Semiautomatic segmentation for prostate brachytherapy: Dosimetric evaluation

PurposeTo demonstrate that manual prostate segmentation in transrectal ultrasound images can be replaced with semiautomatic segmentation.Methods and MaterialsSemiautomatic segmentation using a tapered ellipsoid model was applied to transrectal ultrasound images. Region-based volumetric evaluation was performed between original and physician-reviewed semiautomatic contours. For dosimetric assessment, treatment plans generated on semiautomatic contours were overlaid on physician-reviewed semiautomatic contours and dose parameters were computed. To establish a threshold for the acceptable amount of dosimetric degradation below which the adoption of semiautomatic planning is unacceptable, the range of variability in dosimetric quality attributed to manual variability was obtained and compared with that of semiautomatic contours.ResultsAn average volume error (1—Dice similarity coefficient) of less than 7% between semiautomatic and manual volumes (140 cases) was obtained. The difference between the mean V₁₀₀ of plans created for semiautomatic contours then overlaid on physician-reviewed semiautomatic contours and the original V₁₀₀ values, that is, before overlaying on the physician-reviewed contours (41 cases) was lower than 5%. An average total duration of 2–4 min, which includes algorithm initialization, 11.67 ± 3.57 s algorithm time, and contour modification is required per case. This algorithm is being used at the British Columbia Cancer Agency and to this date has been applied for the treatment of more than 600 patients.ConclusionsIn terms of volumetric and dosimetric accuracy, the proposed algorithm is a suitable replacement for manual segmentation in the context of our planning technique. The benefits are shorter segmentation times; greater consistency; less reliance on user experience; and smooth, symmetric contours.     

Dosimetric effect of interfractional needle displacement in prostate high-dose-rate brachytherapy

PurposeTo quantify the dosimetric deviations that would arise from delivering subsequent prostate high-dose-rate fractions with only needle readjustment and no replanning after the first fraction.Methods and MaterialsPatients were treated with either two implant sessions (two 9.5-Gy fractions per session) separated by 2–4 weeks or with one implant session and external beam radiotherapy. After needle placement, needle positions were adjusted under CT guidance, after which dosimetric planning was performed before each fraction. To evaluate the consequence of not replanning before the second fraction, we analyzed the dosimetric parameters of 45 consecutive implants (26 patients). Needles with optimized dwell positions from the first fraction were transferred to the needle positions in the second fraction. Needle displacement between fractions was assessed as well as changes in plan metrics.ResultsAfter adjustment, the mean interfractional needle displacement was 3.5 mm. If replanned, the probability of planning target volume D_90% ≥95% is 100%, prostate V_100% ≥95% is 87%, and urethra V₁₁₅% ≤10% is 78%. If treated without replanning, the probability of planning target volume D_90% ≥95% is 82%, prostate V_100% ≥95% is 53%, and urethra V_115% ≤10% is 69%. Even for implants with minimal needle displacement (<3 mm) and minimal prostate volume change (<3 cc), the dosimetric consequence of not replanning the second fraction would result in 46% of cases with a prostate V_100% <95%.ConclusionThe dosimetric consequences of not replanning the second fraction for prostate high-dose-rate implants results in significantly inferior plan metrics.     

Dosimetric characteristics of 192Ir sources used in interstitial brachytherapy.

Dosimetric quantities of 192Ir seed (5 mm length) and wire (10 mm length) brachytherapy sources have been determined. The quantities were measured based on the protocol introduced by the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) Task Group 43. Quantities such as dose rate constant, (lambda), radial dose function, g(r), and anisotropy function, F(r, theta) were experimentally determined and the geometry function, G(r, theta), was calculated. TLD measurements were made in a polymethyl methacrylate (PMMA) phantom of dimensions 25 cm x 20 cm x 5 cm by means of LiF:Mg,Ti (TLD-100) dosimeters for distances of 1-10 cm for g(r), and the same distances at angles of 0-180 degrees for F(r, theta). Dose rate constant for 192Ir seed and wire were found to be 1.196+/-5 and 1.082+/-5% cGy h(-1) U(-1), respectively (1 U = unit of air Kerma strength = 1 microGy m2 h(-1) = 1 cGy cm2 h(-1)). The obtained results for g(r), G(r, theta) and F(r, theta) are also presented and discussed.     

