Prostate Contours Research Articles

Prostate Brachytherapy With Oblique Needles to Treat Large Glands and Overcome Pubic Arch Interference

First, to show that low-dose-rate prostate brachytherapy plans using oblique needle trajectories are more successful than parallel trajectories for large prostates with pubic arch interference (PAI); second, to test the accuracy of delivering an oblique plan by using a three-dimensional (3D) transrectal ultrasonography (TRUS)-guided mechatronic system. Prostates were contoured for 5 subjects' 3D TRUS images showing a maximum PAI of ≤1 cm and a prostate volume of <50 cc. Two planning studies were done. First, prostate contours were artificially enlarged to 45 to 80 cc in 5- to 10-cc increments for a single subject. Second, all subject prostate contours were enlarged to 60 cc. For each study, three types of plans were manually created for comparison: a parallel needle template (PT) plan, a parallel needle no-template (PNT) plan, and an oblique needle no-template (OBL) plan. Needle positions and angles were not discretized for nontemplate plans. European Society for Therapeutic Radiology and Oncology dose-volume histogram guidelines, iodine-125 (145-Gy prescription, 0.43 U), and needle angles of <15° were used. An OBL plan was delivered to a pubic arch containing a 60-cc prostate phantom that mimicked the anatomy of the subject with the greatest PAI (23% by volume). In the increasing-prostate volume study, OBL plans were successful for prostates of ≤80 cc, and PT plans were successful for prostates of <65 cc. In paired, one-sided t tests for the 60-cc volume study, OBL plans showed dosimetric improvements for all organs compared to both of the parallel type plans (p < 0.05); PNT plans showed a benefit only in planning target volumes receiving more than 100 Gy compared to PT plans. A computed tomography scan of the phantom showed submillimeter seed placement accuracy in all directions. OBL plans were significantly better than parallel plans, and an OBL plan was accurately delivered to a 60-cc prostate phantom with 23% PAI by volume.

Radical prostatectomy reporting: consensus and controversy

Standardisation of the reporting of prostate cancer in radical prostatectomy (RP) specimens has resulted from the development of synoptic reporting protocols by groups in the US, Canada, UK and the RCPA. The recommendations for this were informed by the International Society of Urological Pathology Consensus Conference convened in 2009 which made a comprehensive series of recommendations regarding sampling and reporting. From this conference it was recommended that the weight of the prostate without seminal vesicles should be recorded. It was also agreed that the apical and basal excision margins should be examined by the cone method and that, while total sampling of the prostate was preferred, partial sampling was acceptable. For RPs without carcinoma on initial evaluation, a formal protocol was developed. It was agreed at the Conference that pT2 substaging was, in its present form, illogical and its use should be optional. It was similarly agreed that morphometric measurement of tumour volume should be optional, although some assessment of tumour volume should be provided. Infiltration of extraprostatic fat or desmoplastic bulging of the prostatic contour was considered diagnostic of extraprostatic extension of tumour. There was less agreement regarding the features of extraprostatic extension at the prostatic apex or bladder neck, although infiltration of an inked margin was considered positive by a majority of respondents. It was agreed that the site of positive surgical margin(s) should be provided and that the extent of the positive margin involvement should be measured in millimetres. There was agreement that all lymph nodes from any associated pelvic lymph node dissection should be blocked and an attempt made to quantify the proportion of positive nodes. In response to recent prognostic evidence it was also agreed that the diameter of the largest lymph node metastasis should be measured. There was consensus that lymphovascular invasion should be reported, although there was debate regarding the significance of presence of lymphovascular invasion within the seminal vesicle, with 56% reporting this as pT3b. Muscular infiltration of the extraprostatic, but not intraprostatic seminal vesicle, was considered by a majority to be diagnostic of pT3b tumours.

