Permanent Prostate Implants Research Articles

Preplan Dosimetry as a Predictor of Change in Urinary Symptoms After 125I Permanent Prostate Implants (PPI)

Preplan Dosimetry as a Predictor of Change in Urinary Symptoms After 125I Permanent Prostate Implants (PPI)

Segmental Analysis of Permanent Prostate Implant Dosimetry and Effects on Long-term Clinical Outcomes

Segmental Analysis of Permanent Prostate Implant Dosimetry and Effects on Long-term Clinical Outcomes

Comparison of Day-0 Ultrasound real-time dosimetry with Day 0 and Day 30 CT-based dosimetry for permanent prostate implants using 125I single seeds – Initial Study

Comparison of Day-0 Ultrasound real-time dosimetry with Day 0 and Day 30 CT-based dosimetry for permanent prostate implants using 125I single seeds – Initial Study

Optimization of image fusion in permanent prostate implants

Optimization of image fusion in permanent prostate implants

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.

SU‐E‐T‐692: Validation of Permanent Interstitial I‐125 Implant Pre‐Plans Against An Experimentally Derived Nomogram Can Potentially Reduce Variability of Day 0 Post‐Implant Dosimetry

Purpose: To evaluate whether standardization of total pre‐plan activity against a nomogram reduces variability of post‐implant dosimetry for I‐125 permanent interstitial prostate implants (PIPI). Methods: Between January 2007 and December 2011, 843 patients underwent TRUS guided PIPI (MPD 108Gy) in combination with external irradiation. The implants and all contouring for pre‐and post‐plans were performed by a single physician. Post‐implant dosimetry was determined based on CTs taken on the day of implant. Implants were grouped into the following categories based on their post‐implant dosimetry: 1.Excellent (90%< D90 <120% and V150>70%); 2.Hot (D90>120% and V150>70%); 3. Cold (D90<80%); and 4.Other. A known‐effective activity versus volume nomogram was derived from a fit of the data from the excellent implants. The actual activities used in the hot and cold implant groups were evaluated for deviations from the predictions of the known‐effective nomogram. The potential impact of imposing a tolerance window was investigated. Results: Of all implants, 347, 81, 37 and 378 were excellent, hot, cold and other, respectively. The activity versus volume data of the excellent implants were well fit with a power law (R^2=0.972). The average and standard deviation of the differences between actual implanted activity and the prediction of the known‐effective nomogram was 3.8% +/− 4.0 and −0.6% +/− 5.0 for the hot and cold groups, respectively. The implanted activity agreed with the known‐effective nomogram prediction within +/−4% for 514 plans. For these plans, 44%, 7.0% and 3.9% were excellent, hot and cold, respectively. Outside this tolerance window, the corresponding values were 36%, 14% and 5.2%. Conclusion: Imposing a quality assurance requirement that total pre‐plan activity agree with the prediction of a known‐effective nomogram should improve homogeneity of post‐implant dosimetric parameters by increasing the frequency of excellent implants and reducing the frequency of hot implants.

A Novel Method to Register Post-Implant Magnetic Resonance Imaging to Pre-Plan Ultrasound Images to Evaluate Needle Passage through Neurovascular Bundles during Permanent Prostate Implant

Transrectal ultrasound (TRUS) is the most common image modality used in permanent prostate brachytherapy (PPI). Although providing excellent prostate visualization, TRUS images are inferior to MRI images for identifying critical structures such as neurovascular bundles (NVB). To preserve erectile function after PPI, the traumatic injury resulting from needles passing through NVB is worth investigating. Our goals were to develop a novel method to register post-implant MRI to pre-plan ultrasound (US) images and demonstrate its usage on evaluating needle passages to NVB.

