The influence of tissue composition uncertainty on dose distributions in brachytherapy

169Yb-based high dose rate intensity modulated brachytherapy for focal treatment of prostate cancer

PURPOSE

This study compares conventional 192Ir-based high dose rate brachytherapy (HDR-BT) with 169Yb-based HDR intensity modulated brachytherapy (IMBT) for focal prostate cancer treatment. Additionally, the study explores the potential to generate less invasive treatment plans with IMBT by reducing the number of catheters needed to achieve acceptable outcomes.

METHODS AND MATERIALS

A retrospective dosimetric study of ten prostate cancer patients initially treated with conventional 192Ir-based HDR-BT and 5-14 catheters was employed. RapidBrachyMCTPS, a Monte Carlo-based treatment planning system was used to calculate and optimize dose distributions. For 169Yb-based HDR IMBT, a custom 169Yb source combined with 0.8 mm thick platinum shields placed inside 6F catheters was used. Furthermore, dose distributions were investigated when iteratively removing catheters for less invasive treatments.

RESULTS

With IMBT, the urethra D10 and D0.1cc decreased on average by 15.89 and 15.65 percentage points (pp) and the rectum V75 and D2cc by 1.53 and 11.54 pp, respectively, compared to the conventional clinical plans. Similar trends were observed when the number of catheters decreased. On average, there was an observed increase in PTV V150 from 2.84 pp with IMBT when utilizing all catheters to 8.83 pp when four catheters were removed. PTV V200 increased from 0.42 to 2.96 pp on average. Hotspots in the body were however lower with IMBT compared to conventional clinical plans.

CONCLUSIONS

169Yb-based HDR IMBT for focal treatment of prostate cancer has the potential to successfully deliver clinically acceptable, less invasive treatment with reduced dose to organs at risk.

Impact on the estimated dose of different tissue assignment strategies during partial breast irradiations with INTRABEAM

PURPOSE

Partial breast irradiations with electronic brachytherapy or kilovoltage intraoperative radiotherapy devices such as Axxent or INTRABEAM are becoming more common every day. Breast is mainly composed of glandular and adipose tissues, which are not always clearly disentangled in planning breast CTs. In these cases, breast tissues are replaced with an average soft tissue, or even water. However, at kilovoltage energies, this may lead to large differences in the delivered dose, due to the dominance of photoelectric effect. Therefore, the aim of this work was to study the effect on the dose prescribed in breast with the INTRABEAM device using different soft tissue assignment strategies that would replace the adipose and glandular tissues that constitute the breast in cases where these tissues cannot be adequately distinguished in a CT scan.

METHODS AND MATERIALS

Dose was computed with a Monte Carlo code in five patients with a 3 cm diameter INTRABEAM spherical applicator. Tissues within the breast were assigned following six different strategies: one based on the TG-43 recommendations, representing the whole breast as water of unity density, another one also water-based but with CT derived density, and the other four also based on CT-derived densities, using a single tissue resulting from different mixes of glandular and adipose tissues. These were compared against the reference dose computed in an accurately segmented CT, following TG-186 recommendations. Relative differences and dose ratios between the reference and the other tissue assignment strategies were obtained in three regions of interest inside the breast.

RESULTS AND CONCLUSIONS

Dose planning in water-based tissues was found inaccurate for breast treatment with INTRABEAM, as it would incur in up to 30% under-prescription of dose. If accurate soft tissue assignments in the breast cannot be safely done, a single-tissue composition of 80% adipose and 20% glandular tissue, or even a 100% adipose tissue, would be recommended to avoid dose under-prescription.

