Prostate stereotactic ablative radiation therapy using volumetric modulated arc therapy to dominant intraprostatic lesions

Optimizing the dose-averaged linear energy transfer for the dominant intraprostatic lesions in high-risk localized prostate cancer patients

Background and purpose

Radiotherapy for localized prostate cancer often targets the entire prostate with a uniform dose despite the presence of high-risk dominant intraprostatic lesions (DILs). This study investigated the feasibility of focal dose-averaged linear energy transfer (LETd) boost for prostate carbon-ion radiotherapy to deposit higher LETd to DILs while ensuring desired relative biological effectiveness weighted dose coverage to targets and sparing organs at risk (OARs).

Materials and methods

A retrospective planning study was conducted on 15 localized prostate cancer cases. The DILs were identified on multiparametric MRI and used to define the boost target (PTVboost). Two treatment plans were designed for each patient: 1) conventional plan optimized by the single-field uniform dose technique, and 2) boost plan optimized by the multifield optimization and LET painting technique, to achieve LETd boost within the PTVboost. Dose and LETd metrics of the targets and OARs were compared between the two plans.

Results

Compared to the conventional plans, the boost plans delivered clinically acceptable dose coverage (D90% and D50%) to the target (PTV2) within 1% differences while significantly increasing the minimum LETd by 16 ∼ 24 keV/μm for the PTVboost (63.9 ± 2.8 vs. 44.0 ± 1.3 keV/μm, p < 0.001). Furthermore, these improvements were consistent across all cases, irrespective of their anatomical features, including the boost volume's size, location, and shape.

Conclusion

Focal LETd boost was a feasible strategy for prostate carbon-ion radiotherapy. This investigation demonstrated its superiority in delivering LETd boost without depending on tumor location and volume across different cases.

Stereotactic Body Radiation Therapy (SBRT) in prostate cancer in the presence of hip prosthesis - is it a contraindication? A narrative review

Hip replacement is a common orthopedic surgery in the aging population. With the rising incidence of prostate cancer, metallic hip prosthetics can cause considerable beam hardening and streak artifacts, leading to difficulty in identifying the target volumes and planning process for radiation treatment. The growing use of Stereotactic Body Radiation Therapy (SBRT) to treat prostate cancer is now well established. However, the use of this treatment modality in the presence of a hip prosthesis is poorly understood. There is enough literature on planning for external beam radiation treatment without any difficulties in the presence of hip prosthesis with conventional or Hypofractionated treatment. However, there is a shortage of literature on the impact of the prosthesis in SBRT planning, and there is a need for further understanding and measures to mitigate the obstacles in planning for SBRT in the presence of hip prosthesis. We present our review of the intricacies that need to be understood while considering SBRT in the presence of hip prostheses in prostate cancer treatment.

Dosimetric evaluation of ultrafractionated dose escalation with simultaneous integrated boost to intraprostatic lesion using 1.5-Tesla MR-Linac in localized prostate cancer

BACKGROUND

We analyzed a dose escalation of 36.25 Gy to the entire prostate and a dose increment up to 40 Gy with 1.25 Gy increments to intraprostatic lesion (IPL) using simultaneous integrated boost (SIB) in five fractions.

MATERIALS AND METHODS

Eighteen low- and intermediate-risk prostate cancer patients treated with 1.5T MR-Linac were retrospectively evaluated. The same planning computed tomography (CT) images generated four plans: no SIB, 37.5 Gy SIB, 38.75 Gy SIB, and 40 Gy SIB. In four plans, planning target volume (PTV) doses, organ at risk (OAR) doses, and PTV-SIB homogeneity index (HI), gradient index (GI) and conformity index (CI) were compared.

RESULTS

All plans met the criteria for PTV and PTV-SIB coverage. PTV 40 Gy plan has higher maximum PTV and PTV-SIB doses than other plans. The PTV HI was significantly higher in the SIB 40 Gy plan (0.135 ± 0.007) compared to SIB 38.75 Gy plan (0.099 ± 0.007; p = 0.001), SIB 37.5 Gy (0.067 ± 0.008; p < 0.001), and no SIB plan (0.049 ± 0.010; p < 0.001), while there were no significant differences in HI, GI and CI for PTV-SIB between three plans. Four rectum and bladder plans had similar dosimetric parameters. The urethra D5 was significantly higher in SIB 40 Gy plan compared to no SIB plan (37.7 ± 1.1 Gy vs. 37.0 ± 0.7 Gy; p = 0.009) and SIB 37.5 Gy plan (36.9 ± 0.8 Gy; p = 0.008). There was no significant difference in monitor units between the four consecutive plans.

CONCLUSIONS

Ultra-hypofractionated dose escalation to IPL up to 40 Gy in 5 fractions with a 1.5-T MR-linac is dosimetrically feasible, potentially paving the way for clinical trials.

Achievable Dosimetric Constraints in Stereotactic Reirradiation for Recurrent Prostate Cancer

PURPOSE

Stereotactic body radiation therapy has been proposed as a salvage treatment for recurrent prostate cancer after irradiation. One crucial issue is choosing appropriate dose-volume constraints (DVCs) during planning. The objectives of this study were to (1) quantify the proportion of patients respecting the DVCs according to the Urogenital Tumor Study Group GETUG-31 trial, testing 36 Gy in six fractions, (2) explain geometrically why the DVCs could not be respected, and (3) propose the most suitable DVCs.

METHODS AND MATERIALS

This retrospective dosimetric analysis included 141 patients treated for recurrent prostate cancer with Cyberknife (Accuray), according to GETUG-31 DVCs: V95% ≥ 95% for the planning target volume (PTV), V12Gy < 20% and V27Gy < 2 cc for the rectum, and V12Gy < 15% and V27Gy < 5 cc for the bladder. The percentage of patients not respecting the DVCs was quantified. Correlations between the DVCs and anatomic structures were examined. New DVCs were proposed.

RESULTS

Only 19% of patients respected all DVCs, with a mean PTV of 18.5 cc (range, 3-48 cc), although the mean PTV was 40.5 cc (range, 3-174 cc) in the whole series. A total of 98% of the patients with a clinical target volume (CTV)/prostate ratio >0.5 could not respect the DVCs in the organs at risk. The target coverage and organ-at-risk sparing decreased significantly with increase in the values of PTV, CTV, CTV/prostate ratio, the overlapping volume between the PTV and bladder wall and between the PTV and rectal wall. Threshold values of PTV, >20 cc and 40 cc, allowed for the PTV and bladder DVCs, respectively. To improve DVC respect in case of large target volume, we proposed the following new DVCs: V12Gy < 25% and 25% and V27Gy < 2 cc and 5 cc for the rectum and bladder, respectively.