Changes in radiobiological parameters in 131Cs permanent prostate implants

AbstractIn prostate permanent implants using 131Cs seeds, the prostatic edema developed during the implantation procedure, increases the separation between the seeds. This leads to a decrease in the prostate coverage and thus causes an edema induced dose reduction, which results in an increase in tumour cell surviving fraction (SF) with a corresponding decrease in tumour control probability (TCP). To investigate the impact of edema on the SF and the TCP, the expression of the SF of the linear quadratic (LQ) model was extended to account for the effects of edema using the exponential nature of edema resolution and the dose delivered to the edematous prostate. The SF and the TCP for edematous prostate implants were calculated for 31 patients who underwent real time 131Cs permanent seed implantation. The dose delivered to the edematous prostate was calculated to compute the SF and the TCP for these patients for edema half lives (EHL) ranging from 4 days to 34 days and for edemas of magnitudes (M0) varying from 5 to 60% of the actual prostate volume.A reduction in the dose delivered to the edematous prostate was found with the increase of EHL and edema magnitude which results in an increase of the SF, and corresponding decrease in the TCP. The dose reductions in 131Cs implants varied from 1.1% (for EHL = 4 days and M0 = 5%) to 32.3% (for EHL = 34 days and M0 = 60%). These are higher than the dose reduction in 125I implants, which vary from 0.3% (for EHL = 4 days and M0 = 5%) to 17.5% (for EHL = 34 days and M0 = 60%). As edema half life increased from 4 days to 34 days and edema magnitude increased from 5 to 60% the SF increased by 4.57 log, and the TCP decreased by 0.80. Compensation of edema induced increase in the SF and decrease in the TCP in 131Cs seed implants should be carefully done by redefining seed positions with the guidance of post-needle plans. The presented model in this study can be used to estimate the SF or the TCP for pre plan or real time permanent prostate implants using day 0 post-implant CT images.

Multiparametric MRI-based Dosimetric Parameters Best Predict Short-term Time Course of PSA After Iodine 125 Permanent Prostate Implantation for Localized Prostate Cancer

D90% and V150% of the entire prostate are recognized as the best dosimetric predictors of outcome after 125 I permanent prostate implantation (PPI). The purpose of this study was 2-fold: 1) to determine the relationship between dose-volume parameters of the Dominant Intraprostatic Lesion (DIL) when compared to the prostate and early biochemical outcome after PPI; 2) to define if dose-volume parameters of the central gland (CG), the peripheral zone (PZ) and the DIL could best predict PSA bounce occurrence. The time course of PSA and mechanisms of bounces still remain unclear after PPI. Patients who had a higher dose in the DIL had a worse PSA level at 1 year which is in keeping with previous reports with a longer follow-up suggesting that patients who will achieve a very low PSA are more likely to require a protracted time after PPI. A strong relationship was found between V150% in the CG and PSA bounce. These pioneering results require further investigations.

Preplan Dosimetry as a Predictor of Clinical Outcome for 125I Permanent Prostate Implants (PPI)

Patients with post-implant D90≥140 Gy have a higher freedom from biochemical failure rate than those with D90<140 Gy. We previously reported that on PPI pre-plans, an increased number of seeds in the Inferior-Anterior (I-A) quarter and larger superior (S) margins independently predict for post-implant D90≥140 Gy. The goal of the current study was to determine PPI pre-plan dosimetric parameters that directly predict biochemical no evidence of disease (bNED).

Feasibility of MR Imaging/MR Spectroscopy-Planned Focal Partial Salvage Permanent Prostate Implant (PPI) for Localized Recurrence After Initial PPI for Prostate Cancer