Penalty weight tuning in high dose rate brachytherapy using multi-objective Bayesian optimization

Objective.Treatment plan optimization in high dose rate brachytherapy often requires manual fine-tuning of penalty weights for each objective, which can be time-consuming and dependent on the planner's experience. To automate this process, this study used a multi-criteria approach called multi-objective Bayesian optimization with q-noisy expected hypervolume improvement as its acquisition function (MOBO-qNEHVI).Approach.The treatment plans of 13 prostate cancer patients were retrospectively imported to a research treatment planning system, RapidBrachyMTPS, where fast mixed integer optimization (FMIO) performs dwell time optimization given a set of penalty weights to deliver 15 Gy to the target volume. MOBO-qNEHVI was used to find patient-specific Pareto optimal penalty weight vectors that yield clinically acceptable dose volume histogram metrics. The relationship between the number of MOBO-qNEHVI iterations and the number of clinically acceptable plans per patient (acceptance rate) was investigated. The performance time was obtained for various parameter configurations.Main results.MOBO-qNEHVI found clinically acceptable treatment plans for all patients. With increasing the number of MOBO-qNEHVI iterations, the acceptance rate grew logarithmically while the performance time grew exponentially. Fixing the penalty weight of the tumour volume to maximum value, adding the target dose as a parameter, initiating MOBO-qNEHVI with 25 parallel sampling of FMIO, and running 6 MOBO-qNEHVI iterations found solutions that delivered 15 Gy to the hottest 95% of the clinical target volume while respecting the dose constraints to the organs at risk. The average acceptance rate for each patient was 89.74% ± 8.11%, and performance time was 66.6 ± 12.6 s. The initiation took 22.47 ± 7.57 s, and each iteration took 7.35 ± 2.45 s to find one Pareto solution.Significance.MOBO-qNEHVI combined with FMIO can automatically explore the trade-offs between treatment plan objectives in a patient specific manner within a minute. This approach can reduce the dependency of plan quality on planner's experience and reduce dose to the organs at risk.

Applying the column generation method to the intensity modulated high dose rate brachytherapy inverse planning problem

Objective.Intensity modulated high dose rate brachytherapy (IMBT) is a rapidly developing application of brachytherapy where anisotropic dose distributions can be produced at each source dwell position. This technique is made possible by placing rotating metallic shields inside brachytherapy needles or catheters. By dynamically directing the radiation towards the tumours and away from the healthy tissues, a more conformal dose distribution can be obtained. The resulting treatment planning involves optimizing dwell position and shield angle (DPSA). The aim of this study was to investigate the column generation method for IMBT treatment plan optimization.Approach.A column generation optimization algorithm was developed to optimize the dwell times and shield angles. A retrospective study was performed on 10 prostate cases using RapidBrachyMCTPS. At every iteration, the plan was optimized with the chosen DPSA which would best improve the cost function that was added to the plan. The optimization process was stopped when the remaining DPSAs would not add value to the plan to limit the plan complexity.Main results.The average number of DPSAs and voxels were 2270 and 7997, respectively. The column generation approach yielded near-optimal treatment plans by using only 11% of available DPSAs on average in ten prostate cases. The coverage and organs at risk constraints passed in all ten cases.Significance.The column generation method produced high-quality deliverable prostate IMBT plans. The treatment plan quality reached a plateau, where adding more DPSAs had a minimal effect on dose volume histogram parameters. The iterative nature of the column generation method allows early termination of the treatment plan creation process as soon as the dosimetric indices from dose volume histogram satisfy the clinical requirements or if their values stabilize.

Dosimetric impact of a robust optimization approach to mitigate effects from rotational uncertainty in prostate intensity-modulated brachytherapy

BACKGROUND

Intensity-modulated brachytherapy (IMBT) is an emerging technology for cancer treatment, in which radiation sources are shielded to shape the dose distribution. The rotatable shields provide an additional degree of freedom, but also introduce an additional, directional, type of uncertainty, compared to conventional high-dose-rate brachytherapy (HDR BT).

PURPOSE

We propose and evaluate a robust optimization approach to mitigate the effects of rotational uncertainty in the shields with respect to planning criteria.

METHODS

A previously suggested prototype for platinum-shielded prostate ¹⁶⁹ Yb-based dynamic IMBT is considered. We study a retrospective patient data set (anatomical contours and catheter placement) from two clinics, consisting of six patients that had previously undergone conventional ¹⁹² Ir HDR BT treatment. The Monte Carlo-based treatment planning software RapidBrachyMCTPS is used for dose calculations. In our computational experiments, we investigate systematic rotational shield errors of ±10° and ±20°, and the same systematic error is applied to all dwell positions in each scenario. This gives us three scenarios, one nominal and two with errors. The robust optimization approach finds a compromise between the average and worst-case scenario outcomes.