Comparison of prostate volume, shape, and contouring variability determined from preimplant magnetic resonance and transrectal ultrasound images

PurposeTo compare preimplant prostate contours and contouring variability between magnetic resonance (MR) and transrectal ultrasound images. Methods and MaterialsTwenty-three patients were imaged using ultrasound (US) and MR before permanent brachytherapy treatment. Images were anonymized, randomized, and duplicated, and the prostate was independently delineated by five radiation oncologists. Contours were compared in terms of volume, dimensions, posterior rectal indentation, and observer variability. The Jaccard index quantified spatial overlap between contours from duplicated images. ResultsThe mean US/MR volume ratio was 0.99±0.08 (p=0.5). The width, height, and length ratios for the prostate were 0.98±0.06 (p=0.09), 0.99±0.08 (p=0.4), and 1.05±0.14 (p=0.1). Rectal indentation was larger on US by 0.18mL (p=0.01) and correlated with prostate volume (p<0.01). MR and US interobserver variability in volume were similar at 3.5±1.7 and 3.3±1.9mL (p=0.6). Intraobserver variability was smaller on US at 1.4±1.1mL compared with MR at 2.4±2.2mL (p=0.01). Local intraobserver variability was lower on US at the midgland slice (p<0.01) but lower on MR at the base (p<0.01) and apex (p<0.01) slices. ConclusionsUS is comparable to MR for preimplant prostate delineation, with no significant difference in volume and dimensions. Rectal indentation because of the transrectal ultrasound probe was measurable, although the effects were small. Intraobserver variability was lower on US for the prostate volume but was lower on MR locally at the base and apex. However, the difference was not observed for the interobserver variability, which was similar between MR and US.

Expansion/De-expansion Tool to Quantify the Accuracy of Prostate Contours

Accurate delineation of the prostate gland on computed tomography (CT) remains a persistent challenge and continues to introduce geometric uncertainty into the planning and delivery of external beam radiotherapy. We, therefore, developed an expansion/de-expansion tool to quantify the contour errors and determine the location of the deviations. A planning CT scan and magnetic resonance imaging scan were prospectively acquired for 10 patients with prostate cancer. The prostate glands were contoured by 3 independent observers using the CT data sets with instructions to contour the prostate without underestimation but to minimize overestimation. The standard prostate for each patient was defined using magnetic resonance imaging and CT on multiple planes. After registration of the CT and magnetic resonance imaging data sets, the CT-defined prostates were scored for accuracy. The contours were defined as ideal if they were within a 2.5-mm expansion of the standard without underestimation, acceptable if they were within a 5.0-mm expansion and a 2.5-mm de-expansion, and unacceptable if they extended >5.0 mm or underestimated the prostate by >2.5 mm. A total of 636 CT slices were individually analyzed, with the vast majority scored as ideal or acceptable. However, none of the 30 prostate contour sets had all the contours scored as ideal or acceptable. For all 3 observers, the unacceptable contours were more likely from underestimation than overestimation of the prostate. The errors were more common at the base and apex than the mid-gland. The expansion/de-expansion tool allows for directed feedback on the location of contour deviations, as well as the determination of over- or underestimation of the prostate. This metric might help improve the accuracy of prostate contours.

Evaluations of an adaptive planning technique incorporating dose feedback in image‐guided radiotherapy of prostate cancer

Online image guidance (IG) has been used to effectively correct the setup error and inter-fraction rigid organ motion for prostate cancer. However, planning margins are still necessary to account for uncertainties such as deformation and intra-fraction motion. The purpose of this study is to investigate the effectiveness of an adaptive planning technique incorporating offline dose feedback to manage inter-fraction motion and residuals from online correction. Repeated helical CT scans from 28 patients were included in the study. The contours of prostate and organs-at-risk (OARs) were delineated on each CT, and online IG was simulated by matching center-of-mass of prostate between treatment CTs and planning CT. A seven beam intensity modulated radiation therapy (IMRT) plan was designed for each patient on planning CT for a total of 15 fractions. Dose distribution at each fraction was evaluated based on actual contours of the target and OARs from that fraction. Cumulative dose up to each fraction was calculated by tracking each voxel based on a deformable registration algorithm. The cumulative dose was compared with the dose from initial plan. If the deviation exceeded the pre-defined threshold, such as 2% of the D₉₉ to the prostate, an adaptive planning technique called dose compensation was invoked, in which the cumulative dose distribution was fed back to the treatment planning system and the dose deficit was made up through boost radiation in future treatment fractions. The dose compensation was achieved by IMRT inverse planning. Two weekly compensation delivery strategies were simulated: one intended to deliver the boost dose in all future fractions (schedule A) and the other in the following week only (schedule B). The D₉₉ to prostate and generalized equivalent uniform dose (gEUD) to rectal wall and bladder were computed and compared with those without the dose compensation. If only 2% underdose is allowed at the end of the treatment course, then 11 patients fail. If the same criteria is assessed at the end of each week (every five fractions), then 14 patients fail, with three patients failing the 1st or 2nd week but passing at the end. The average dose deficit from these 14 patients was 4.4%. They improved to 2% after the weekly compensation. Out of these 14 patients who needed dose compensation, ten passed the dose criterion after weekly dose compensation, three patients failed marginally, and one patient still failed the criterion significantly (10% deficit), representing 3.6% of the patient population. A more aggressive compensation frequency (every three fractions) could successfully reduce the dose deficit to the acceptable level for this patient. The average number of required dose compensation re-planning per patient was 0.82 (0.79) per patient for schedule A (B) delivery strategy. The doses to OARs were not significantly different from the online IG only plans without dose compensation. We have demonstrated the effectiveness of offline dose compensation technique in image-guided radiotherapy for prostate cancer. It can effectively account for residual uncertainties which cannot be corrected through online IG. Dose compensation allows further margin reduction and critical organs sparing.

