Evaluation of a collapsed-cone convolution algorithm for esophagus and surface mold 192Ir brachytherapy treatment planning.

PURPOSE: TG43 does not account for a lack of scatter and tissue and applicator heterogeneities. The advanced collapsed-cone engine (ACE) algorithm available for use in the Oncentra Brachy treatment planning system (Elekta AB, Stockholm, Sweden) can model these conditions more accurately and is evaluated for esophageal and surface mold brachytherapy treatments. METHODS AND MATERIALS: ACE was commissioned for use then compared against TG43 for five esophageal and five surface mold treatment plans. Dosimetric differences between each algorithm were assessed using superimposed comparisons and dose-volume histogram statistics. RESULTS: Esophagus (6 Gy per fraction): Compared with TG43, ACE demonstrated up to a 0.63% and 0.05 Gy reduction in planning target volume (PTV) V100% and PTV D98, respectively. Lung D2cc and bone D2cc deviated by up to 0.09 Gy and 0.03 Gy, respectively. Lung D0.1 cc and bone D0.1 cc both deviated by up to 0.12 Gy. Surface mold (4.5 Gy per fraction): Compared with TG43, ACE demonstrated up to a 12.5% and 0.18 Gy reduction in PTV V80% and PTV D98, respectively. Bone D2cc and D0.1 cc both reduced by up to 0.2 Gy when modeled with ACE. Increasing mold size laterally increased the dosimetric differences between TG43 and ACE. CONCLUSIONS: TG43 generally overestimated dose delivered to the target volume and organs at risk for the sites investigated. Dosimetric differences observed for esophageal treatments were minimal; however, surface mold treatments would benefit from the increased dosimetric accuracy offered by ACE. Implementation should be considered for surface mold 192Ir treatment planning, but increased calculation time, additional contouring, and mass density assignment requirements should be scrutinized with regard to their potentially negative impact on current clinical practice.

Error detection thresholds for routine real time in vivo dosimetry in HDR prostate brachytherapy.

PURPOSE: Routine real time in vivo dosimetry (IVD) is performed in HDR prostate brachytherapy to independently verify dose delivery. This study investigates impact of position uncertainty on error detection thresholds for IVD. METHODS: IVD is implemented using a microMOSFET placed centrally in the prostate using an additional needle. 144 IVD measurements were made for 15 Gy or 19 Gy single fraction treatments. Needle insertion and treatment planning used real-time trans-rectal ultrasound. Source-MOSFET position thresholds of ±1, ±2 and ±3 mm were used to calculate per-needle and total plan error detection thresholds for the measured dose using an uncertainty analysis based on the treatment plan data. RESULTS: The median dose difference from 144 total plan measurements was -5.2% (range +7.4% to -17.3%). 3 plans measured outside the total plan error detection threshold for position threshold ±1 mm, no plans measured outside the total plan error detection threshold for larger position thresholds. For 2233 individual needle measurements, for position thresholds of ±1 mm, ±2mm and ±3 mm the number of needles outside the per-needle error detection threshold was 103, 25 and 10 respectively and the number of treatments that would have required interruption based on these thresholds for real-time IVD was 66, 16 and 8 respectively. CONCLUSION: IVD in HDR prostate brachytherapy using a microMOSFET provides a high level of confidence that we are correctly delivering the planned dose to our patients. A ±2-3 mm position threshold gives an appropriate balance between error detection and avoiding unnecessary treatment interruptions.

Importance of dynamic contrast enhanced magnetic resonance imaging for targeting biopsy and salvage treatments after prostate cancer recurrence.