To assess the feasibility of magnetic resonance imaging (MRI)-planned partial salvage permanent prostate implant (psPPI) among patients with biopsy-proven local recurrence after initial PPI without evidence of distant disease. From 2003-2009, 15 patients underwent MRI/magnetic resonance spectroscopy (MRS) planning for salvage brachytherapy (psPPI, I-125 [n=14; 144 Gy]; Pd-103 [n=1; 125 Gy]) without hormone therapy. Full dose was prescribed to areas of recurrence and underdosage, without entire prostate implantation. Limiting urethral and rectal toxicity was prioritized. Follow-up was from salvage date to prostate-specific antigen (PSA) concentration failure (Phoenix criteria = nadir + 2.0; ASTRO = 3 consecutive rises), recurrence, distant metastases, or last follow-up PSA level. Progression-free survival (PFS) was defined as no PSA failure or biopsy-proven recurrence without all-cause mortality. Toxicity was scored using Common Terminology Criteria for Adverse Events version 4.0. At salvage, median age was 68 years, and PSA concentration was 3.5 ng/mL (range, 0.9-5.6 ng/mL). Abnormal MRI/MRS findings were evident in 40% of patients. Biopsy-proven recurrences consisted of a single focus (80%) or 2 foci (20%). At recurrence, Gleason score was 6 (67%) or ≥7 (27%). Median interval between initial and salvage implantation was 69 months (range, 28-132 months). psPPI planning characteristics limited doses to the rectum (mean V100 = 0.5% [0.07 cc]) and urethra (V100 = 12% [0.3 cc]). At median follow-up (23.3 months; range, 8-88 months), treatment failure (n=2) resulted only in localized recurrence; both patients underwent second psPPI with follow-up PSA tests at 12 and 26 months, resulting in 0.6 and 0.7 ng/mL, respectively. American Society for Radiation Oncology PFS rates at 1, 2, and 3 years were 86.7%, 78.4%, and 62.7%, respectively, with 5 patients for whom treatment failed (n=3 with negative transrectal ultrasound-guided biopsy results). Phoenix PFS rates at 1, 2, and 3 years were 100%, 100%, and 71.4%. 73%, respectively; achieved PSA nadir of <0.5 ng/mL; and 47% of patients had a nadir of <0.1 ng/mL. Treatment-related toxicity was minimal, with no operative interventions, fistulas, or other grade ≥3 gastrointestinal (GI)/genitourinary (GU) toxicity. Thirteen percent had grade 1 GI and 33% had grade 2 GU toxicities. Postsalvage, 20% of patients had no erectile dysfunction, 67% of patients had medication-responsive erectile dysfunction, and 13% of patients had erectile dysfunction refractory to medication. Focal psPPI with MR-planning in highly selected patients is feasible with short-term control comparable to conventional salvage, with less toxicity. Longer follow-up is needed to confirm its impact on quality of life and treatment.

SU-E-T-07: Edema Induced Changes in Tumor Cell Survival Fraction and Tumor Control Probability in 131Cs Permanent Prostate Implant Patients.

The purpose of this study is to estimate the effect of edema, developed during implant procedure, on tumor cell surviving fraction(SF) and tumor control probability(TCP) in the patients of prostate cancer who underwent 131 Cs permanent seed implants. The impact of edema on SF and TCP, was calculated using LQ equation extended to account for exponential nature of edema decay, dose delivered to dematous prostate and inhomogeneous dose distribution. Where (1) S(D)=(1/V)Σi=1n [Vpi{1+M0 exp(-λe t)}Si (D)] Si (D)=exp[-αRi (0)∫0t [exp(- λt)/{1+M0 exp(-λe t)}τ/3]dt -βq(t){Ri (0)∫0t [exp(-λt)/{1+M0 exp(-λe t)}τ/3]dt }2 ] and (2) TCP=exp[-ρVpS(D)] Following parameters, α=0.15Gy-1 , β=0.05Gy-2 , α/β=3.0Gy, Tp=42days, μ=61.6d-1 and ρ=1×106 are used to calculate SF and TCP for 31 patients of 131 Cs permanent seed implants for edema half lives(EHL) ranging from 4 days to 34 days and for edemas of magnitudes(M0 ) varying from 5% to 60% of the actual prostate volume. The dose reductions in 131 Cs implants varied from 1.1% (for EHL=4 days and M0 =5%) to 32.3% (for EHL= 34 days and M0 = 60%). These are higher than the dose reduction in 125 I implants, which vary from 0.3% (for EHL= 4 days and M0 = 5%) to 17.5% (for EHL= 34 days and M0 = 60%). As edema half life increased from 4 days to 34 days and edema magnitude increased from 5% to 60% the SF increased by 4.57 log, and the TCP decreased by 0.80. Compensation of edema induced increase in the SF and decrease in the TCP in 131 Cs seed implants should be carefully done by redefining seed positions with the guidance of post needle plans. The presented model in this study can be used to estimate the SF or the TCP for pre plan or real time permanent prostate implants using day 0 post implant CT images.