Combining Prior Day Contours to Enhance Automated Prostate Segmentation

To improve the accuracy of automatically generated prostate, rectum and bladder contours, required for daily online adaptive therapy. This is achieved by using contours from image guidance (IGRT) scans of prior treatment days to enhance the automated contour generation of the current scan via the proposed algorithm. Six IGRT prostate patients with daily kilovoltage (kV) cone beam CT (CBCT) had their original plan CT and 9 CBCTs contoured by the same physician. Three types of automated contours were produced for analysis: (1) STANDARD: by deformably registering the plan CT to each CBCT, and then using the resulting deformation field to morph the plan contours to match the CBCT anatomy; (2) PRDAY: the contour set drawn by the physician on the previous day CBCT, is similarly deformed to match CBCT anatomy; and (3) STAPLE: the contours drawn by the physician, on each prior CBCT, are deformed to match the CBCT anatomy to produce multiple contour sets. These sets are combined using an algorithm, STAPLE, into one optimal set (e.g., day 3 CBCT combines contours produced using the plan plus day 1 and 2 CBCTs). STAPLE computes a probabilistic estimate of the true contour from this collection of contours based on maximizing the sensitivity and specificity of each organ's contour. ABAS (CMS Inc.) was used for this purpose. It utilizes an intra-patient deformable registration algorithm with a normalized sum of squared differences block matching phase followed by dense local correlation coefficient based registration, implemented on a Graphics Processor Unit (GPU). Compared to STANDARD, and PRDAY, STAPLE improved the average Dice's Coefficient (DC) between the original physician drawn CBCT contours with DC of; bladder: 81 ± 13%, 91 ± 6%, and 92 ± 6%; prostate: 75 ± 8%, 82 ± 5%, 84 ± 5%; and rectum: 79 ± 6%, 81 ± 6%, 85 ± 4%, respectively. The STAPLE results are within intra-observer consistency, determined here by the physician blindly recontouring a subset of CBCTs, e.g., prostate 84 ± 5%. STAPLE gave the best DC comparison for 79% of all CBCT organ contours. Using one scan takes ∼19 s, fives scans ∼110 s, on a GTX 460 GPU. Combining the plan and all prior days via the STAPLE algorithm to produce the current day contours is superior to the current standard of deforming only the plan contours to the daily CBCT, while maintaining a feasible treatment timescale. Using just the previous day treatment contours provides some improvement. STAPLE also improves the precision, with a substantial decrease in SD, a key for adaptive therapy. There is an immediate gain in accuracy by combining the initial plan with the first day's CBCT (from a second simulation), increasing with more image sets but only marginal improvement beyond the inclusion of 5 CBCTs.