CONCLUSIONS

GETUG-31 DVCs are achievable only for small target volumes (CTV more than half of the prostate). For a larger target volume, new DVCs have been proposed.

Focal Boost in Prostate Cancer Radiotherapy: A Review of Planning Studies and Clinical Trials

BACKGROUND

Focal boost radiotherapy was developed to deliver elevated doses to functional sub-volumes within a target. Such a technique was hypothesized to improve treatment outcomes without increasing toxicity in prostate cancer treatment.

PURPOSE

To summarize and evaluate the efficacy and variability of focal boost radiotherapy by reviewing focal boost planning studies and clinical trials that have been published in the last ten years.

METHODS

Published reports of focal boost radiotherapy, that specifically incorporate dose escalation to intra-prostatic lesions (IPLs), were reviewed and summarized. Correlations between acute/late ≥G2 genitourinary (GU) or gastrointestinal (GI) toxicity and clinical factors were determined by a meta-analysis.

RESULTS

By reviewing and summarizing 34 planning studies and 35 trials, a significant dose escalation to the GTV and thus higher tumor control of focal boost radiotherapy were reported consistently by all reviewed studies. Reviewed trials reported a not significant difference in toxicity between focal boost and conventional radiotherapy. Acute ≥G2 GU and late ≥G2 GI toxicities were reported the most and least prevalent, respectively, and a negative correlation was found between the rate of toxicity and proportion of low-risk or intermediate-risk patients in the cohort.

CONCLUSION

Focal boost prostate cancer radiotherapy has the potential to be a new standard of care.

Prostate Stereotactic Body Radiation Therapy With a Focal Simultaneous Integrated Boost: 5-Year Toxicity and Biochemical Recurrence Results From a Prospective Trial

PURPOSE

Stereotactic body radiation therapy (SBRT) is increasingly used as a definitive treatment option for patients with prostate adenocarcinoma. The aim of this study was to assess the late toxicity, patient-reported quality of life outcomes, and biochemical recurrence rates after prostate SBRT with simultaneous integrated boost (SIB) targeting lesions defined by magnetic resonance imaging (MRI).

METHODS AND MATERIALS

Patients were eligible if they had biopsy-proven low- or intermediate-risk prostate adenocarcinoma, one or more focal lesions on MRI, and an MRI-defined total prostate volume of <120 mL. All patients received SBRT delivered to the entire prostate to a dose of 36.25 Gy in 5 fractions with an SIB to the lesions seen on MRI to 40 Gy in 5 fractions. Late toxicity was defined as any possible treatment-related adverse event occurring after 3 months from the completion of SBRT. Patient-reported quality of life was ascertained using standardized patient surveys.

RESULTS

A total of 26 patients were enrolled. Six patients (23.1%) had low-risk disease and 20 patients had intermediate-risk disease (76.9%). Seven patients (26.9%) received androgen deprivation therapy. Median follow-up was 59.5 months. No biochemical failures were observed. Three patients (11.5%) experienced late grade 2 genitourinary (GU) toxicity requiring cystoscopy, and 7 patients (26.9%) had late grade 2 GU toxicity requiring oral medications. Three patients (11.5%) had late grade 2 gastrointestinal toxicity characterized by hematochezia requiring colonoscopy and steroids per rectum. There were no grade 3 or higher toxicity events observed. The patient-reported quality-of-life metrics at the time of last follow-up were not significantly different than the pre-treatment baseline.

CONCLUSIONS

The results of this study support that SBRT to the entire prostate to a dose of 36.25 Gy in 5 fractions with focal SIB to 40 Gy in 5 fractions has excellent biochemical control and is not associated with undue late gastrointestinal or GU toxicity or long-term quality of life decrement. Focal dose escalation with an SIB planning approach may be an opportunity to improve biochemical control while limiting dose to nearby organs at risk.

Radiotherapy planning in a prostate cancer phantom model with intraprostatic dominant lesions using stereotactic body radiotherapy with volumetric modulated arcs and a simultaneous integrated boost

Aim

In the treatment of prostate cancer with radiation therapy, the addition of a simultaneous integrated boost (SIB) to the dominant intraprostatic lesions (DIL) may improve local control. In this study, we aimed to determine the optimal radiation strategy in a phantom model of prostate cancer using volumetric modulated arc therapy for stereotactic body radiotherapy (SBRT-VMAT) with a SIB of 1-4 DILs.

Methods

We designed and printed a three-dimensional anthropomorphic phantom pelvis to simulate individual patient structures, including the prostate gland. A total of 36.25 Gy (SBRT) was delivered to the whole prostate. The DILs were irradiated with four different doses (40, 45, 47.5, and 50 Gy) to assess the influence of different SIB doses on dose distribution. The doses were calculated, verified, and measured using both transit and non-transit dosimetry for patient-specific quality assurance using a phantom model.

Results

The dose coverage met protocol requirements for all targets. However, the dose was close to violating risk constraints to the rectum when four DILs were treated simultaneously or when the DILs were located in the posterior segments of the prostate. All verification plans passed the assumed tolerance criteria.

Conclusions

Moderate dose escalation up to 45 Gy seems appropriate in cases with DILs located in posterior prostate segments or if there are three or more DILs located in other segments.

3D-printed patient-specific pelvis phantom for dosimetry measurements for prostate stereotactic radiotherapy with dominant intraprostatic lesion boost

AIM

Developing and assessing the feasibility of using a three-dimensional (3D) printed patient-specific anthropomorphic pelvis phantom for dose calculation and verification for stereotactic ablative radiation therapy (SABR) with dose escalation to the dominant intraprostatic lesions.

MATERIAL AND METHODS

A 3D-printed pelvis phantom, including bone-mimicking material, was fabricated based on the computed tomography (CT) images of a prostate cancer patient. To compare the extent to which patient and phantom body and bones overlapped, the similarity Dice coefficient was calculated. Modular cylindrical inserts were created to encapsulate radiochromic films and ionization chamber for absolute dosimetry measurements at the location of prostate and at the boost region. Gamma analysis evaluation with 2%/2mm criteria was performed to compare treatment planning system calculations and measured dose when delivering a 10 flattening filter free (FFF) SABR plan and a 10FFF boost SABR plan.

RESULTS

Dice coefficients of 0.98 and 0.91 were measured for body and bones, respectively, demonstrating agreement between patient and phantom outlines. For the boost plans the gamma analysis yielded 97.0% of pixels passing 2%/2mm criteria and these results were supported by the chamber average dose difference of 0.47 ± 0.03%. These results were further improved when overriding the bone relative electron density: 97.3% for the 2%/2mm gamma analysis, and 0.05 ± 0.03% for the ionization chamber average dose difference.