PURPOSE: Evaluate T2 weighted MRI (T2W), diffusion weighted imaging (DWI), and dynamic contrast enhanced MRI (DCE-MRI) for determining areas of prostate cancer recurrence to target biopsy or salvage treatment in patients previously treated with I-125 seed brachytherapy. MATERIAL AND METHODS: MRI data from 15 patients, whose primary treatment was I-125 seed brachytherapy and who were subsequently treated with partial gland salvage high-dose-rate brachytherapy were retrospectively analyzed. Two radiologists independently reviewed imaging on two occasions blinded to clinical and biopsy information. At first review, the T2W and DWI sequences were assessed for likely presence of tumor and at second review, the additional DCE-MRI sequence was assessed. Results were recorded and compared on a prostate diagram divided into 12 sectors (quadrants at each of base, mid-gland, and apex) plus seminal vesicles (SV). RESULTS: Number of patients for whom recurrence was visible was 7/15 for T2W, 6.5/15 for DWI, and 15/15 for DCE-MRI (average of results for the two radiologists). Approximately, half of the sectors identified as showing recurrence were at the anterior base of the prostate. CONCLUSIONS: In prostate cancer patients previously treated with I-125 permanent seed implants, DCE-MRI is superior to T2W and DWI in defining areas of recurrence, and should be used to target biopsy and for treatment planning of focal salvage therapies.

Real-time in vivo dosimetry in high dose rate prostate brachytherapy.

BACKGROUND AND PURPOSE: Single fraction treatments of 15Gy or 19Gy are common in HDR prostate brachytherapy. In vivo dosimetry (IVD) is therefore important to ensure patient safety. This study assesses clinical IVD and investigates error detection thresholds for real-time treatment monitoring. MATERIALS AND METHODS: IVD was performed for 40 treatments planned using intra-operative trans-rectal ultrasound (TRUS) with a MOSFET inserted into an additional needle. Post-treatment TRUS images were acquired for 20 patients to assess needle movement. Monte Carlo simulations of treatment plans were performed for 10 patients to assess impact of heterogeneities. Per-needle and total plan uncertainties were estimated and retrospectively applied to the measured data as error detection thresholds. RESULTS: The mean measured dose was -6.4% compared to prediction (range +5.1% to -15.2%). Needle movement and heterogeneities accounted for -1.8% and -1.6% of this difference respectively (mean values for the patients analysed). Total plan uncertainty (k=2) ranged from 11% to 17% and per needle uncertainty (k=2) ranged from 18% to 110% (mean 31%). One out of 40 plans and 5% of needles were outside k=2 error detection threshold. CONCLUSIONS: IVD showed good agreement with predicted dose within measurement uncertainties, providing reassurance in the accuracy of dose delivery. Thresholds for real-time error detection should be calculated on an individual plan/needle basis.

Comparison of focal boost high dose rate prostate brachytherapy optimisation methods.

For HDR prostate brachytherapy treatments of 15 Gy to the whole gland plus focal boost, optimisation to either tumour plus margin (F-PTV) or involved sectors was compared. For 15 patients median F-PTV D90 and V150 were 21.0 Gy and 77.2% for F-PTV optimisation and 19.8 Gy and 75.6% for sector optimisation.

Dosimetry Modeling for Focal Low-Dose-Rate Prostate Brachytherapy.

PURPOSE: Focal brachytherapy targeted to an individual lesion(s) within the prostate may reduce side effects experienced with whole-gland brachytherapy. The outcomes of a consensus meeting on focal prostate brachytherapy were used to investigate optimal dosimetry of focal low-dose-rate (LDR) prostate brachytherapy targeted using multiparametric magnetic resonance imaging (mp-MRI) and transperineal template prostate mapping (TPM) biopsy, including the effects of random and systematic seed displacements and interseed attenuation (ISA). METHODS AND MATERIALS: Nine patients were selected according to clinical characteristics and concordance of TPM and mp-MRI. Retrospectively, 3 treatment plans were analyzed for each case: whole-gland (WG), hemi-gland (hemi), and ultra-focal (UF) plans, with 145-Gy prescription dose and identical dose constraints for each plan. Plan robustness to seed displacement and ISA were assessed using Monte Carlo simulations. RESULTS: WG plans used a mean 28 needles and 81 seeds, hemi plans used 17 needles and 56 seeds, and UF plans used 12 needles and 25 seeds. Mean D90 (minimum dose received by 90% of the target) and V100 (percentage of the target that receives 100% dose) values were 181.3 Gy and 99.8% for the prostate in WG plans, 195.7 Gy and 97.8% for the hemi-prostate in hemi plans, and 218.3 Gy and 99.8% for the focal target in UF plans. Mean urethra D10 was 205.9 Gy, 191.4 Gy, and 92.4 Gy in WG, hemi, and UF plans, respectively. Mean rectum D2 cm(3) was 107.5 Gy, 77.0 Gy, and 42.7 Gy in WG, hemi, and UF plans, respectively. Focal plans were more sensitive to seed displacement errors: random shifts with a standard deviation of 4 mm reduced mean target D90 by 14.0%, 20.5%, and 32.0% for WG, hemi, and UF plans, respectively. ISA has a similar impact on dose-volume histogram parameters for all plan types. CONCLUSIONS: Treatment planning for focal LDR brachytherapy is feasible. Dose constraints are easily met with a notable reduction to organs at risk. Treating smaller targets makes seed positioning more critical.