Dosimetric characterization of 142Pr glass seeds for brachytherapy.

A glass bead consisted of the beta(-)-emitting (142)Pr is proposed for brachytherapy treatment of prostate cancer. Appropriate radionuclide and seed dimensions were selected and sample seeds were manufactured. For the quantitative dosimetric parameters, two-dimensional dose distributions were calculated using the MCNP5 Monte Carlo code and measured using radiochromic film. The computational results compared well with measurements. Dose at 0.6 cm from the seed center can exceed 130 Gy. The reference dose rate, radial dose function and the anisotropy function were derived.     

Dosimetric consequences of seed placement accuracy in permanent breast seed implant brachytherapy

PurposeThis study quantifies the dosimetric impact of implant accuracy and derives a quantitative relationship relating implant accuracy to target dosimetry.Methods and MaterialsA framework was developed to simulate multiple implants for error combinations. Spherical clinical target volumes (CTVs) were modeled with volumes 1.4 cm³, 9.2 cm³, and 20.6 cm³, representing the range seen clinically. Each CTV was expanded 10 mm isotropically to create a planning target volume (PTV).. Random and systematic seed placement errors were simulated by shifting needles from their planned positions. Implant errors were simulated over the range of clinically practical errors in permanent breast seed implant. The relative effect on target coverage was evaluated. Regression analysis was performed to derive relationships between CTV dosimetry and the magnitude of implant accuracy. The validity of the clinically used 10 mm PTV margin for each of the CTVs was assessed.ResultsIntroducing practical implant errors resulted in CTV V_90% median (10th and 90th percentile) of 97.7% (85.9% and 100%), 96.2% (86.8% and 99.7%), and 100% (77.8% and 100%) for the typical, large, and small CTV, respectively. All CTVs show similar trends in target coverage. Polynomials were derived relating seed placement accuracy to median (R² = 0.82) and 10th percentile (R² = 0.78) CTV V_90%._.ConclusionsThis work quantitatively describes the relationship between implant accuracy and CTV coverage. Based on simulations, the 10 mm PTV margin is adequate to maintain target coverage. These equations can be used with institutional seed placement accuracy to estimate coverage.     

Dosimetric and clinical outcome in image-based high-dose-rate interstitial brachytherapy for anal cancer

PurposeTo evaluate dosimetric and clinical outcome in patients of anal cancer treated with image-based interstitial high-dose-rate brachytherapy following chemoradiation.Methods and MaterialsSixteen patients with anal cancer were treated with chemoradiation followed by brachytherapy boost with image-based high-dose-rate interstitial brachytherapy from January 2007 to June 2011. Two brachytherapy dose schedules were used: 21 Gy in seven fractions and 18 Gy in six fractions depending on response to chemoradiation. CT scan was done after placement of needles for confirmation of placement and treatment planning. Target volume was contoured on CT scans. Volumetric quality indices and dose parameters were calculated.ResultsThe mean clinical target volume was 17.7 ± 4.98 cm³, and the median overall tumor size was 4.2 cm (3.4–5 cm). The mean values of coverage index, dose homogeneity index, overdose volume index, dose non-uniformity ratio, and conformal index were 0.94, 0.83, 0.21, 0.37, and 0.88, respectively. With a median followup of 41 months (range, 20–67.2 months), preservation of the anal sphincter was achieved in 14 patients. The 1- and 2-year local control rates were 93.8% and 87.5%, respectively. Treatment was well tolerated and none of the patients developed Grade 3 or higher late toxicity.ConclusionsThe combination of external beam radiotherapy with interstitial brachytherapy increases the dose to the tumor volume and limits the volume of irradiated normal tissue, thereby decreasing late toxicity. The use of image-based treatment planning provides better dose conformality with reduced toxicity and helps to prevent a geographic miss.     