Approche hybride combinant champs de Markov et modèle statistique de forme pour l’extraction des contours de la prostate en IRM

Recent progress in magnetic resonance imaging (MRI) has enabled new prostate cancer diagnosis techniques. The newest challenges in this field are to enhance image-based tumours detection. In such a context, the extraction of prostate's contours is a crucial step in the interpretation of MR images, and is usually carried out by an expert radiologist. This is though a tedious time consuming task, especially in 3D images (like CT and MRI). In addition, manual delineation is not reproducible because of differences between observers. In this paper, we introduce a novel method for automatic segmentation of prostate MRI that could help physicians in extracting 3D outlines of the gland. First a deformable shape model is used to obtain a first segmentation. The latter is refined using intensity information and Markov Random Fields modelling of regions. We use the Iterative Conditional Mode for optimising voxels’ labelling according to a Maximum A Posteriori criterion. Results from evaluation on patients’ data show that the method is satisfyingly accurate, fast and robust which makes it suitable for use in a clinical context. A multicentric validation and transfer to the industry would bring the contributions of this method to clinical routine and help improving diagnosis of prostate cancer.

Definition of medical event is to be based on the total source strength for evaluation of permanent prostate brachytherapy: A report from the American Society for Radiation Oncology

PurposeThe Nuclear Regulatory Commission deems it to be a medical event (ME) if the total dose delivered differs from the prescribed dose by 20% or more. A dose-based definition of ME is not appropriate for permanent prostate brachytherapy as it generates too many spurious MEs and thereby creates unnecessary apprehension in patients, and ties up regulatory bodies and the licensees in unnecessary and burdensome investigations. A more suitable definition of ME is required for permanent prostate brachytherapy. Methods and MaterialsThe American Society for Radiation Oncology (ASTRO) formed a working group of experienced clinicians to review the literature, assess the validity of current regulations, and make specific recommendations about the definition of an ME in permanent prostate brachytherapy. ResultsThe working group found that the current definition of ME in §35.3045 as “the total dose delivered differs from the prescribed dose by 20 percent or more” was not suitable for permanent prostate brachytherapy since the prostate volume (and hence the resultant calculated prostate dose) is dependent on the timing of the imaging, the imaging modality used, the observer variability in prostate contouring, the planning margins used, inadequacies of brachytherapy treatment planning systems to calculate tissue doses, and seed migration within and outside the prostate. If a dose-based definition for permanent implants is applied strictly, many properly executed implants would be improperly classified as an ME leading to a detrimental effect on brachytherapy. The working group found that a source strength-based criterion, of >20% of source strength prescribed in the post-procedure written directive being implanted outside the planning target volume is more appropriate for defining ME in permanent prostate brachytherapy. ConclusionsASTRO recommends that the definition of ME for permanent prostate brachytherapy should not be dose based but should be based upon the source strength (air-kerma strength) administered.

SU-E-T-616: Shifting MLC to Follow the Prostate Movements while Concurrently Treating Pelvic Lymph Nodes

Purpose: To evaluate an MLC tracking method and compare it to conventional iso‐center shift methods using daily kilo‐voltage cone beam CT (KV‐CBCT) for concurrent prostate and lymph nodes treatment. Methods: A total of 124 kV‐CBCT images from six patients were studied, and three verification plans were created for each KV‐CBCT. Contours of prostate, bladder and rectum were manually delineated, while contours of lymph nodes were transferred from the planning CT after bone‐based rigid image registration. The daily prostate displacements were obtained using dual‐image registrations: alignment to the bone and alignment to the prostate. For each segment of the initial plan, an in‐house program automatically shifted MLC leaf pairs that were collimated to the prostate according to measured prostate displacements. The MLC shifted plans were calculated based on shifted MLCs that applied to corresponding KV‐CBCT. The conventional iso‐center shifted plans were created based on original MLC shapes with shifted iso‐centers according to prostate contour‐based (iso‐shift‐contour plans) or bone‐based registration (iso‐shift‐bone plans). The dose to 95% (D95) of prostate and lymph nodes, and dose to 5% (D5) of rectum and bladder, were compared for all verification plans. Results: Using D95 within 3% of the prescription dose as a criterion, 97%, 97.6%, and 95% of 124 fractions met the criterion for prostate in the MLC‐shift, iso‐shift‐contour, and iso‐shift‐bone plans, respectively. Accordingly, 96%, 91%, and 97.6 of 124 fractions met the criterion for lymph nodes. For bladder, D5 of the bladder < 102% of the planned dose was observed in 67%, 56%, and 70% of the fractions, and D5 of the rectum <105% of the planned dose was observed in 85%, 88%, and 86% of the fractions, respectively. Conclusions: MLC‐shift method is an effective strategy for concurrent treatment of two independent moving targets, especially for patients with large organ motion.