CONCLUSIONS

The modular patient-specific 3D-printed pelvis phantom has proven to be a highly attractive and versatile tool to validate prostate SABR boost plans using multiple detectors.

Proton vs. photon radiotherapy for MR-guided dose escalation of intraprostatic lesions

BACKGROUND

Dose escalation has been associated with improved biochemical control for prostate cancer. Focusing the high dose on the MRI-defined intraprostatic lesions (IL) could spare the surrounding organs at risk and hence allow further escalation. We compare treatment efficacy between state-of-the-art focally-boosted proton and photon-based radiotherapy, and investigate possible predictive guidelines regarding individualized treatment prescriptions.

MATERIAL AND METHODS

Ten prostate cancer patients with well-defined ILs were selected. Multiparametric MRI was used to delineate ILs, which were transferred to the planning CT via image registration. Pencil beam scanning proton therapy and volumetric modulated arc therapy treatment plans, were created for each patient. Each modality featured 6 plans: (1) moderately hypofractionated dose: 70 Gy to the prostate in 28 fractions, (2)-(6) plan 1 plus additional simultaneous-integrated-boost to ILs to 75.6, 81.2, 86.6, 98 and 112 Gy in 28 fractions. Equivalent dose to 2 Gy-per-fraction (EqD2) was used to calculate tumor control (TCP) and normal tissue complication probabilities (NTCP) for ILs and organs-at-risk.

RESULTS

For both modalities, the maximum necessary dose to achieve TCP > 99% was 98 Gy for very high-risk ILs. For lower risk ILs lower doses were sufficient. NTCP was <25% and 35% for protons and photons at the maximum dose escalation, respectively. For the cases and beam characteristics considered, proton therapy was dosimetrically superior when IL was >4 cc or located <2.5 mm from the rectum.

CONCLUSION

This work demonstrated the potential role for proton therapy in the setting of prostate focal dose escalation. We propose that anatomical characteristic could be used as criteria to identify patients who would benefit from proton treatment.

Ultra hypofractionated extended nodal irradiation using volumetric modulated arc therapy for oligorecurrent pelvic nodal prostate cancer

Prostate cancer (PCa) may recur after primary treatment but no standard of care exists for patients with pelvic nodal relapse. Based on obervational data, Extended Nodal Irradiation (ENI) might be associated with fewer treatment failures than Stereotactic Ablative Radiotherapy (SABR) to the involved node(s) alone. Ultra hypofractionated ENI is yet to be evaluated in this setting, but it could provide a therapeutic advantage if PCa has a low α/β ratio in addition to patient convenience/resource benefits. This volumetric modulated arc therapy (VMAT) planning study developed a class solution for 5-fraction Extended Nodal Irradiation (ENI) plus a simultaneous integrated boost (SIB) to involved node(s). Ten patients with oligorecurrent nodal disease after radical prostatectomy/post-operative prostate bed radiotherapy were selected. Three plans were produced for each dataset to deliver 25 Gy in 5 fractions ENI plus SIBs of 40, 35 and 30 Gy. The biologically effective dose (BED) formula was used to determine the remaining dose in 5 fractions that could be delivered to re-irradiated segments of organs at risk (OARs). Tumour control probability (TCP) and normal tissue complication probability (NTCP) were calculated using the LQ-Poisson Marsden and Lyman-Kutcher-Burman models respectively. Six patients had an OAR positioned within planning target volume node (PTVn), which resulted in reduced target coverage to PTV node in six, five and four instances for 40, 35 and 30 Gy SIB plans respectively. In these instances, only 30 Gy SIB plans had a median PTV coverage >90% (inter-quartile range 90-95). No OAR constraint was exceeded for 30 Gy SIB plans, including where segments of OARs were re-irradiated. Gross tumour volume node (GTVn) median TCP was 95.7% (94.4-96), 90.7% (87.1-91.2) and 78.6% (75.8-81.1) for 40, 35 and 30 Gy SIB plans respectively, where an α/β ratio of 1.5 was assumed. SacralPlex median NTCP was 43.2% (0.7-61.2), 12.1% (0.6-29.7) and 2.5% (0.5-5.1) for 40, 35 and 30 Gy SIB plans respectively. NTCP for Bowel_Small was <0.3% and zero for other OARs for all three plan types. Ultra hypofractionated ENI planning for pelvic nodal relapsed PCa appears feasible with encouraging estimates of nodal TCP and low estimates of NTCP, especially where a low α/β ratio is assumed and a 30 Gy SIB is delivered. This solution should be further evaluated within a clinical trial and compared against SABR to involved node(s) alone.

Voxel-level Classification of Prostate Cancer on Magnetic Resonance Imaging: Improving Accuracy Using Four-Compartment Restriction Spectrum Imaging

BACKGROUND

Diffusion magnetic resonance imaging (MRI) is integral to detection of prostate cancer (PCa), but conventional apparent diffusion coefficient (ADC) cannot capture the complexity of prostate tissues and tends to yield noisy images that do not distinctly highlight cancer. A four-compartment restriction spectrum imaging (RSI₄ ) model was recently found to optimally characterize pelvic diffusion signals, and the model coefficient for the slowest diffusion compartment, RSI₄ -C₁ , yielded greatest tumor conspicuity.

PURPOSE

To evaluate the slowest diffusion compartment of a four-compartment spectrum imaging model (RSI₄ -C₁ ) as a quantitative voxel-level classifier of PCa.

STUDY TYPE

Retrospective.

SUBJECTS

Forty-six men who underwent an extended MRI acquisition protocol for suspected PCa. Twenty-three men had benign prostates, and the other 23 men had PCa.

FIELD STRENGTH/SEQUENCE

A 3 T, multishell diffusion-weighted and axial T2-weighted sequences.

ASSESSMENT

High-confidence cancer voxels were delineated by expert consensus, using imaging data and biopsy results. The entire prostate was considered benign in patients with no detectable cancer. Diffusion images were used to calculate RSI₄ -C₁ and conventional ADC. Classifier images were also generated.

STATISTICAL TESTS

Voxel-level discrimination of PCa from benign prostate tissue was assessed via receiver operating characteristic (ROC) curves generated by bootstrapping with patient-level case resampling. RSI₄ -C₁ was compared to conventional ADC for two metrics: area under the ROC curve (AUC) and false-positive rate for a sensitivity of 90% (FPR₉₀ ). Statistical significance was assessed using bootstrap difference with two-sided α = 0.05.

RESULTS

RSI₄ -C₁ outperformed conventional ADC, with greater AUC (mean 0.977 [95% CI: 0.951-0.991] vs. 0.922 [0.878-0.948]) and lower FPR₉₀ (0.032 [0.009-0.082] vs. 0.201 [0.132-0.290]). These improvements were statistically significant (P < 0.05).