Dosimetric impact of inter-observer catheter reconstruction variability in ultrasound-based high-dose-rate prostate brachytherapy

PurposeTo investigate the dosimetric impact of interobserver catheter reconstruction variability in transrectal ultrasound–guided prostate high-dose-rate (HDR) brachytherapy.Methods and MaterialsTwenty consecutive patients with intermediate- or high-risk prostate cancer were treated with a single, 15-Gy HDR brachytherapy boost as part of this study. The treated plan was used as the study reference plan (P_R). Three expert treatment planners (observers) manually reconstructed the catheter paths on the static three-dimensional transrectal ultrasound images, and new plans were generated from the updated positions (P_OBS); subsequently, the dwell time and positions from the P_OBS plans were superimposed on the P_R catheter paths to evaluate the dosimetric effect of the interobserver variations (P_EVAL). Plans from each group were stratified by observer and by number of catheters (12 or 16) and then compared using a one-way Kruskal–Wallis H test with post hoc Mann–Whitney U tests reserved for significant variations (α = 0.05).ResultsGreater than 98.9% of catheter reconstruction variations were <3 mm. When stratified by observer, there was a significant decrease (p << 0.05) in planning target volume (PTV) V_100% and increases in the urethral D_max between the P_OBS plans propagated to the P_R catheter paths and dosimetry evaluated and P_R plans only. Stratification of plans by catheter number showed nonclinically significant decreases in PTV V_100%, and D_90% and increases in urethral D_max for the 12-catheter plans.ConclusionsLimiting interobserver variability, and its effects on prostate HDR brachytherapy plan quality, is critical to achieving good dosimetric outcomes; small variations in catheter reconstruction may translate to inadequate PTV coverage, excessive urethral dose, or both.     

Dosimetric evaluation of MRI-to-ultrasound automated image registration algorithms for prostate brachytherapy

PurposeIdentifying dominant intraprostatic lesions (DILs) on transrectal ultrasound (TRUS) images during prostate high-dose-rate brachytherapy treatment planning remains a significant challenge. Multiparametric MRI (mpMRI) is the tool of choice for DIL identification; however, the geometry of the prostate on mpMRI and on the TRUS may differ significantly, requiring image registration. This study assesses the dosimetric impact attributed to differences in DIL contours generated using commonly available MRI to TRUS automated registration: rigid, semi-rigid, and deformable image registration, respectively.Methods and MaterialsTen patients, each with mpMRI and TRUS data sets, were included in this study. Five radiation oncologists with expertise in TRUS-based high-dose-rate brachytherapy were asked cognitively to transfer the DIL from the mpMRI images of each patient to the TRUS image. The contours were analyzed for concordance using simultaneous truth and performance level estimation (STAPLE) algorithm. The impact of DIL contour differences due to registration variability was evaluated by comparing the STAPLE-DIL dosimetry from the reference (STAPLE) plan with that from the evaluation plans (manual and automated registration) for each patient. The dosimetric impact of the automatic registration approach was also validated using a margin expansion that normalizes the volume of the autoregistered DILs to the volumes of the STAPLE-DILs. Dose metrics including D₉₀, D_mean, V₁₅₀, and V₂₀₀ to the prostate and DIL were reported. For urethra and rectum, D₁₀ and V₈₀ were reported.ResultsSignificant differences in DIL coverage between reference and evaluation plans were found regardless of the algorithm methodology. No statistical difference was reported in STAPLE-DIL dosimetry when manual registration was used. A margin of 1.5 ± 0.8 mm, 1.1 ± 0.8 mm, and 2.5 ± 1.6 mm was required to be added for rigid, semi-rigid, and deformable registration, respectively, to mitigate the difference in STAPLE-DIL coverage between the evaluation and reference plans.ConclusionThe dosimetric impact of integrating an MRI-delineated DIL into a TRUS-based brachytherapy workflow has been validated in this study. The results show that rigid, semi-rigid, and deformable registration algorithms lead to a significant undercoverage of the DIL D₉₀ and D_mean. A margin of at least 1.5 ± 0.8 mm, 1.1 ± 0.8 mm, and 2.5 ± 1.6 mm is required to be added to the rigid, semi-rigid, and deformable DIL registration to be suitable for DIL-boosting during prostate brachytherapy.     