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‐J‐121: Assessment of Different Approaches of Treatment Plan Selection in a Multiple Plans IGRT Strategy

Purpose: To assess treatment plan selection in a multiple plans adaptive IGRT strategy in routine clinical workflow. Methods: Continuous Offline Replanning (COR) is an adaptive IGRT treatment strategy where patient treatment plan is selected from a pool of offline previously calculated plans based on the patient repositioning CBCT images. A treatment sequence with n=19 fractions was simulated and, with COR strategy, different selection approaches based on numerical analysis of DVH extracted values (COR_PTVV95, COR_CF) or on qualitative human selection (COR_H) were investigated. In numerical analysis, PTV coverage and multi‐objectives cost function were investigated. Each COR strategy was compared with the standard IGRT strategy using prostate center of mass translations (IGRT_0), and an optimal online replanning strategy (IGRT_Opt). In COR_H, operators were asked to select the most appropriate plan available from a screen display, priorizing geometrical fit of prostate, rectum and bladder contours on CBCT images. Results: Cumulative average values of PTV V95 range from 92.9 to 95.5% for COR_H; 93.2 to 97.7 % for COR_PTVV95; 92.7 to 93.2% for IGRT_0 and 99.9 to 100% for IGRT_Opt, which ranks IGRT_Opt as the best treatment option and the COR_PTVV95 as second best option. Values of the multi‐objectives cost function were consistent with PTV coverage and were in the range 0.71 to 2.62; 0.36 to 2.40; 1.74 to 2.69 and 0.014 to 0.021 respectively for COR_H, COR_CF, IGRT_0 and IGRT_Opt. Operators were asked to choose preferentially among the first 10 contour sets to see if 10 plans were sufficient to represent the diversity of anatomical configurations routinely seen. They succeeded for 17/19 fractions. Conclusions: For multiple plans adaptive prostate IGRT treatment, numerical selection process of daily plan based on target dose coverage or cost function values appear to be superior to human visual selection. Further development aim to minimize time selection procedure.Part of this work was financially supported by Varian Medical Sytems France.

SU-E-J-39: Daily Prostate Rotation Should Be Compensated in Translational Correction and Not to Be Ignored

Purpose: To quantify magnitude of the prostate rotation during daily treatment and evaluate strategies to effectively compensate the rotation if a six‐degree table is not available. Methods: Six patients, with forty‐four kilo‐voltage cone beam CT (KV‐CBCT), were selected for this study. On these KV‐CBCTs, the prostate was contoured by a physician. These patients had three transponders implanted for non‐imaging daily prostate localization using the Calypso system. To determine the prostate movement, we registered each KV‐CBCT with the planning CT in four different ways: (a) manually align to the bones with translation only (Bone_T); (b) automatically align to the markers with both translation and rotation (Marker_TR); (c) manually align to the markers with translation only (Marker_T); (d) manually align to the prostate contour with translation only (Contour_T). The prostate rotations from the second method were recorded. Using the contour registration as a benchmark, two center of mass distances (CMD) between the Contour_T and Marker_TR/Marker_T registrations were calculated. Only translations were used for the CMD calculation. Results: Detected from the Marker_TR method, the mean and standard deviation of the prostate rotations about the transverse, anterior‐posterior, and longitudinal axes were 3.21±6.26°, ‐1.39±2.91°, and ‐0.94±2.75°, respectively. The mean CMD between the Contour_T and the Marker_TR/Marker_T shifted prostate were 6.6 and 4.0 mm, respectively. When the rotation is greater than 10°, the differences in CMD were greater than 5 mm in 7 out of 8 (87.5%) of such fractions. When the rotation is greater than 6°, the differences were greater than 4 mm in 13 out of 19 (68.4%) cases. Conclusions: Without a six‐degree table, prostate rotations were often not corrected. Compensating for the rotations with translational shifts is effective, and superior to ignoring rotations. Simply ignoring large rotations may lead to increased planning margins or inaccurate prostate localization.