DATA CONCLUSION

RSI₄ -C₁ yielded a quantitative, voxel-level classifier of PCa that was superior to conventional ADC. RSI classifier images with a low false-positive rate might improve PCa detection and facilitate clinical applications like targeted biopsy and treatment planning.

EVIDENCE LEVEL

3 TECHNICAL EFFICACY: Stage 2.

Visualizing the urethra by magnetic resonance imaging without usage of a catheter for radiotherapy of prostate cancer

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Post urination MRI is useful for urethra-sparing radiotherapy treatment planning.

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This prospective clinical trial included 11 prostate cancer patients.

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Post urination MRI is the identification method of prostatic urinary tract in non-invasive manner.

The urethra position may shift due to the presence/absence of the catheter. Our proposed post-urination-magnetic resonance imaging (PU-MRI) technique is possible to identify the urethra without catheter. We aimed to verify the inter-operator difference in contouring the urethra by PU-MRI. The mean values of the evaluation indices of dice similarity coefficient, mean slice-wise Hausdorff distance, and center coordinates were 0.93, 0.17 mm, and 0.36 mm for computed tomography, and 0.75, 0.44 mm, and 1.00 mm for PU-MRI. Therefore, PU-MRI might be useful for identifying the prostatic urinary tract without using a urethral catheter.

Clinical trial registration: Hokkaido University Hospital for Clinical Research (018-0221).

Deformable MR-CBCT prostate registration using biomechanically constrained deep learning networks

BACKGROUND AND PURPOSE

Radiotherapeutic dose escalation to dominant intraprostatic lesions (DIL) in prostate cancer could potentially improve tumor control. The purpose of this study was to develop a method to accurately register multiparametric magnetic resonance imaging (MRI) with CBCT images for improved DIL delineation, treatment planning, and dose monitoring in prostate radiotherapy.

METHODS AND MATERIALS

We proposed a novel registration framework which considers biomechanical constraint when deforming the MR to CBCT. The registration framework consists of two segmentation convolutional neural networks (CNN) for MR and CBCT prostate segmentation, and a three-dimensional (3D) point cloud (PC) matching network. Image intensity-based rigid registration was first performed to initialize the alignment between MR and CBCT prostate. The aligned prostates were then meshed into tetrahedron elements to generate volumetric PC representation of the prostate shapes. The 3D PC matching network was developed to predict a PC motion vector field which can deform the MRI prostate PC to match the CBCT prostate PC. To regularize the network's motion prediction with biomechanical constraints, finite element (FE) modeling-generated motion fields were used to train the network. MRI and CBCT images of 50 patients with intraprostatic fiducial markers were used in this study. Registration results were evaluated using three metrics including dice similarity coefficient (DSC), mean surface distance (MSD), and target registration error (TRE). In addition to spatial registration accuracy, Jacobian determinant and strain tensors were calculated to assess the physical fidelity of the deformation field.

RESULTS

The mean and standard deviation of our method were 0.93 ± 0.01, 1.66 ± 0.10 mm, and 2.68 ± 1.91 mm for DSC, MSD, and TRE, respectively. The mean TRE of the proposed method was reduced by 29.1%, 14.3%, and 11.6% as compared to image intensity-based rigid registration, coherent point drifting (CPD) nonrigid surface registration, and modality-independent neighborhood descriptor (MIND) registration, respectively.

CONCLUSION

We developed a new framework to accurately register the prostate on MRI to CBCT images for external beam radiotherapy. The proposed method could be used to aid DIL delineation on CBCT, treatment planning, dose escalation to DIL, and dose monitoring.

Detectability of fiducials' positions for real-time target tracking system equipping with a standard linac for multiple fiducial markers

PURPOSE

To investigate the detectability of fiducial markers' positions for real-time target tracking system equipping with a standard linac. The hypothesis is that the detectability depends on the type of fiducial marker and the gantry angle of acquired triggered images.

METHODS

Three types of ball fiducials and four slim fiducials with lengths of 3 and 5 mm were prepared for this study. Triggered images with three similar fiducials were acquired at every 10° during the conformal arc irradiation to detect the target position. Although only one type of arrangement was prepared for the ball fiducials, a three-type arrangement was prepared for the slim fiducials, such as parallel, orthogonal, and oblique with 45° to the gantry-couch direction. To measure the detectability of the real-time target tracking system for each fiducial and arrangement, detected marker positions were compared with expected marker positions at every angle of acquired triggered images.

RESULTS

For the ball-type fiducial, the maximum difference between the detected marker positions and expected marker positions was 0.3 mm in all directions. For the slim fiducial arranged parallel and oblique with 45°, the maximum difference was 0.4 mm in all directions. When each slim fiducial was arranged orthogonal to the gantry-couch direction, the maximum difference was 1.5 mm for the length of 3 mm, and 3.2 mm for the length of 5 mm.

CONCLUSIONS

The detectability of fiducial markers' positions for the real-time target tracking system equipping with a standard linac depends on the form and insertion angles of the fiducials.

A planning study of focal dose escalations to multiparametric MRI-defined dominant intraprostatic lesions in prostate proton radiation therapy

OBJECTIVES

The purpose of this study is to investigate the dosimetric effect and clinical impact of delivering a focal radiotherapy boost dose to multiparametric MRI (mp-MRI)-defined dominant intraprostatic lesions (DILs) in prostate cancer using proton therapy.

METHODS

We retrospectively investigated 36 patients with pre-treatment mp-MRI and CT images who were treated using pencil beam scanning (PBS) proton radiation therapy to the whole prostate. DILs were contoured on co-registered mp-MRIs. Simultaneous integrated boost (SIB) plans using intensity-modulated proton therapy (IMPT) were created based on conventional whole-prostate-irradiation for each patient and optimized with additional DIL coverage goals and urethral constraints. DIL dose coverage and organ-at-risk (OAR) sparing were compared between conventional and SIB plans. Tumor control probability (TCP) and normal tissue complication probability (NTCP) were estimated to evaluate the clinical impact of the SIB plans.

RESULTS

Optimized SIB plans significantly escalated the dose to DILs while meeting OAR constraints. SIB plans were able to achieve 125, 150 and 175% of prescription dose coverage in 74, 54 and 17% of 36 patients, respectively. This was modeled to result in an increase in DIL TCP by 7.3-13.3% depending on α / β and DIL risk level.

CONCLUSION

The proposed mp-MRI-guided DIL boost using proton radiation therapy is feasible without violating OAR constraints and demonstrates a potential clinical benefit by improving DIL TCP. This retrospective study suggested the use of IMPT-based DIL SIB may represent a strategy to improve tumor control.

