS ≥15 group exhibited lower rates of CSC-12 (13% v. 57%, p=0.05) but not CSC-6 (55% v. 80%, p=0.12). We did not find a relationship between urethral dose and IPSS elevation, AUR or US. We did identify a significantly lower change in IPSS at 12 months for those with baseline IPSS ≥15 compared to those with low baseline scores.

Review of Clinical and Technological Consideration for MRI-Guided Robotic Prostate Brachytherapy

Low Dose Rate Brachytherapy (LDR-BT) is a technique for treating localized prostate cancer by implanting radioactive seeds. In conventional practice, the delivery of seeds is performed using transrectal ultrasonography (TRUS) imaging for implant guidance and checked using computed-tomography for post-implant dosimetry. In the case of TRUS, accuracy can be compromised due to sub-optimal imaging. Magnetic Resonance Imaging (MRI), however, is known to provide better soft-tissue contrast, therefore, increasing the ability to detect small lesions; for that reason, the integration of intraoperative MRI in BT workflows has been investigated over the last two decades. The fusion of preoperative MR-images during TRUS-brachytherapy is possible. However, the image registration process introduces a source of uncertainty. Manual, real-time intra-operative LDR-BT is challenging under MRI due to confined space and procedural workflows. This motivates the development of MRI-compatible robots for prostate BT, with potential advantages of improved source placement accuracy and final dosimetry. In this paper, the state-of-art of technological components in MRI compatible robots, especially for LDR-BT, has been presented. This systematic review helps us to position an ongoing Cooperative Brachytherapy project, developing a real-time MRI-guided robot for adaptive LDR-BT. The design approach includes integrating separate modules: imaging, dose planning, needles, and robot.

Patient Reported Quality of Life Outcomes After Definitive Radiation Therapy With Absorbable Spacer Hydrogel for Prostate Cancer

PurposeSpaceOAR is a device approved for conventional radiation in prostate cancer. We sought to observe prospectively how SpaceOAR Hydrogel effected quality of life and dosimetry to organs at risk at our institution. Methods and MaterialsWe prospectively enrolled patients with low risk or favorable-intermediate risk localized prostate cancer. Baseline Expanded Prostate Cancer Index Composite (EPIC-26) scores along with baseline American Urology Association Symptom Index (AUA-SI) scores were collected. SpaceOAR was placed for all patients who then received stereotactic body radiation therapy, low dose rate brachytherapy, conventionally fractionated radiation therapy, or moderately hypofractionated radiation therapy. We evaluated postimplant dosimetry to critical structures, and prospectively collected follow-up EPIC-26 and AUA-SI scores. We performed a repeated measures analysis of variance to compare patient-specific responses and correlated survey data with dosimetric metrics by generating linear regression models. ResultsWe enrolled 59 patients in this study with a median follow-up of 366 days (interquartile range, 507). At final follow-up, the "?>prostate-specific antigen had a significant decline compared with baseline (P < .0001). There were no grade 3 toxicities on treatment. There were no significant changes in the AUA-SI score (P = .69) at final follow-up compared with baseline, nor was there any change in EPIC-26 domain scores (P = .19) during the course of the study period. There were no significant associations between AUA scores and EPIC-26 scores and the dose to the rectum, bladder, or urethra with the exception being dose to the 2 mL rectum correlated with decline in EPIC-26 rectal score (β, −0.002; P = .006). Patient-reported declines in bowel domains were less than previously reported data. ConclusionsUse of SpaceOAR results in favorable dosimetry to the organs at risk and portends excellent short-term quality of life as measured by the association with the patient reported outcome measures. Longer-term follow-up is ongoing and necessary to assess the long-term effect and association of the hydrogel.

Comprehensive assessment of the effect of eye plaque tilt on tumor dosimetry

PURPOSE: Tilting of the posterior plaque margin during eye plaque brachytherapy can lead to tumor underdosing and increased risk of local recurrence. We performed a quantitative analysis of the dosimetric effects of plaque tilt as a function of tumor position, basal dimension, height and plaque type using 3D treatment planning software.MATERIALS AND METHODS: Posterior and anterior tumors with largest basal dimensions of 6, 12 and 18 mm and heights of 4, 7 and 10 mm were modeled. Both Eye Physics and COMS plaques were simulated and uniformly loaded. Plans were normalized to 85 Gy at the tumor apex. Posterior plaque tilts of 1, 2, 3 and 4 mm were simulated.RESULTS: Volumetric coverage is more sensitive to tilt than the area coverage. Wide, flat tumors are more susceptible to tilt. Apical dose changed significantly as a function of tumor height and diameter. No other parameter exhibited significant differences. Posterior tumors are slightly more susceptible to tilt due to the use of notched plaques. Plaque type does not significantly alter the effect of plaque tilt.CONCLUSIONS: Wide, flat tumors are the most susceptible to plaque tilt. Tumor location or plaque type does not have a significant effect on dosimetry changes from plaque tilt. Robust clinical procedures such as the use of mattress sutures, pre- and post-implant ultrasound and post-implant dosimetry can all mitigate the risk associated with plaque tilt.

PO39: The Effect of Timing of Hydrogel Spacer Placement on Prostate and Rectal Dosimetry Following PD-103 Brachytherapy

Purpose: Although a rare event, the physical and emotional consequences of rectal injuries following the definitive treatment of prostate cancer are devastating and often permanent. In this study, we evaluated the impact of hydrogel spacer (SpaceOar™) on rectal and prostate dosimetry in patients with and without spacer placement and investigated whether the timing of hydrogel spacer placement effects post-implant dosimetry to include the rectum, prostate and urethra.