Dosimetric influence of seed spacers and end-weld thickness for permanent prostate brachytherapy

PurposeThe aim of this study was to analyze the dosimetric influence of conventional spacers and a cobalt chloride complex contrast (C4) agent, a novel marker for MRI that can also serve as a seed spacer, adjacent to ¹⁰³Pd, ¹²⁵I, and ¹³¹Cs sources for permanent prostate brachytherapy.Methods and MaterialsMonte Carlo methods for radiation transport were used to estimate the dosimetric influence of brachytherapy end-weld thicknesses and spacers near the three sources. Single-source assessments and volumetric conditions simulating prior patient treatments were computed. Volume–dose distributions were imported to a treatment planning system for dose–volume histogram analyses.ResultsSingle-source assessment revealed that brachytherapy spacers primarily attenuated the dose distribution along the source long axis. The magnitude of the attenuation at 1 cm on the long axis ranged from −10% to −5% for conventional spacers and approximately −2% for C4 spacers, with the largest attenuation for ¹⁰³Pd. Spacer perturbation of dose distributions was less than manufacturing tolerances for brachytherapy sources as gleaned by an analysis of end-weld thicknesses. Volumetric Monte Carlo assessment demonstrated that TG-43 techniques overestimated calculated doses by approximately 2%. Specific dose–volume histogram metrics for prostate implants were not perturbed by inclusion of conventional or C4 spacers in clinical models.ConclusionsDosimetric perturbations of single-seed dose distributions by brachytherapy spacers exceeded 10% along the source long axes adjacent to the spacers. However, no dosimetric impact on volumetric parameters was noted for brachytherapy spacers adjacent to ¹⁰³Pd, ¹²⁵I, or ¹³¹Cs sources in the context of permanent prostate brachytherapy implants.     

乌德勒支施源器用于局部晚期宫颈癌三维近距离放疗的剂量学研究

目的 探讨在使用乌德勒支施源器治疗局部晚期宫颈癌时,不同的管道组合设计与靶区、危及器官（OAR）的剂量关系。方法 调取2017—2018年于河北省沧州中西医结合医院放疗科接受乌德勒支施源器进行近距离治疗的17例局部晚期宫颈癌患者数据,原计划为腔内联合组织间插植近距离放疗（IC/IS）,在原勾画轮廓与照射剂量不变的情况下,去除插植针完成腔内近距离放疗（IC）计划设计,对IC/IS计划和IC计划的剂量参数进行对比分析,并评价加入IS插植针对剂量的影响。结果 IC/IS和IC计划的高危临床靶区（HR-CTV）总的D₉₀分别（88.68±1.84）和（85.54±0.54）Gy,差异具有统计学意义（t=6.200,P<0.05）。此外,在靶区的CI指数上,IC/IS计划的明显高于IC计划（0.58±0.08,0.43±0.05）,差异具有统计学意义（t=1.010,P<0.05）。在两种计划中危及器官（OAR）的受照剂量差异无统计学意义（P>0.05）。结论 对于局部晚期宫颈癌的近距离治疗,使用乌德勒支施源器可满足其处方剂量覆盖,而加入IS针,提高了计划的优化空间,在满足OAR受照剂量限值的基础上,显著提高HR-CTV的受照剂量,且提升了剂量分布均匀性。     

Dosimetric study of CO-60 source step size in uterine cervix intracavitary HDR brachytherapy

Purpose

The present work reports effects of source step sizes on dose distribution in patients treated with cobalt-60 (Co-60) high-dose-rate afterloading brachytherapy in carcinoma cervix (Ca-cx).

江苏省核查放射治疗参考与非参考条件下剂量学参数方法验证

目的 研究用热释光剂量计（TLD）方法核查放射治疗参考条件和非参考条件下剂量学参数的可靠性验证。方法 在参考条件和非参考条件下,用建立的TLD方法,核查5家医院的10条6 MV光子线束剂量随深度、源皮距离、照射野大小和45°楔形板等变化,5条9 MeV电子线束轴向最大剂量点处等剂量学参数,TLD估算结果与剂量仪测量结果进行对比。结果 6 MV 光子线束轴上非参考条件和离轴非参考条件下的TLD监测结果与指型电离室监测结果的相对偏差范围分别为-1.7%~5.4%、-6.3%~-0.6%,符合IAEA要求的≤±7%;电子射线束TLD估算结果与平行板电离室测量结果相对偏差范围为-2.3%~3.7%,符合IAEA要求的≤±5%。结论 用TLD核查参考条件和非参考条件下放射治疗剂量学参数方法可靠,简单易行。     