ADVANCES IN KNOWLEDGE

This study investigated the planning of mp-MRI-guided DIL boost in prostate proton radiation therapy and estimated its clinical impact with respect to TCP and NTCP.

Volumetric and Voxel-Wise Analysis of Dominant Intraprostatic Lesions on Multiparametric MRI

Introduction: Multiparametric MR imaging (mpMRI) has shown promising results in the diagnosis and localization of prostate cancer. Furthermore, mpMRI may play an important role in identifying the dominant intraprostatic lesion (DIL) for radiotherapy boost. We sought to investigate the level of correlation between dominant tumor foci contoured on various mpMRI sequences. Methods: mpMRI data from 90 patients with MR-guided biopsy-proven prostate cancer were obtained from the SPIE-AAPM-NCI Prostate MR Classification Challenge. Each case consisted of T2-weighted (T2W), apparent diffusion coefficient (ADC), and K^trans images computed from dynamic contrast-enhanced sequences. All image sets were rigidly co-registered, and the dominant tumor foci were identified and contoured for each MRI sequence. Hausdorff distance (HD), mean distance to agreement (MDA), and Dice and Jaccard coefficients were calculated between the contours for each pair of MRI sequences (i.e., T2 vs. ADC, T2 vs. K^trans, and ADC vs. K^trans). The voxel wise spearman correlation was also obtained between these image pairs. Results: The DILs were located in the anterior fibromuscular stroma, central zone, peripheral zone, and transition zone in 35.2, 5.6, 32.4, and 25.4% of patients, respectively. Gleason grade groups 1-5 represented 29.6, 40.8, 15.5, and 14.1% of the study population, respectively (with group grades 4 and 5 analyzed together). The mean contour volumes for the T2W images, and the ADC and K^trans maps were 2.14 ± 2.1, 2.22 ± 2.2, and 1.84 ± 1.5 mL, respectively. K^trans values were indistinguishable between cancerous regions and the rest of prostatic regions for 19 patients. The Dice coefficient and Jaccard index were 0.74 ± 0.13, 0.60 ± 0.15 for T2W-ADC and 0.61 ± 0.16, 0.46 ± 0.16 for T2W-K^trans. The voxel-based Spearman correlations were 0.20 ± 0.20 for T2W-ADC and 0.13 ± 0.25 for T2W-K^trans. Conclusions: The DIL contoured on T2W images had a high level of agreement with those contoured on ADC maps, but there was little to no quantitative correlation of these results with tumor location and Gleason grade group. Technical hurdles are yet to be solved for precision radiotherapy to target the DILs based on physiological imaging. A Boolean sum volume (BSV) incorporating all available MR sequences may be reasonable in delineating the DIL boost volume.

Multiparametric MRI-guided dose boost to dominant intraprostatic lesions in CT-based High-dose-rate prostate brachytherapy

OBJECTIVE

The purpose of this study is to investigate the dosimetric feasibility of delivering focal dose to multiparametric (mp) MRI-defined DILs in CT-based high-dose-rate (HDR) prostate brachytherapy with MR/CT registration and estimate its clinical benefit.

METHODS

We retrospectively investigated a total of 17 patients with mp-MRI and CT images acquired pre-treatment and treated by HDR prostate brachytherapy. 21 dominant intraprostatic lesions (DILs) were contoured on mp-MRI and propagated to CT images using a deformable image registration method. A boost plan was created for each patient and optimized on the original needle pattern. In addition, separate plans were generated using a virtually implanted needle around the DIL to mimic mp-MRI guided needle placement. DIL dose coverage and organ-at-rick (OAR) sparing were compared with original plan results. Tumor control probability (TCP) was estimated to further evaluate the clinical impact on the boost plans.

RESULTS

Overall, optimized boost plans significantly escalated dose to DILs while meeting OAR constraints. The addition of mp-MRI guided virtual needles facilitate increased coverage of DIL volumes, achieving a V150 > 90% in 85 % of DILs compared with 57 % of boost plan without an additional needle. Compared with original plan, TCP models estimated improvement in DIL control by 28 % for patients with external-beam treatment and by 8 % for monotherapy patients.

CONCLUSION

With MR/CT registration, the proposed mp-MRI guided DIL boost in CT-based HDR brachytherapy is feasible without violating OAR constraints, and indicates significant clinical benefit in improving TCP of DIL. It may represent a strategy to personalize treatment delivery and improve tumor control.

ADVANCES IN KNOWLEDGE

This study investigated the feasibility of mp-MRI guided DIL boost in HDR prostate brachytherapy with CT-based treatment planning, and estimated its clinical impact by TCP and NTCP estimation.

Prostate Stereotactic Body Radiation Therapy With a Focal Simultaneous Integrated Boost: Acute Toxicity and Dosimetry Results From a Prospective Trial

Purpose

This study aimed to report the early toxicity results of a prospective clinical trial of prostate stereotactic body radiation therapy (SBRT) to the entire prostate with a simultaneous integrated boost (SIB) to magnetic resonance imaging (MRI)-defined focal lesions.

Methods and materials

Eligible patients included men with biopsy-proven prostate stage T1c to T2c adenocarcinoma, a Gleason score ≤7, and prostate-specific antigen values of ≤20 ng/mL, who had at least 1 focal lesion visible on MRI and a total prostate volume no greater than 120 cm³. SBRT consisted of a dose of 36.25 Gy to the entire prostate with an SIB of 40 Gy to the MRI-defined lesions, delivered in 5 fractions. The primary purpose of the study was to confirm the feasibility of treatment planning/delivery and to estimate the rate of urinary retention requiring placement of a Foley catheter within 90 days of treatment. This study was to be considered successful if urinary retention occurred in no more than 15% of cases, with a planned enrollment of at least 25 patients.

Results

A total of 26 men were enrolled, and all underwent SBRT as planned. Twenty patients (77%) had intermediate-risk features, and the remainder were low risk. A treatment plan that met the protocol-defined goals for all cases was developed. Two patients (7.7%) developed acute urinary symptoms that required the temporary placement of a Foley catheter. No grade 3+ toxicity events were observed.

Conclusions

Planning and delivery of prostate SBRT with a whole prostate dose of 36.25 Gy and a focal 40 Gy SIB is feasible. Early follow-up suggests that this treatment is not associated with undue morbidity.

Toward planning target volume margin reduction for the prostate using intrafraction motion correction with online kV imaging and automatic detection of implanted gold seeds

PURPOSE

The imaging application Auto Beam Hold (ABH) allows for the online analysis of 2-dimensional kV images acquired during treatment. ABH can automatically detect fiducial markers and initiate a beam interrupt. In this study, we investigate the practical use and results of this intrafraction monitoring tool for patients with prostate cancer who have implanted gold seeds treated with a RapidArc technique.