RESULTS

We compare dose plans obtained from standard models and their robust counterparts. With dwell times obtained from a linear penalty model (LPM), for 10° errors, the dose to urethra ( D 0.1 c c $D_{0.1cc}$ ) and rectum ( D 0.1 c c $D_{0.1cc}$ and D 1 c c $D_{1cc}$ ) increase with up to 5% and 7%, respectively, in the worst-case scenario, while with the robust counterpart, the corresponding increases were 3% and 3%. For all patients and all evaluated criteria, the worst-case scenario outcome with the robust approach had lower deviation compared to the standard model, without compromising target coverage. We also evaluated shield errors up to 20° and while the deviations increased to a large extent with the standard models, the robust models were capable of handling even such large errors.

CONCLUSIONS

We conclude that robust optimization can be used to mitigate the effects from rotational uncertainty and to ensure the treatment plan quality of IMBT.

Evaluation of dosimetric effects of metallic artifact reduction and tissue assignment on Monte Carlo dose calculations for 125 I prostate implants

PURPOSE

Monte Carlo (MC) simulation studies, aimed at evaluating the magnitude of tissue heterogeneity in ¹²⁵ I prostate permanent seed implant brachytherapy (BT), customarily use clinical post-implant CT images to generate a virtual representation of a realistic patient model (virtual patient model). Metallic artifact reduction (MAR) techniques and tissue assignment schemes (TAS) are implemented on the post-implant CT images to mollify metallic artifacts due to BT seeds and to assign tissue types to the voxels corresponding to the bright seed spots and streaking artifacts, respectively. The objective of this study is to assess the combined influence of MAR and TAS on MC absorbed dose calculations in post-implant CT-based phantoms. The virtual patient models used for ¹²⁵ I prostate implant MC absorbed dose calculations in this study are derived from the CT images of an external radiotherapy prostate patient without BT seeds and prostatic calcifications, thus averting the need to implement MAR and TAS.

METHODS

The geometry of the IsoSeed I25.S17plus source is validated by comparing the MC calculated results of the TG-43 parameters for the line source approximation with the TG-43U1S2 consensus data. Four MC absorbed dose calculations are performed in two virtual patient models using the egs_brachy MC code: (1) TG-43-based D_{w,w-TG 43} , (2) D_w,w-MBDC that accounts for interseed scattering and attenuation (ISA), (3) D_m,m that examines ISA and tissue heterogeneity by scoring absorbed dose in tissue, and (4) D_w,m that unlike D_m,m scores absorbed dose in water. The MC absorbed doses (1) and (2) are simulated in a TG-43 patient phantom derived by assigning the densities of every voxel to 1.00 g cm^-3 (water), whereas MC absorbed doses (3) and (4) are scored in the TG-186 patient phantom generated by mapping the mass density of each voxel to tissue according to a CT calibration curve. The MC absorbed doses calculated in this study are compared with VariSeed v8.0 calculated absorbed doses. To evaluate the dosimetric effect of MAR and TAS, the MC absorbed doses of this work (independent of MAR and TAS) are compared to the MC absorbed doses of different ¹²⁵ I source models from previous studies that were calculated with different MC codes using post-implant CT-based phantoms generated by implementing MAR and TAS on post-implant CT images.

RESULTS

The very good agreement of TG-43 parameters of this study and the published consensus data within 3% validates the geometry of the IsoSeed I25.S17plus source. For the clinical studies, the TG-43-based calculations show a D₉₀ overestimation of more than 4% compared to the more realistic MC methods due to ISA and tissue composition. The results of this work generally show few discrepancies with the post-implant CT-based dosimetry studies with respect to the D₉₀ absorbed dose metric parameter. These discrepancies are mainly Type B uncertainties due to the different ¹²⁵ I source models and MC codes.

CONCLUSIONS

The implementation of MAR and TAS on post-implant CT images have no dosimetric effect on the ¹²⁵ I prostate MC absorbed dose calculation in post-implant CT-based phantoms.