Preoperative treatment planning with intraoperative optimization can achieve consistent high-quality implants in prostate brachytherapy

Advances in brachytherapy treatment planning systems have allowed the opportunity for brachytherapy to be planned intraoperatively as well as preoperatively. The relative advantages and disadvantages of each approach have been the subject of extensive debate, and some contend that the intraoperative approach is vital to the delivery of optimal therapy. The purpose of this study was to determine whether high-quality permanent prostate implants can be achieved consistently using a preoperative planning approach that allows for, but does not necessitate, intraoperative optimization. To achieve this purpose, we reviewed the records of 100 men with intermediate-risk prostate cancer who had been prospectively treated with brachytherapy monotherapy between 2006 and 2009 at our institution. All patients were treated with iodine-125 stranded seeds; the planned target dose was 145 Gy. Only 8 patients required adjustments to the plan on the basis of intraoperative findings. Consistency and quality were assessed by calculating the correlation coefficient between the planned and implanted amounts of radioactivity and by examining the mean values of the dosimetric parameters obtained on preoperative and 30 days postoperative treatment planning. The amount of radioactivity implanted was essentially identical to that planned (mean planned radioactivity, 41.27 U vs. mean delivered radioactivity, 41.36 U; R2 = 0.99). The mean planned and day 30 prostate V100 values were 99.9% and 98.6%, respectively. The mean planned and day 30 prostate D90 values were 186.3 and 185.1 Gy, respectively. Consistent, high-quality prostate brachytherapy treatment plans can be achieved using a preoperative planning approach, mostly without the need for intraoperative optimization. Good quality assurance measures during simulation, treatment planning, implantation, and postimplant evaluation are paramount for achieving a high level of quality and consistency.

Impact of edema and seed movement on the dosimetry of prostate seed implants

This article summarizes current knowledge concerning the characterization of prostatic edema and intra-prostatic seed movement as these relate to dosimetry of permanent prostate implants, and reports the initial application to clinical data of a new edema model used in calculating pre- and post-implant dose distributions. Published edema magnitude and half-life parameters span a broad range depending on implant technique and measurement uncertainty, hence clinically applicable values should be determined locally. Observed intra-prostatic seed movements appear to be associated with particular aspects of implant technique and could be minimized by technique modification. Using an extended AAPM TG-43 formalism incorporating the new edema model, relative dose error RE associated with neglecting edema was calculated for three I-125 seed implants (18.9 cc, 37.6 cc, 60.2 cc) performed at our center. Pre- and post-plan RE average values and ranges in a 50 × 50 × 50 mm3 calculation volume were similar at ~2% and ~0–3.5%, respectively, for all three implants; however, the spatial distribution of RE varied for different seed configurations. Post-plan values of D90 and V100 for prostate were reduced by ~2% and ~1%, respectively. In cases where RE is not clinically negligible as a consequence of large edema magnitude and / or use of Pd-103 seeds, the dose calculation method demonstrated here can be applied to account for edema explicitly and there by improve the accuracy of clinical dose estimates.

Feasibility of MR-Guided Partial Permanent Prostate Implant (PPI) Salvage of Localized Recurrence after Initial PPI (iPPI)-treated Prostate Cancer (PCa)

Thirty percent of patients have locally recurrent PCa after iPPI and can be treated palliatively with hormone therapy (HT) or curatively with cryotherapy, surgery, or brachytherapy (BT). Because (1) exact location of recurrent tumor is generally unknown, (2) iPPI causes volumetric changes over time, and (3) difficulty recreating iPPI isodose coverage, conventional PPI salvage involves the entire prostate (EP), with GI/GU Grade 3+ toxicity ∼30% and ∼13% requiring colostomy/urostomy for fistulas. Our goal was to assess feasibility, control, and toxicity in eligible patients for partial salvage PPI (sPPI) using MRI/MR-spectroscopy and MR deformation.

Rectal toxicity and rectal dosimetry in low-dose-rate 125I permanent prostate implants: A long-term study in 1006 patients