Prostate Contouring Variation: Can It Be Fixed?

To assess whether an education program on CT and MRI prostate anatomy would reduce inter- and intraobserver prostate contouring variation among experienced radiation oncologists. Three patient CT and MRI datasets were selected. Five radiation oncologists contoured the prostate for each patient on CT first, then MRI, and again between 2 and 4 weeks later. Three education sessions were then conducted. The same contouring process was then repeated with the same datasets and oncologists. The observer variation was assessed according to changes in the ratio of the encompassing volume to intersecting volume (volume ratio [VR]), across sets of target volumes. For interobserver variation, there was a 15% reduction in mean VR with CT, from 2.74 to 2.33, and a 40% reduction in mean VR with MRI, from 2.38 to 1.41 after education. A similar trend was found for intraobserver variation, with a mean VR reduction for CT and MRI of 9% (from 1.51 to 1.38) and 16% (from 1.37 to 1.15), respectively. A well-structured education program has reduced both inter- and intraobserver prostate contouring variations. The impact was greater on MRI than on CT. With the ongoing incorporation of new technologies into routine practice, education programs for target contouring should be incorporated as part of the continuing medical education of radiation oncologists.

Regulatory evaluation of prostate volume implants: Pitfalls of a retrospective assessment

Purpose Evaluate for regulatory compliance the prostate implants from the Philadelphia Veterans Medical Center applying both an activity-based and volume-corrected D 90 (the maximum dose delivered to 90% of the prostate volume) metrics. Methods and Materials Dosimetry from 107 prostate implants performed at the Philadelphia Veterans Medical Center used immediate postprocedural CT image sets. D 90 values were adjusted for volume differences from planning volumes. Medical events (MEs) determined from the volume-corrected data were compared with an activity-based metric. Results Examination of images using original and third-party reviewed prostate contours revealed 56 and 62 cases with D 90 values <80% of the prescription dose, respectively. Because postprocedural prostate volumes were on average 55.7% larger than the planned volume, clinical nomogram-based doses using the implanted activity and actual volumes found 34–47 implants failing to achieve doses greater than 80% of the prescription dose. Volume correction identified 20 MEs, 9 cases with D 90 values within 4% of the ME threshold and 11 significantly inferior cases with median D 90 values <52% of the prescribed dose. Eleven implants also had 20% or more seeds beyond the treatment site according to an activity metric recommended by the VHA Blue Ribbon Panel. Ten of these 11 cases were also identified by volume-corrected D 90 metric. The remaining 96 cases, however, had 95% (±6%) of seeds placed within the treatment site. Conclusions Of the cases reported to the United States Nuclear Regulatory Commission (NRC) on the basis of Day-1 D 90 values, many appear to have been acceptable implants relative to standard-of-practice clinical criteria. The activity-based dose metric, endorsed by the NRC Advisory Committee on the Medical Uses of Isotopes in 2005 and recommended by the VHA Blue Ribbon Panel for Prostate Brachytherapy yields a more robust determination of ME for this population of implants.

Expertise Does Not Mitigate Significant Intra- and Inter-Observer Variations in Contouring of Prostate and Associated Dosimetric Outcomes

Phenomenon of intra- and inter-observer variations (IOV) in delineation of prostate is well known and poorly understood. 7,980 articles recovered through Google search engine give controversial assessment of IOV magnitude and clinical importance, with some authors reporting major impact on the dose distribution. Proposed remedies included use of formalized contouring protocols and atlases, addition of MRI, assistance of diagnostic radiologists, and computer contouring algorithm; all- empiric, none - focused on root cause of IOV.