METHODS AND MATERIALS

A total of 105 patients were included. For setup, the seeds were lined up using 2 orthogonal 2-dimensional kV images. After the setup procedure, ABH was applied at an interval of 3 seconds. The software requires a maximum-allowed deviation to be defined for each seed, which is referred to as a deviation limit (DL). Online, the ABH application evaluates the position of the seeds and indicates for each seed whether or not it exceeds the DL. Patients were divided in 3 groups. For the first group ABH was used with the DL at 6 mm, which corresponds to the planning target volume (PTV) margin. For the second group, the DL was set at 5 mm with an unchanged PTV margin of 6 mm. For the third group, the PTV margin was reduced to 5 mm with a DL of 5 mm. Offline, we performed an analysis of the number of beam stops and resulting re-setups.

RESULTS

ABH initiated a beam interrupt 223 times (13%) during a total of 1736 sessions. By decreasing the DL from 6 mm to 5 mm, the amount of workload for re-setups increased from 6% (group 1) to 14% (groups 2 and 3). Re-setup, 3-dimensional shifts larger than the PTV margin were found in 44%, 35%, and 45% for groups 1,2, and 3, respectively.

CONCLUSIONS

Intrafraction imaging of prostate position during treatment using automatic detection of implanted gold seeds was successfully implemented. PTV margins were safely reduced from 6mm to 5mm without a substantial increase in workload.

Characterization of a novel liquid fiducial marker for multimodal image guidance in stereotactic body radiotherapy of prostate cancer

PURPOSE

Liquid fiducial markers have shown to be a promising alternative to solid gold markers in terms of imaging artifact reduction, patient comfort, and compatibility with different imaging modalities. This study aims to investigate the performance of the novel BioXmark^® liquid marker for state-of-the-art multimodal imaging used in prostate cancer (PCa) radiotherapy, encompassing kV CT/CBCT, multiparametric MRI, and kV x-ray imaging. In addition, automatic detection of the liquid markers in x-ray imaging for prostate motion monitoring during treatment was investigated.

METHODS

A total of eight BioXmark^® liquid markers with varying volumes (range 5-300 μL) were casted on a square grid into a gelatin phantom insert. A cylindrical gold marker (QLRAD, length = 7 mm, Ø = 1 mm) was inserted for reference. Liquid marker visibility and streaking artifacts in CT/CBCT imaging were evaluated by placing the gelatin phantom into a CIRS anthropomorphic phantom. Relevant MRI characteristics such as the T2 and T1 relaxation times, the ADC value, and the relative proton density (ρH) were quantified by placing the gelatin phantom insert next to a T1MES mapping phantom and a water-filled syringe for reference. Ex vivo multiparametric MRI images were acquired by placing the gelatin phantom next to a resected prostate specimen. Anterior-posterior x-ray projection images were obtained by placing the gelatin phantom insert on top of an anthropomorphic pelvic phantom with internal pelvic bony structures and were acquired for five positions relative to the bony anatomy and 24 clinically relevant x-ray exposure settings. To quantify individual automatic marker detection, single markers were artificially isolated in the x-ray images using postprocessing.

RESULTS

Markers of all sizes were clearly visible on CT and CBCT images with only the largest marker volumes (100-300 μL) displaying artifacts similar in size to the gold fiducial marker. Artifact size increased with increasing liquid marker volume. Liquid markers displayed good contrast in ex vivo T1-weighted and ρH-weighted images. The markers were not visible in the ex vivo T2-weighted image. The liquid markers induced a chemical shift artifact in the obtained ADC-map. Automated detection in x-ray imaging was feasible with high detection success (four of five positions) for marker volumes in the range of 25-200 μL. None of the liquid markers were detected successfully when superimposed on a bony edge, independent of their size.

CONCLUSIONS

This study is the first to show the compatibility of BioXmark^® liquid markers with multimodal image-guided radiotherapy for PCa. Compared to a solid gold marker, they had favorable results in both visibility and induced imaging artifacts. Liquid marker visibility in MRI imaging of the prostate does not solely depend on the low ρH value (not visible on T2-weighted image) but is also influenced by its relaxation times. Automated marker detection in x-ray images was feasible but better adapted marker detection algorithms are necessary for marker localization in the presence of bony edges. Hence, the liquid marker provides a minimally invasive (fine needles) and highly applicable alternative to current solid gold markers for multimodal image-guided prostate radiotherapy treatments.

Efficacy of a rectal spacer with prostate SABR-first UK experience

OBJECTIVE

This study assessed the use of implanted hydrogel rectal spacers for stereotactic ablative radiotherapy-volumetric modulated arc therapy (SABR-VMAT) patients, investigating practicality, dosimetric impact, normal tissue complication probability (NTCP) and early toxicity.

METHODS

Data from the first 6 patients treated within a prostate SABR and rectal spacer trial were examined to determine spacer insertion tolerability, resultant changes in treatment planning and dosimetry and early toxicity effects. CT scans acquired prior to spacer insertion were used to generate SABR plans which were compared to post-insertion plans. Plans were evaluated for target coverage, conformity, and organs at risk doses with NTCPs also determined from resultant dose fluences. Early toxicity data were also collected.

RESULTS

All patients had successful spacer insertion under local anaesthetic with maximal Grade 1 toxicity. All plans were highly conformal, with no significant differences in clinical target volume dose coverage between pre- and post-spacer plans. Substantial improvements in rectal dose metrics were observed in post-spacer plans, e.g. rectal volume receiving 36 Gy reduced by ≥42% for all patients. Median NTCP for Grade 2 + rectal bleeding significantly decreased from 4.9 to 0.8% with the use of a rectal spacer (p = 0.031). To date, two episodes of acute Grade 1 proctitis have been reported following treatment.

CONCLUSION

The spacer resulted in clinically and statistically significant reduction in rectal doses for all patients. Advances in knowledge: This is one of the first studies to investigate the efficacy of a hydrogel spacer in prostate SABR treatments. Observed dose sparing of the rectum is predicted to result in meaningful clinical benefit.

MRI micturating urethrography for improved urethral delineation in prostate radiotherapy planning: a case study

Stereotactic ablative body radiotherapy is used in prostate cancer to deliver a high dose of radiation to the tumour over a small number of treatments. This involves the simulation of the patient using both CT and MRI. Current practice is to insert an indwelling catheter (IDC) during CT to assist with visualisation of the urethra and subsequently minimise dose to this highly critical structure. However, this procedure is invasive and has an associated risk of infection. This is a case study, which demonstrates our initial experience of using a real-time non-invasive MRI technique to replace the use of IDC for prostate cancer patients. The patient was scanned on a dedicated 3T MRI and was instructed to micturate in their own time whereupon a sagittal T2 weighted HASTE sequence was acquired every 5 s. This was subsequently followed by T2 weighted axial imaging at the level of mid prostate to provide improved urethral definition. Acquired images showed bladder voidance in real-time and an increase in signal intensity in the proximal urethra post voiding allowing for delineation of the urethra. The dimension and shape of the proximal urethra was well visualised and accumulation time of urine in the urethra was sufficient to enable optimum timing of the scanning technique. We have presented for the first time a micturating urethography technique using MRI, which has allowed us to visualise the urethra without contrast and with minimal invasiveness to the patient.