ObjectiveTo describe the acute and late rectal toxicity in 1006 prostate brachytherapy patients implanted 1998–2003. To determine whether rectal dose–volume histogram as well as patient and treatment factors were associated with rectal toxicity. Methods and materialsMedian followup was 60.7 months. Rectal dosimetry was calculated as dose–volume histogram of the rectum using Day 28 CT-based dosimetry and expressed as volume of the rectum in cc receiving 50%, 100%, and 150% of the prescription dose (VR50cc, VR100cc, and VR150cc, respectively). Univariate and multivariate analyses were performed to examine the influence of patient, implant, dosimetry, and learning curve factors on the development of acute and late toxicities using a modified Radiation Therapy Oncology Group (RTOG) scale. Acute toxicity was analyzed using logistic regression and late toxicity using Cox proportional hazards regression. Analysis of variance was used to examine the association between rectal toxicity and rectal dose. ResultsRectal dosimetry in 93.5% and rectal toxicity in 96.2% have been recorded. Median VR100=1.05cc. Late RTOG Grades 0, 1, 2, 3, and 4 were recorded in 68%, 23%, 7.3%, 0.9%, and 0.2% patients, respectively. On multivariate analysis, acute RTOG ≥2 rectal toxicity was associated with urinary retention (p=0.036) and learning curve (p=0.015); late RTOG ≥2 was associated with the presence of acute toxicity (p=0.0074), higher VR100 (p=0.030) and learning curve (p=0.027). ConclusionsLate rectal RTOG ≥2 rectal toxicity in this cohort was 8%. Increased VR100, presence of acute rectal toxicity, and learning curve were associated with higher rate of late RTOG ≥2 toxicity. Severe late rectal toxicity after prostate brachytherapy was rare.

WE‐E‐BRC‐08: Evaluation of a Probabilistic Non‐Rigid Registration Method for Improved Intra‐Operative Target Definition in 125‐I Permanent Prostate Implants

Purpose: To evaluate the use of a non‐rigid registration method for improved intra‐operative target definition in 125‐I permanent prostate implants. Methods: The validation dataset was created from 10 MRI‐guided prostate biopsy patients with both diagnostic (with endorectal coil) and intra‐procedural (without endorectal coil) 3 Tesla MRI scans under an IRB approved protocol. A biomechanical‐based probabilistic non‐rigid registration method was adapted to register diagnostic to intra‐procedural images by matching the contoured prostate boundaries. The probabilistic framework provides a collection of prostate configurations under deformation, or a marginal probability map for a given location to be identified as inside the prostate by the registration. The marginal probability map was compared to the intra‐procedural prostate contour. 125‐I treatment plans were generated for the intra‐procedural scans. Results: In two cases we find that ∼5% of the prostate volume within the 50th percentile of the marginal deformed probability is not included in the intra‐procedural contour, and 3–4% of the intra‐procedural prostate volume is not included in any of the deformed configurations. Different regions of the prostate have varying uncertainties; preliminary results show margins of 1–2 mm near mid‐gland and 3–4 mm around the apex. We present geometric uncertainties and the resulting variation of dosimetric quantities in the base, mid‐gland, apex and peripheral zone of the prostate. Conclusions: We study the feasibility of using a probabilistic non‐rigid registration method for supplementing intra‐procedural images with diagnostic MR images. The method is evaluated geometrically and dosimetrically, which quantifies the methodˈs ability to provide intra‐procedural estimation of the accuracy of the deformed configurations. This method will be applied to registration of diagnostic MR with intra‐procedural transrectal ultrasound images to improve visualization of the prostate apex and substructure, which could provide improved prostate boundary definitions with uncertainty margins to guide the development of intra‐operative brachytherapy treatment plans.This work was supported by NIH grants 1U41RR019703‐01A2, 1R01CA111288‐01A1,P01‐CA67165,andU01CA151261.

SU-E-T-391: Is 1D Anisotropy Function Recommended by TG-43U1 Protocol a Good Candidate for Clinical Brachytherapy Dosimetry?

Purpose: The effect of the anisotropy on dose distributions for real permanent prostate implants with 125I seeds has been evaluated and compared with Monte Carlo (MC) calculations. Methods: soSeed (Bebig) was used. Dose distribution of the prostate implants was calculated using MCNP5 code and compared to TG43 calculation. A DVH comparison between 2D and 1D approximations was performed for prostate, urethra and rectum for 20 patients. Results: A DVHs comparison between the use of 1D & 2D anisotropy functions are presented for 20 prostates implants. For prostate, Dmean, D90 and D98 are underestimated by 1D approximation by about 3%, 1.5% and 1% respectively. It leads also to inconsistent results for Dmin and D100. 1D approximation overestimates the dose for about of 15% in planes situated at the two ends of implant containing no implanted seeds while it underestimates dose about 5% in the other planes. This yields a compensating effect in the resulting DVH's. These over dosage and under dosage must be attributed to an averaging effect between 2D and 1D anisotropy function. It can be noticed that the effect of 1D approximation on D98 and Dmin of prostate varies from patient to another. This is due to the many factors, for example, prostate volume, number of seeds and seeds positions relative to prostate…etc. D90 is always underestimated by 1D approximation where for 70% of patients D90 exceeds 185 Gy. For the urethra, 1D approximation underestimates the Dmax in which 60% of patients, their maximum dose for the urethra exceeds the limits. Finally, 1D approximation underestimates V100 about 50% les than 2D approximation Conclusions: The use of the 2D anisotropy function is preferred to the 1D anisotropy function as this approximation yields more consistent results for the 20 patients studied. The observed overestimation and the variability among the patients can influence dose—outcome.