Evaluation of Dose Volume Histograms for Rectal and Prostate Cancer Patients Undergoing CBCT Based IGRT

Advances in radiotherapy treatment planning led to improved local tumor control and better healthy tissues sparing. Without image guidance at the time of therapy (IGRT), changes in position, shape and intra-fractional motions may prevent desired dose from being precisely delivered to the planning target. The rapid fall-off in dose outside the target in highly conformal radiotherapy techniques impose demand for accurate target localization at the time of the treatment to avoid underdosing of disease. Daily kV cone-beam computed tomography (CBCT) acquisition can provide accurate 3D volumetric patient's anatomy prior to every treatment fraction and it is suitable for adaptive corrections of treatment positioning variations. This study retrospectively assesses dose volume histograms (DVHs) with and without IGRT employing CBCT. We also report on additional radiation skin dose to patients during CBCT. Pre-treatment 3D imaging of 20 patients in treatment position was acquired using Pelvis CBCT protocol on on-board imaging (OBI) system (v. 1.4 Varian Medical Systems, Palo Alto) on a daily basis and twice a week for prostate and rectum cancer patients respectively. The obtained reconstructed CBCT images were co-registered with planning CT data using bony landmarks for rectal patients and prostate soft tissue visibility and prostate contours for prostate cancer patients. Obtained shifts were used in the Eclipse Treatment planning system to mimic the non-IGRT guided treatment. We calculated the dose distribution for original and simulated plan and obtained DVHs for targets and organs at risk were compared. Prior to CBCT scan, film pieces were taped on patient skin. Films were scanned before and after irradiation with the Epson Expression 10000XL document scanner in reflective mode. Response of the reference film dosimetry system was calibrated in terms of air kerma in air. Our preliminary results show small difference in PTV coverage for rectum patients with better sparing of the bladder in high dose region. For prostate patients, DVH of simulated plan shows 5% worse PTV coverage when compared to the original plan while the bladder sparing increases significantly. Measured anterior dose during CBCT amounts to 1.60 cGy in larger and up to 3.17 cGy in smaller patients, while the lateral dose ranges from 1.63 cGy in larger and up to 2.68 cGy in smaller patients. Our preliminary data indicate better sparing of bladder in high dose regions, and possible margin reduction for PTV definition if daily CBCT was used. This could lead to better tumor control and decrease NTCP, but the benefits must be weighted against the dose delivered to patients during CBCT.

Quadrant dosimetry as a predictor of biochemical relapse in 125I prostate brachytherapy

Purpose To evaluate region-specific dose metrics as predictors of biochemical relapse in prostate brachytherapy patients. Methods and Materials In a cohort of 1006 low-risk and “low-tier” intermediate-risk prostate brachytherapy patients treated to a planned dose of 144 Gy mPD (minimal peripheral dose), 30 of 32 with biochemical relapse (nadir + 2 ng/mL definition) had postimplant CT scans available for retrospective analysis. These were matched to nonrelapsing controls from the same era. Three copies of each CT were created and, after randomization and deletion of identifiers and original contours, were re-contoured by three radiation oncologists. Prostate contours were then divided into quadrants: Anterior-Superior (ASQ), Posterior-Superior (PSQ), Anterior-Inferior (AIQ), and Posterior-Inferior (PIQ), and dosimetric parameters calculated. Results were analyzed using mixed-effects linear regression and multivariate logistic regression. Results Whole prostate volume of the prostate receiving at least 100% of the prescribed dose ( V 100) and minimum dose, as a percentage of the prescribed dose, received by 90% of the prostate volume ( D 90) were similar for relapses and controls ( p = 0.40 and 0.48, respectively). Among the quadrants, the largest differences between relapses and controls were seen for the AIQ. Mean AIQ V 100s were 91.2% (relapses) and 95.5% (controls) ( p = 0.096), and D 90s were 112.8% (relapses) and 119.3% (controls) ( p = 0.145). Overall, the lowest doses were in the ASQ, but were not very different for relapses and controls ( V 100 = 76.5% and 78.5%, respectively) ( p = 0.54). On multivariate analysis along with various clinical parameters, AIQ metrics approached significance at the p ≤ 0.05 level in models that also included initial prostate-specific antigen, androgen suppression, and risk group. Conclusions Although whole prostate dose metrics did not predict for biochemical relapse in our data set, dose to the AIQ was predictive in multivariate analysis.