The delineation of intraprostatic boost regions for radiotherapy using multimodality imaging

Dose escalation to the prostate improves tumor control but at the expense of increased rectal toxicity. Modern imaging can be used to detect the most common site of recurrence, the intraprostatic lesion (IPL), which has led to the concept of focusing dose escalation to the IPL in order to improve the therapeutic ratio. Imaging must be able to detect lesions with adequate sensitivity and specificity to accurately delineate the IPL. This information must be carefully integrated into the radiotherapy planning process to ensure the dose is targeted to the IPL. This review will consider the role and challenges of multiparametric MRI and PET computed tomography in delineating a tumor boost to be delivered by external beam radiotherapy.

A validated tumor control probability model based on a meta-analysis of low, intermediate, and high-risk prostate cancer patients treated by photon, proton, or carbon-ion radiotherapy

PURPOSE

A fully heterogeneous population averaged mechanistic tumor control probability (TCP) model is appropriate for the analysis of external beam radiotherapy (EBRT). This has been accomplished for EBRT photon treatment of intermediate-risk prostate cancer. Extending the TCP model for low and high-risk patients would be beneficial in terms of overall decision making. Furthermore, different radiation treatment modalities such as protons and carbon-ions are becoming increasingly available. Consequently, there is a need for a complete TCP model.

METHODS

A TCP model was fitted and validated to a primary endpoint of 5-year biological no evidence of disease clinical outcome data obtained from a review of the literature for low, intermediate, and high-risk prostate cancer patients (5218 patients fitted, 1088 patients validated), treated by photons, protons, or carbon-ions. The review followed the preferred reporting item for systematic reviews and meta-analyses statement. Treatment regimens include standard fractionation and hypofractionation treatments. Residual analysis and goodness of fit statistics were applied.

RESULTS

The TCP model achieves a good level of fit overall, linear regression results in a p-value of <0.000 01 with an adjusted-weighted-R(2) value of 0.77 and a weighted root mean squared error (wRMSE) of 1.2%, to the fitted clinical outcome data. Validation of the model utilizing three independent datasets obtained from the literature resulted in an adjusted-weighted-R(2) value of 0.78 and a wRMSE of less than 1.8%, to the validation clinical outcome data. The weighted mean absolute residual across the entire dataset is found to be 5.4%.

CONCLUSIONS

This TCP model fitted and validated to clinical outcome data, appears to be an appropriate model for the inclusion of all clinical prostate cancer risk categories, and allows evaluation of current EBRT modalities with regard to tumor control prediction.

Twenty Fraction Prostate Radiotherapy with Intra-prostatic Boost: Results of a Pilot Study

AIMS

For patients with high-risk, locally bulky prostate cancer, an intra-prostatic boost to tumour volumes (dose-painting) offers a risk-adapted dose escalation. We evaluated the feasibility of hypofractionated dose-painting radiotherapy and the associated toxicity. The possibility to streamline a radiobiologically optimised planning protocol was also investigated.

MATERIALS AND METHODS

Twenty-eight patients were treated using a dose-painting approach; boost volumes were identified with functional magnetic resonance imaging scans. The prostate dose outside the boost volume was 60 Gy in 20 fractions, and the maximum integrated boost dose was set to 68 Gy, provided that the dose constraints to the organs at risk could be fulfilled. Rotational intensity-modulated radiotherapy was used with daily image guidance and fiducial markers.

RESULTS

The boost dose was escalated to 68 Gy for 25 patients (median dose 69 Gy, range 68-70 Gy); for three patients the boost dose was 67 Gy, due to the proximity of the urethra and/or the rectum. The mean normal tissue complication probability for rectal bleeding was 4.7% (range 3.4-5.8%) and was 3.5% for faecal incontinence (range 2.3-5.0%). At a median follow-up of 38 months (range 32-45) there was no grade 3 toxicity. Two patients developed grade 2 genitourinary toxicity (7.1%) and none developed grade 2 gastrointestinal toxicity. The mean prostate-specific antigen (PSA) for 23 patients who had stopped the adjuvant hormone therapy with a normal testosterone was 0.27 ng/ml (0.02-0.72) at follow-up; two patients have suppressed PSA and testosterone after stopping 3 year adjuvant hormone and three patients have relapsed (one pelvic node, two PSA only) at 36, 12 and 42 months, respectively.

CONCLUSIONS

A hypofractionated radiotherapy schedule, 60 Gy in 20 fractions with intra-prostatic boost dose of 68 Gy, can be achieved without exceeding dose constraints for organs at risk. Hypofractionated dose-painting escalated radiotherapy has an acceptable safety profile. The same planning protocol was used in a phase II single-arm trial (BIOPROP20: ClinicalTrials.gov identifier NCT02125175) and will further be used in a large phase III randomised trial (PIVOTALboost): patients will be randomised standard radiotherapy (60 Gy in 20 fractions) with or without lymph node radiotherapy versus dose-painting radiotherapy with or without lymph node radiotherapy; the trial will be opened for recruitment in summer 2017.

SBRT for prostate cancer: Challenges and features from a physicist prospective

Emerging data are showing the safety and the efficacy of Stereotactic Body Radiation Therapy (SBRT) in prostate cancer management. In this context, the medical physicists are regularly involved to review the appropriateness of the adopted technology and to proactively study new solutions. From the physics point of view there are two major challenges in prostate SBRT: (1) mitigation of geometrical uncertainty and (2) generation of highly conformal dose distributions that maximally spare the OARs. Geometrical uncertainties have to be limited as much as possible in order to avoid the use of large PTV margins. Furthermore, advanced planning and delivery techniques are needed to generate maximally conformal dose distributions. In this non-systematic review the technology and the physics aspects of SBRT for prostate cancer were analyzed. In details, the aims were: (i) to describe the rationale of reducing the number of fractions (i.e. increasing the dose per fraction), (ii) to analyze the features to be accounted for performing an extreme hypo-fractionation scheme (>6-7Gy), and (iii) to describe technological solutions for treating in a safe way. The analysis of outcomes, toxicities, and other clinical aspects are not object of the present evaluation.