Dosimetric effect of external beam planning preceding combined high-dose-rate brachytherapy of the prostate

Purpose

The sequencing of external beam radiotherapy (EBRT) and a high-dose-rate brachytherapy (HDRB) boost is often interchangeable in clinical practice. When given before EBRT, HDRB could induce volume alterations in the prostate, which may have significant implications for EBRT dosimetry. We aimed to assess the influence of HDRB on prostate volume and, hence, prostate dosing via subsequent EBRT.

Methods and Materials

Fifteen men had both pre- and post-HDRB CT performed followed by EBRT. After deidentification, the clinical target volume (CTV) was defined on each CT by a single-blinded observer. Volumes were compared for the pre- and post-HDRB scans in each patient. Radiotherapy planning was performed using the prebrachytherapy volumes aiming for the planning target volume (PTV) to be covered by 43.7 Gy. After soft-tissue coregistration, this plan was also applied to the postbrachytherapy volumes.

Results

Median volume increase was 35.4% for the CTV after HDRB. No patient experienced a decrease in CTV volume (range, 0-79% volume increase; p-value < 0.001). Median volume increase was 26.1% for the PTVs, with no volume decrease observed (range, 8-56%; p < 0.001). PTV proportion achieving dose target (V43.7 Gy) decreased by median of 7% (range, 0-21.5%; p = 0.004). The minimum dose to the PTV (D₁₀₀%) decreased by a median of 6 Gy (range, 0.5-16 Gy; p < 0.001).

Conclusions

Insertion of HDRB catheters exerts substantial acute volumetric distortion on the prostate. EBRT planning performed on the basis of pre-HDRB imaging only inherently risks underdosing tumor. Planning adjustments based on repeat CT, or dedicated post-HDBT planning, is warranted for men managed with HDRB before EBRT.     

Dosimetric characterization of a brachytherapy applicator using MCNP5 modelisation and in-phantom measurements

A gynaecological applicator consisting of a metallic intra-uterine tube with a plastic vaginal applicator and an HDR Ir-192 source have been simulated with MCNP5 (Monte Carlo code). A solid phantom has been designed to perform measurements around the applicator with radiochromic films. The isodose curves obtained are compared with curves calculated with the F4MESH tally of MCNP5 with a good agreement. A pinpoint ionization chamber has been used to evaluate dose at some reference points.     

Dosimetric comparison of optimization methods for multichannel intracavitary brachytherapy for superficial vaginal tumors

PurposeMultichannel vaginal applicators allow treatment of a more conformal volume compared with a single, central vaginal channel. There are several optimization methods available for use with multichannel applicators, but no previous comparison of these has been performed in the treatment of superficial vaginal tumors. Accordingly, a feasibility study was completed to compare inverse planning by simulated annealing (IPSA), dose point optimization (DPO), and graphical optimization for high-dose-rate brachytherapy using a multichannel, intracavitary vaginal cylinder.Methods and MaterialsThis comparative study used CT data sets from five patients with superficial vaginal recurrences of endometrial cancer treated with multichannel intracavitary high-dose-rate brachytherapy. Treatment plans were generated using DPO, graphical optimization, surface optimization with IPSA (surf IPSA), and two plans using volume optimization with IPSA. The plans were evaluated for target coverage, conformal index, dose homogeneity index, and dose to organs at risk.ResultsBest target coverage was achieved by volume optimization with IPSA 2 and surf IPSA with mean V100 values of 93.89% and 91.87%, respectively. Doses for the most exposed 2-cm³ of the bladder (bladder D2cc) was within tolerance for all optimization methods. Rectal D2cc was above tolerance for one DPO plan. All volume optimization with IPSA plans resulted in higher vaginal mucosa doses for all patients. Greatest homogeneity within the target volume was seen with surf IPSA and DPO. Highest conformal indices were seen with surf IPSA and graphical optimization.ConclusionsOptimization with surf IPSA was user friendly for the generation of treatment plans and achieved good target coverage, conformity, and homogeneity with acceptable doses to organs at risk.     