Characterization of Dosimetric Differences in Strut-Adjusted Volume Implant Treatment Plans Calculated With TG-43 Formalism and a Model-Based Dose Calculation Algorithm

PURPOSE

To comprehensively characterize dosimetric differences between calculations with a commercial model-based dose calculation algorithm (MBDCA) and the TG-43 formalism in application to accelerated partial breast irradiation (APBI) with the strut-adjusted volume implant (SAVI) applicator.

METHODS

Dose for 100 patients treated with the SAVI applicator was recalculated with an MBDCA for comparison to dose calculated via TG-43. For every pair of dose calculations, dose-volume histogram (DVH) metrics including V90%, V95%, V100%, V150%, and V200% for the PTV_EVAL were compared. Features were defined for each case including (1) applicator size, (2) ratio between PTV_EVAL contour and 1-cm rind surrounding SAVI applicator, (3) ratio between dwell time in central catheter and total dwell time, and (4) mean computed tomography (CT) number within the lumpectomy cavity. Wilcoxon rank sum tests were performed to test whether treatment plans could be stratified according to feature values into groups with statistically significant dosimetry differences between MBDCA and TG-43.

RESULTS

For all DVH metrics, differences between TG-43 and MBDCA calculations were statistically significant (P < .05). Minimum (maximum) relative percent differences between the MBDCA and TG-43 for V90%, V95%, and V100% were -2.1% (0.1%), -3.1% (-0.1%), and -5.0% (-0.5%), respectively. The median relative percent difference in mean PTV_EVAL dose between the MBDCA and TG-43 was -3.9%, with minimum (maximum) difference of -6.5% (-1.8%). For V90%, V95%, and V100%, plan quality worsened beyond defined thresholds in 26, 23, and 31 cases with no instances of coverage improvement. Features 1, 2, and 4 were shown to be able to stratify treatment plans into groups with statistically significant differences in dosimetry metrics between MBDCA and TG-43.

CONCLUSIONS

Investigated dose metrics for SAVI treatments were found to be systematically lower with MBDCA calculation in comparison to TG-43. Plans could be stratified according to several features by the magnitude of dosimetric differences between these calculations.

A review of dosimetric impact of implementation of model-based dose calculation algorithms (MBDCAs) for HDR brachytherapy

To obtain dose distributions more physically representative to the patient anatomy in brachytherapy, calculation algorithms that can account for heterogeneity are required. The current standard AAPM Task Group No 43 (TG-43) dose calculation formalism has some clinically relevant dosimetric limitations. Lack of tissue heterogeneity and scattered dose corrections are the major weaknesses of the TG-43 formalism and could lead to systematic dose errors in target volumes and organs at risk. Over the last decade, model-based dose calculation algorithms (MBDCAs) have been clinically offered as complementary algorithms beyond the TG43 formalism for high dose rate (HDR) brachytherapy treatment planning. These algorithms provide enhanced dose calculation accuracy by using the information in the patient's computed tomography images, which allows modeling the patient's geometry, material compositions, and the treatment applicator. Several researchers have investigated the implementation of MBDCAs in HDR brachytherapy for dose optimization, but moving toward using them as primary algorithms for dose calculations is still lagging. Therefore, an overview of up-to-date research is needed to familiarize clinicians with the current status of the MBDCAs for different cancers in HDR brachytherapy. In this paper, we review the MBDCAs for HDR brachytherapy from a dosimetric perspective. Treatment sites covered include breast, gynecological, lung, head and neck, esophagus, liver, prostate, and skin cancers. Moreover, we discuss the current status of implementation of MBDCAs and the challenges.

On the impact of absorbed dose specification, tissue heterogeneities, and applicator heterogeneities on Monte Carlo-based dosimetry of Ir-192, Se-75, and Yb-169 in conventional and intensity-modulated brachytherapy for the treatment of cervical cancer

PURPOSE

The purpose of this study was to evaluate the impact of dose reporting schemes and tissue/applicator heterogeneities for ¹⁹² Ir-, ⁷⁵ Se-, and ¹⁶⁹ Yb-based MRI-guided conventional and intensity-modulated brachytherapy.