Dosimetry modeling for focal high-dose-rate prostate brachytherapy.

PURPOSE: The dosimetry of focal high-dose-rate prostate brachytherapy was assessed. Dose volume histogram parameters, robustness to source position errors, and Monte Carlo (MC) simulations were compared for whole-gland (WG), hemi-gland (HEMI), and ultra-focal (UF) treatment plans. METHODS AND MATERIALS: Tumor volumes were delineated based on MRI and template biopsy results for 9 patients. WG, HEMI, and UF plans were produced assuming 19 Gy single fraction monotherapy treatments. For UF plans, a 6-mm margin was applied to the visible tumor to create a focal-planning target volume (F-PTV). Systematic source position shifts of 1-4 mm were applied to assess plan robustness. The dosimetric impact of steel catheters was assessed using MC simulation. RESULTS: Mean D90 and V100 were 20.4 Gy and 97.9% for prostate in WG plans, 22.2 Gy and 98.1% for hemi-prostate in HEMI plans, and 23.0 Gy and 98.2% for F-PTV in UF plans. Mean urethra D10 was 20.3, 19.7, and 9.2 Gy in WG, HEMI, and UF plans, respectively. Mean rectal D2cc was 12.5, 9.8, and 4.6 Gy in WG, HEMI, and UF plans, respectively. Focal treatment plans were sensitive to source position errors-2 mm systematic shifts reduced mean prostate D90 by 0.7%, hemi-prostate D90 by 2.6%, and F-PTV D90 by 8.3% in WG, HEMI, and UF plans, respectively. MC simulation results were similar for all plan types with most dose volume histogram parameters reduced by <2%. CONCLUSIONS: HEMI and UF treatments can achieve higher D90 values compared with WG treatments with reduced organ at risk dose. Focal treatments are more sensitive to systematic source position errors than WG treatments.

Investigation of interseed attenuation and tissue composition effects in (125)I seed implant prostate brachytherapy.

PURPOSE: To use Monte Carlo (MC) simulation and sector analysis to assess interseed attenuation and scatter (ISA) and tissue effects in permanent seed implant prostate brachytherapy and to compare methods for modeling tissue. METHODS AND MATERIALS: CT-based postimplant plan simulations for 40 patients were evaluated using dose-volume histogram (DVH) parameters and sector analysis. Simulations in water (to evaluate ISA alone) and tissue assigned from contours or CT data, with and without calcifications, were compared. RESULTS: For patients without calcifications, mean combined ISA and tissue effect reduced prostate D90 by 4.2 Gy (2.9%), prostate V100 by 0.5 cm(3) (1.4%), urethra D10 by 8.6 Gy (3.5%), rectal [Formula: see text] by 11.6 Gy (10.5%), and the 100% isodose volume by 4.7 cm(3). Larger differences were observed comparing planned dose to postimplant, mean prostate D90 reduced from 185 to 149.8 Gy (-19%). For patients with calcifications, larger tissue effects were observed, prostate D90 reduced by up to 7.4%. Sector analysis showed dose reductions were larger in anterior and base sectors of the prostate. For patients without calcifications, contour- and CT-based tissue model results agreed within <0.5% for all DVH parameters, with up to 4% difference for patients with calcifications. CONCLUSIONS: Advanced brachytherapy dose calculation methods that take account of ISA and tissue effects show that clinical (125)I implant dose is different from TG-43U1 (AAPM Task Group No. 43 Report-Update 1) calculations, reducing DVH parameter values particularly for patients with calcifications. Peripheral dose and areas of the implant with relatively poorer coverage are particularly affected. However, dose reductions are small compared with other postimplant dose uncertainties.