Dose calculation for permanent prostate implants incorporating spatially anisotropic linearly time‐resolving edema

The objectives of this study were (i) to develop a dose calculation method for permanent prostate implants that incorporates a clinically motivated model for edema and (ii) to illustrate the use of the method by calculating the preimplant dosimetry error for a reference configuration of 125I, 103Pd, and 137Cs seeds subject to edema-induced motions corresponding to a variety of model parameters. A model for spatially anisotropic edema that resolves linearly with time was developed based on serial magnetic resonance imaging measurements made previously at our center to characterize the edema for a group of n = 40 prostate implant patients [R. S. Sloboda et al., "Time course of prostatic edema post permanent seed implant determined by magnetic resonance imaging," Brachytherapy 9, 354-361 (2010)]. Model parameters consisted of edema magnitude, delta, and period, T. The TG-43 dose calculation formalism for a point source was extended to incorporate the edema model, thus enabling calculation via numerical integration of the cumulative dose around an individual seed in the presence of edema. Using an even power piecewise-continuous polynomial representation for the radial dose function, the cumulative dose was also expressed in closed analytical form. Application of the method was illustrated by calculating the preimplant dosimetry error, RE(preplan), in a 5 x 5 x 5 cm3 volume for 125I (Oncura 6711), 103Pd (Theragenics 200), and 131Cs (IsoRay CS-1) seeds arranged in the Radiological Physics Center test case 2 configuration for a range of edema relative magnitudes (delta = [0.1, 0.2, 0.4, 0.6, 1.0]) and periods (T = [28, 56, 84] d). Results were compared to preimplant dosimetry errors calculated using a variation of the isotropic edema model developed by Chen et al. ["Dosimetric effects of edema in permanent prostate seed implants: A rigorous solution," Int. J. Radiat. Oncol., Biol., Phys. 47, 1405-1419 (2000)]. As expected, RE(preplan) for our edema model indicated underdosage in the calculation volume with a clear dependence on seed and calculation point positions, and increased with increasing values of delta and T. Values of RE(preplan) were generally larger near the ends of the virtual prostate in the RPC phantom compared with more central locations. For edema characteristics similar to the population average values previously measured at our center, i.e., delta = 0.2 and T = 28 d, mean values of RE(preplan) in an axial plane located 1.5 cm from the center of the seed distribution were 8.3% for 131Cs seeds, 7.5% for 103Pd seeds, and 2.2% for 125I seeds. Maximum values of RE(preplan) in the same plane were about 1.5 times greater. Note that detailed results strictly apply only for loose seed implants where the seeds are fixed in tissue and move in synchrony with that tissue. A dose calculation method for permanent prostate implants incorporating spatially anisotropic linearly time-resolving edema was developed for which cumulative dose can be written in closed form. The method yields values for RE(preplan) that differ from those for spatially isotropic edema. The method is suitable for calculating pre- and postimplant dosimetry correction factors for clinical seed configurations when edema characteristics can be measured or estimated.

Inferring Postimplant Dosimetry and Determining Radiation Dose Cold Spots Using Images Obtained at Time of Recurrence After Permanent Prostate Implant Brachytherapy: A Pragmatic Study

Purpose: Nearly 30% of the patients (Pts) with a clinically localized prostate cancer who underwent a primary permanent prostate implantation (PPI) have a biochemical failure. Hypothesizing that the recurrence could be correlated with the “cold” areas on the preoperative or postoperative PPI dosimetric plans, the purpose of this work is to a) compensate volume change and project the MR based dose distribution back to the time of post-implant assessment, b) identify cold spots and c) infer the cause for failure.