METHODS AND MATERIALS

Treatment plans using a variety of dose reporting and tissue/applicator segmentation schemes were generated for a cohort (n = 10) of cervical cancer patients treated with ¹⁹² Ir-based Venezia brachytherapy. Dose calculations were performed using RapidBrachyMCTPS, a Geant4-based research Monte Carlo treatment planning system. Ultimately, five dose calculation scenarios were evaluated: (a) dose to water in water (D_w,w ); (b) D_w,w taking the applicator material into consideration (D_w,wApp ); (c) dose to water in medium (D_w,m ); (d and e) dose to medium in medium with mass densities assigned either nominally per structure (D_{m,m (Nom)} ) or voxel-by-voxel (D_m,m ).

RESULTS

Ignoring the plastic Venezia applicator (D_w,wApp ) overestimates D_m,m by up to 1% (average) with high energy source (¹⁹² Ir and ⁷⁵ Se) and up to 2% with ¹⁶⁹ Yb. Scoring dose to water (D_w,wApp or D_w,m ) generally overestimates dose and this effect increases with decreasing photon energy. Reporting dose other than D_m,m (or D_{m,m Nom} ) for ¹⁶⁹ Yb-based conventional and intensity-modulated brachytherapy leads to a simultaneous overestimation (up to 4%) of CTV_HR D₉₀ and underestimation (up to 2%) of bladder D_2cc due to a significant dip in the mass-energy absorption ratios at the depths of nearby targets and OARs. Using a nominal mass-density assignment per structure, rather than a CT-derived voxel-by-voxel assignment for MRI-guided brachytherapy, amounts to a dose error up to 1% for all radionuclides considered.

CONCLUSIONS

The effects of the considered dose reporting schemes trend correspondingly between conventional and intensity-modulated brachytherapy. In the absence of CT-derived mass densities, MRI-only-based dosimetry can adequately approximate D_m,m by assigning nominal mass densities to structures. Tissue and applicator heterogeneities do not significantly impact dosimetry for ¹⁹² Ir and ⁷⁵ Se, but do for ¹⁶⁹ Yb; dose reporting must be explicitly defined since D_w,m and D_w,w may overstate the dosimetric benefits.

Clinical Implication of Dosimetry Formalisms for Electronic Low-Energy Photon Intraoperative Radiation Therapy

PURPOSE

Intraoperative radiation therapy (IORT) using the INTRABEAM, a miniature x-ray source, has shown to be effective in treating breast cancer. However, recent investigations have suggested a significant deviation between the reported and delivered doses. In this work, the dose delivered by INTRABEAM in the TARGIT breast protocol was investigated, along with the dose from the Xoft Axxent, another source used in breast IORT.

METHODS AND MATERIALS

The absorbed dose from the INTRABEAM was determined from ionization chamber measurements using: (a) the manufacturer-recommended formula (Zeiss V4.0 method), (b) a Monte Carlo calculated chamber conversion factor (C_Q method), and (c) the formula consistent with the TARGIT breast protocol (TARGIT method). The dose from the Xoft Axxent was determined from ionization chamber measurements using the Zeiss V4.0 method and calculated using the American Association of Physicists in Medicine TG-43 formalism.

RESULTS

For a nominal TARGIT prescription of 20 Gy, the dose at the INTRABEAM applicator surface ranged from 25.2 to 31.7 Gy according to the C_Q method for the largest (5 cm) and smallest (1.5 cm) diameter applicator, respectively. The Zeiss V4.0 method results were 7% to 10% lower (23.2 to 28.6 Gy). At 1 cm depth, the C_Q and Zeiss V4.0 absorbed doses were also larger than those predicted by the TARGIT method. The dose at 1 cm depth from the Xoft Axxent for a surface dose of 20 Gy was slightly less than INTRABEAM (3%-7% compared with C_Q method). An exception was for the 3 cm applicator, where the Xoft dose was appreciably lower (31%).

CONCLUSIONS

The doses delivered in the TARGIT breast protocol with INTRABEAM were significantly greater than the prescribed 20 Gy and depended on the size of spherical applicator used. Breast IORT treatments with the Xoft Axxent received less dose compared with TARGIT INTRABEAM, which could have implications for studies comparing clinical outcomes between the 2 devices.