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

PURPOSE: To investigate boosting dominant intraprostatic lesions (DILs) in the context of stereotactic ablative radiation therapy (SABR) and to examine the impact on tumor control probability (TCP) and normal tissue complication probability (NTCP). METHODS AND MATERIALS: Ten prostate datasets were selected. DILs were defined using T2-weighted, dynamic contrast-enhanced and diffusion-weighted magnetic resonance imaging. Four plans were produced for each dataset: (1) no boost to DILs; (2) boost to DILs, no seminal vesicles in prescription; (3) boost to DILs, proximal seminal vesicles (proxSV) prescribed intermediate dose; and (4) boost to DILs, proxSV prescribed higher dose. The prostate planning target volume (PTV) prescription was 42.7 Gy in 7 fractions. DILs were initially prescribed 115% of the PTV(Prostate) prescription, and PTV(DIL) prescriptions were increased in 5% increments until organ-at-risk constraints were reached. TCP and NTCP calculations used the LQ-Poisson Marsden, and Lyman-Kutcher-Burman models respectively. RESULTS: When treating the prostate alone, the median PTV(DIL) prescription was 125% (range: 110%-140%) of the PTV(Prostate) prescription. Median PTV(DIL) D50% was 55.1 Gy (range: 49.6-62.6 Gy). The same PTV(DIL) prescriptions and similar PTV(DIL) median doses were possible when including the proxSV within the prescription. TCP depended on prostate α/ß ratio and was highest with an α/ß ratio = 1.5 Gy, where the additional TCP benefit of DIL boosting was least. Rectal NTCP increased with DIL boosting and was considered unacceptably high in 5 cases, which, when replanned with an emphasis on reducing maximum dose to 0.5 cm(3) of rectum (Dmax(0.5cc)), as well as meeting existing constraints, resulted in considerable rectal NTCP reductions. CONCLUSIONS: Boosting DILs in the context of SABR is technically feasible but should be approached with caution. If this therapy is adopted, strict rectal constraints are required including Dmax(0.5cc). If the α/ß ratio of prostate cancer is 1.5 Gy or less, then high TCP and low NTCP can be achieved by prescribing SABR to the whole prostate, without the need for DIL boosting.

Multi-parametric MRI-guided focal tumor boost using HDR prostate brachytherapy: a feasibility study.

PURPOSE: This study investigates the feasibility of delivering focal boost dose to tumor regions, identified with multi-parametric MRI, in high-dose-rate prostate brachytherapy. METHODS AND MATERIALS: T2-weighted, diffusion-weighted, and dynamic-contrast-enhanced MRI were acquired the day before treatment and analyzed retrospectively for 15 patients. Twelve patients had hormone therapy before the MRI scan. The tumor was delineated on MRI by a radiologist and registered to treatment planning transrectal ultrasound images. A margin based on analysis of delineation and registration uncertainties was applied to create a focal boost planning target volume (F-PTV). Delivered treatment plans were compared with focal boost plans optimized to increase F-PTV dose as much as allowed by urethral and rectal dose constraints. RESULTS: Tumors were delineated in all patients with volumes 0.4-23.0cc. The margin for tumor delineation and image registration uncertainties was estimated to be 4.5 mm. For F-PTV, the focal boost treatment plans increased median D90 from 17.6 to 20.9 Gy and median V150 from 27.3% to 75.9%. CONCLUSIONS: MRI-guided high-dose-rate prostate brachytherapy focal tumor boost is feasible-tumor regions can be identified even after hormone therapy, and focal boost dose can be delivered without violating urethral and rectal dose constraints.