3D打印个体化模具在子宫内膜癌术后三维后装治疗中的剂量学研究

目的 探讨3D打印个体化模具辅助子宫内膜癌术后三维后装治疗中的剂量学优势。方法 回顾性选取宁波大学附属第一医院21例早期子宫内膜癌术后患者的三维后装治疗计划,利用北京科霖众计划系统,在已实施的个体化模具组基础上,为每位患者设计虚拟单通道柱状施源器计划,所有计划均采用三维逆向模拟退火的算法。比较两组计划靶区90%、98%和100%体积接受的最低剂量(D₉₀、D₉₈、D₁₀₀)以及适形指数(CI)、均匀度(HI)、超剂量体积指数(OI);同时,对比两组危及器官膀胱、直肠、小肠和尿道的0.01、1、2以及5 cm³所接受的最大剂量(D_{0.01 cm³}、D_{1 cm³}、D_{2 cm³}以及D_{5 cm³})差异。结果 两组计划都能满足临床需求。在靶区所受剂量方面,两组计划靶区D₉₀、D₉₈、D₁₀₀没有明显差异,但是个体化模具组靶区剂量的CI、HI均优于单通道组靶区,而靶区OI指数小于单通道计划(t=-3.21、-5.99、6.25,P＜0.05)。在危及器官所受剂量方面,个体化模具组膀胱、直肠、尿道所受剂量D_{1 cm³}、D_{2 cm³}和D_{5 cm³}相比单通道组均有明显降低(t=3.18、3.21、3.77、7.97、8.92、10.92、2.54、3.46、4.28,P＜0.05);小肠由于距离靶区比较远,在两组计划中所受剂量差异不明显(P＞0.05)。结论 3D打印个体化模具在子宫内膜癌术后三维近距离治疗中靶区均匀度、适形指数更优,而且膀胱、直肠、尿道所受剂量更低,具有推广价值。     

Dosimetric outcomes in prostate brachytherapy: Is downsizing the prostate with androgen deprivation necessary?

ObjectivesA large prostate volume has historically been a relative contraindication to prostate brachytherapy (PB) because of concerns of toxicity and potential pubic arch interference. Common practice has been to downsize large prostates with androgen deprivation therapy (ADT) before proceeding with brachytherapy. The present study compares postimplant dosimetry in patients with prostate volumes >50 cc with those with prostate volumes ≤50 cc.MethodsA review of all patients who underwent PB at our institution from 2001 to 2006 was performed. Postimplant dosimetry was obtained approximately 4 weeks after brachytherapy.ResultsOne-hundred forty-five out of a total of 148 patients had available dosimetry. In the 113 patients with prostate volumes ≤50 cc (mean, 35.4 cc, range, 14.2–49.7 cc); the mean D₉₀ (dose which covers 90% of the prostate), V₁₀₀ (volume of prostate receiving 100% of the prescribed dose), V₁₅₀ (volume of prostate receiving 150% of the prescribed dose), and V₂₀₀ (volume of prostate receiving 200% of the prescribed dose) was 128.9%, 95.6%, 73.9%, and 51.2%, respectively. In the 32 patients with prostate volumes >50 cc (mean 58.1 cc, range 50.2–86.0 cc); the mean D₉₀, V₁₀₀, V₁₅₀, and V₂₀₀ was 125.1%, 95.2%, 68.2%, and 41.7%, respectively. The rectal V₁₀₀ was 1.0 cc for both cohorts. There was no statistically significant difference between the cohorts with respect to postimplant dosimetry for D₉₀, V₁₀₀, and V₁₅₀. The V₂₀₀ for prostate volumes >50 cc was significantly lower (p < 0.05).ConclusionsIn the present study, patients with prostate volumes >50 cc have postimplant dosimetry parameters similar to patients with prostate volumes ≤50 cc for D₉₀, V₁₀₀, and V₁₅₀; and significantly lower values for V₂₀₀. These results suggest that patients with large prostate volumes may not need to be routinely placed on hormonal therapy; sparing patients the side effects of hormonal therapy, and sparing the health care system the costs of luteinizing hormone-releasing hormone agonist injections.