Poster - Thur Eve - 63: Planning and Phantom Study of Oblique Needle Based Low Dose Rate Prostate Brachytherapy (LDRPB) Using 3D Trans-Rectal Ultrasound (TRUS) and Robotic Assistance

The purpose of this study was to create a phantom to validate the Low Dose Rate Prostate Brachytherapy (LDRPB) plans using oblique (angled) needles delivered using robotic guidance in overcoming pubic arch interference for large prostates. Five patient 3D Trans‐rectal Ultra Sound (TRUS)images were selected. The images were required to show a maximum pubic arch interference of <= 1 cm and prostate volume of < 50 cc. The prostate contours were artificially enlarged to 60 cc while keeping the pubic arches constant. The following three different types of plans were manually created for each 60 cc prostate: parallel needle template based, parallel needle non template based, and oblique needle non template based. One sided t‐tests between the three different types of plans were computed for DVH indices recommended by ESTRO for LDRPB. At the 1% significance level, a better DVH index for each organ (PTV, Urethra, and Rectum) was observed for oblique plans compared to the other two techniques. The feasibility of robot and 3D TRUS guided delivery of oblique plans are being tested in a phantom study. We created a phantom based on one of the 60 cc prostate contours with a plastic plate simulating pubic arch interference. Robot and 3D TRUS guided delivery of an oblique plan will be delivered. Results of CTimages of the delivered seeds will be compared to that of the planned position, validating the feasibility of delivering oblique plans.

Interfraction Prostate Rotation Determined from In-Room Computerized Tomography Images

Fiducial markers (FMs) are commonly used as a correction technique for interfraction translations of the prostate. The aim of this investigation was to determine the magnitude of prostate rotations using 2 methods: FM coordinates and the anatomical border of the prostate and rectum. Daily computed tomography (CT) scans ( n = 346) of 10 prostate cancer patients with 3 implanted FMs were acquired using the CT on rails. FM coordinates were used to determine rotation in the sagittal, transverse, and coronal planes, and CT contours of the prostate and rectum were used to determine rotation along the sagittal plane. An adaptive technique based on a subset of images ( n = 6; planning and first 5 treatment CTs) to reduce systematic rotation errors in the sagittal plane was tested. The standard deviation (SD) of systematic rotation from FM coordinates was 7.6°, 7.7°, and 5.0° in the sagittal, transverse and coronal planes. The corresponding SD of random error was 10.2°, 15.8°, and 6.5°. Errors in the sagittal plane, determined from prostate and rectal contours, were 10.1° (systematic) and 7.7° (random). These results did not correlate with rotation computed from FM coordinates (r = −0.017; p = 0.753, n = 337). The systematic error could be reduced by 43% to 5.6° when the mean prostate position was estimated from 6 CT scans. Prostate rotation is a significant source of error that appears to be more accurately determined using the anatomical border of the prostate and rectum rather than FMs, thus highlighting the utility of CT image guidance.

Dosimetric Impact of Interfraction Catheter Movement in High-Dose Rate Prostate Brachytherapy

To evaluate the impact of interfraction catheter movement on dosimetry in prostate high-dose-rate (HDR) brachytherapy. Fifteen patients were treated with fractionated HDR brachytherapy. Implants were performed on day 1 under transrectal ultrasound guidance. A computed tomography (CT) scan was performed. Inverse planning simulated annealing was used for treatment planning. The first fraction was delivered on day 1. A cone beam CT (CBCT) was performed on day 2 before the second fraction was given. A fusion of the CBCT and CT was performed using intraprostatic gold markers as landmarks. Initial prostate and urethra contours were transferred to the CBCT images. Bladder and rectum contours were drawn, and catheters were digitized on the CBCT. The planned treatment was applied to the CBCT dataset, and dosimetry was analyzed and compared to the initial dose distribution. This process was repeated after a reoptimization was performed, using the same constraints used on day 1. Mean interfraction catheter displacement was 5.1 mm. When we used the initial plan on day 2, the mean prostate V100 (volume receiving 100 Gy or more) decreased from 93.8% to 76.2% (p < 0.01). Rectal V75 went from 0.75 cm(3) to 1.49 cm(3) (p < 0.01). A reoptimization resulted in a mean prostate V100 of 88.1%, closer to the initial plan (p = 0.05). Mean rectal V75 was also improved with a value of 0.59 cm(3). There was no significant change in bladder and urethra dose on day 2. A mean interfraction catheter displacement of 5.1 mm results in a significant decrease in prostate V100 and an increase in rectum dose. A reoptimization before the second treatment improves dose distribution.