Dose escalation to dominant intraprostatic lesions with MRI-transrectal ultrasound fusion High-Dose-Rate prostate brachytherapy. Prospective phase II trial

BACKGROUND AND PURPOSE

To demonstrate the feasibility, safety and effectiveness of dose escalation to intraprostatic lesions with MRI-transrectal ultrasound fusion High-Dose-Rate (HDR) brachytherapy.

MATERIALS AND METHODS

15 patients with intermediate-high risk prostate cancer and visible dominant intra-prostatic nodule on mpMRI have been treated. The treatment consisted of combined MRI-TRUS fusion HDR-brachytherapy (1 fraction of 1500cGy) and hypofractionated external beam (3750cGy in 15 fractions). A dose of 1875Gy was delivered to at least 98% of the DIL volume.

RESULTS

Median prostate volume was 23.8cc; median number of needles was 16 (13-18). Dose escalation to DIL was feasible in 14/15 patients (93%) without violating dosimetric constraints and 1 patient presented a minimal deviation of dosimetric restrictions. With a median follow-up of 18months (17-24), none of the patients developed acute urinary retention or grade ⩾3 toxicity. In addition to standard PSA follow-up, response has been assessed by mpMRI at 12months. All patients presented adequate morphological responses on anatomical and functional sequences.

CONCLUSIONS

HDR brachytherapy using MRI-transrectal ultrasound fusion for image guidance is a suitable technique for partial prostate dose escalation. Tolerance and toxicity profiles are excellent and results are encouraging in terms of biochemical, morphological and functional response.

[Interobserver variability study for daily cone beam computed tomography registration of prostate volumetric modulated arc therapy]

PURPOSE

This work evaluated the interobserver variability in cone beam computed tomography (CBCT) registration for prostate cancers treated with intensity-modulated radiotherapy.

MATERIAL AND METHODS

Twelve technologists realized 286 CBCT/CT registrations (bone registration followed by prostate to prostate registration). The registration results were compared to those obtained by two radiation oncologists (reference). Each technologist reported the shifts calculated by the software in all three axes. A statistical analysis allowed us to calculate the minimum threshold under which 95% of the observers found similar values. A variance analysis followed by the post hoc test were used to find differences in interobserver registration variability and determine whether any individual users performed registrations which differed significantly from those of the other users.

RESULTS

The registration differences compared to the reference in the three directions in terms of 95th percentile are: 2.1mm left-right, 3.5mm target-gun, 7.3mm anterior-posterior. In the posterior direction, 4% of the observers have found differences superior to 8mm, margin used in routine without the use of a daily CBCT. The variance test revealed a P-value <0.05 only for target-gun and for all observers there was no significant difference compared to the reference.

CONCLUSION

This study confirmed the interest of a 3D tissue registration for prostate treatments. The registration study showed a good interobserver reproducibility. This showed the importance of a daily CBCT/CT registration in prostate treatment with the possibility of a planning target volume margin reduction in the three directions. An evaluation of a partial delegation of registration to technologists should be done by the radiation oncologists.

Volumetric modulated arc therapy planning for primary prostate cancer with selective intraprostatic boost determined by 18F-choline PET/CT

PURPOSE

This study evaluated expected tumor control and normal tissue toxicity for prostate volumetric modulated arc therapy (VMAT) with and without radiation boosts to an intraprostatically dominant lesion (IDL), defined by (18)F-choline positron emission tomography/computed tomography (PET/CT).

METHODS AND MATERIALS

Thirty patients with localized prostate cancer underwent (18)F-choline PET/CT before treatment. Two VMAT plans, plan79 Gy and plan100-105 Gy, were compared for each patient. The whole-prostate planning target volume (PTVprostate) prescription was 79 Gy in both plans, but plan100-105 Gy added simultaneous boost doses of 100 Gy and 105 Gy to the IDL, defined by 60% and 70% of maximum prostatic uptake on (18)F-choline PET (IDLsuv60% and IDLsuv70%, respectively, with IDLsuv70% nested inside IDLsuv60% to potentially enhance tumor specificity of the maximum point dose). Plan evaluations included histopathological correspondence, isodose distributions, dose-volume histograms, tumor control probability (TCP), and normal tissue complication probability (NTCP).

RESULTS

Planning objectives and dose constraints proved feasible in 30 of 30 cases. Prostate sextant histopathology was available for 28 cases, confirming that IDLsuv60% adequately covered all tumor-bearing prostate sextants in 27 cases and provided partial coverage in 1 case. Plan100-105 Gy had significantly higher TCP than plan79 Gy across all prostate regions for α/β ratios ranging from 1.5 Gy to 10 Gy (P<.001 for each case). There were no significant differences in bladder and femoral head NTCP between plans and slightly lower rectal NTCP (endpoint: grade ≥ 2 late toxicity or rectal bleeding) was found for plan100-105 Gy.

CONCLUSIONS

VMAT can potentially increase the likelihood of tumor control in primary prostate cancer while observing normal tissue tolerances through simultaneous delivery of a steep radiation boost to a (18)F-choline PET-defined IDL.

A sector-based dosimetric analysis of dose heterogeneity in high-dose-rate prostate brachytherapy

PURPOSE

High-dose-rate (HDR) prostate brachytherapy delivers a heterogeneous dose distribution throughout the prostate gland. There is however limited information regarding the spatial distribution of this dose heterogeneity. To this end, we analyzed the magnitude and location of intraprostatic dose heterogeneity in HDR prostate brachytherapy.

METHODS AND MATERIALS

Five consecutive prostate cancer patients treated with HDR were analyzed. Based on CT-simulation images, each prostate was divided into three sections (apex, base, and mid-gland). These were further subdivided into eight symmetrical sections to give a total of 24 sections. Dose-volume histograms were analyzed from V100-V200% for these 24 sections comparing the means of individual regions, left vs right, apex vs base vs mid-gland, lateral vs medial, and anterior vs posterior. A separate analysis on dose as a function of individual region volume was also performed.

RESULTS

Analyses comparing the 24 regions showed a maximum 62% difference (range, 21.9-83.9%) at V130% and 19.9% (1.9-20.8%) at V200%. Seven regions were significantly decreased and one significantly elevated from V130-V180% when compared with the mean. The means for lateral sections were 1.57-fold higher than medial sections from V110-V200% (p < 0.0001). The dose at the base was significantly higher than the rest of the gland from V120-V200 (V150, 35.6 ± 16.2% vs 20.9 ± 13.1%, p < 0.0001).

CONCLUSIONS

There is significant intra-prostatic dose heterogeneity in prostate HDR brachytherapy. This is most notable in the increased dose to base and lateral portions of the gland. Further studies are needed to determine the impact of heterogeneity on clinical outcomes.