Monte Carlo dosimetric characterization of a new high dose rate 169 Yb brachytherapy source and independent verification using a multipoint plastic scintillator detector

PURPOSE

A prototype 169 Yb source was developed in combination with a dynamic rotating platinum shield system (AIM-Brachy) to deliver intensity modulated brachytherapy (IMBT). The purpose of this study was to evaluate the dosimetric characteristics of the bare/shielded 169 Yb source using Monte Carlo (MC) simulations and perform an independent dose verification using a dosimetry platform based on a multipoint plastic scintillator detector (mPSD).

METHODS

The TG-43U1 dosimetric parameters were calculated for the source model using RapidBrachyMCTPS. Real-time dose rate measurements were performed in a water tank for both the bare/shielded source using a custom remote afterloader. For each dwell position, the dose rate was independently measured by the three scintillators (BCF-10, BCF-12, and BCF-60). For the bare source, dose rate was measured at distances up to 3 cm away from the source over a range of 7 cm along the catheter. For the shielded source, measurements were performed with the mPSD placed at 1 cm from the source at four different azimuthal angles ( 0 ∘ , 9 0 ∘ , 18 0 ∘ , and 27 0 ∘ ).

RESULTS

The dosimetric parameters were tabulated for the source model. For the bare source, differences between measured and calculated along-away dose rates were generally below 5-10%. Along the transverse axis, deviations were, on average (range), 3.3% (0.6-6.2%) for BCF-10, 1.7% (0.9-2.9%) for BCF-12, and 2.2% (0.3-4.4%) for BCF-60. The maximum dose rate reduction due to shielding at a radial distance of 1 cm was 88.8 ± 1.2%, compared to 83.5 ± 0.5% as calculated by MC.

CONCLUSIONS

The dose distribution for the bare/shielded 169 Yb source was independently verified using mPSD with good agreement in regions close to the source. The 169 Yb source coupled with the partial-shielding system is an effective technique to deliver IMBT.

Model-Based Dose Calculation Algorithms for Brachytherapy Dosimetry

The purpose of this study was to review the limitations of dose calculation formalisms for photon-emitting brachytherapy sources based on the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) report and to provide recommendations to transition to model-based dose calculation algorithms. Additionally, an overview of these algorithms and approaches is presented. The influence of tissue and seed/applicator heterogeneities on brachytherapy dose distributions for breast, gynecologic, head and neck, rectum, and prostate cancers as well as eye plaques and electronic brachytherapy treatments were investigated by comparing dose calculations based on the TG-43 formalism and model-based dose calculation algorithms.

Feasibility of a GATE Monte Carlo platform in a clinical pretreatment QA system for VMAT treatment plans using TrueBeam with an HD120 multileaf collimator

Purpose

To evaluate the quality of patient‐specific complicated treatment plans, including commercialized treatment planning systems (TPS) and commissioned beam data, we developed a process of quality assurance (QA) using a Monte Carlo (MC) platform. Specifically, we constructed an interface system that automatically converts treatment plan and dose matrix data in digital imaging and communications in medicine to an MC dose‐calculation engine. The clinical feasibility of the system was evaluated.

Materials and Methods

A dose‐calculation engine based on GATE v8.1 was embedded in our QA system and in a parallel computing system to significantly reduce the computation time. The QA system automatically converts parameters in volumetric‐modulated arc therapy (VMAT) plans to files for dose calculation using GATE. The system then calculates dose maps. Energies of 6 MV, 10 MV, 6 MV flattening filter free (FFF), and 10 MV FFF from a TrueBeam with HD120 were modeled and commissioned. To evaluate the beam models, percentage depth dose (PDD) values, MC calculation profiles, and measured beam data were compared at various depths (D_max, 5 cm, 10 cm, and 20 cm), field sizes, and energies. To evaluate the feasibility of the QA system for clinical use, doses measured for clinical VMAT plans using films were compared to dose maps calculated using our MC‐based QA system.

Results

A LINAC QA system was analyzed by PDD and profile according to the secondary collimator and multileaf collimator (MLC). Values for MC calculations and TPS beam data obtained using CC13 ion chamber (IBA Dosimetry, Germany) were consistent within 1.0%. Clinical validation using a gamma index was performed for VMAT treatment plans using a solid water phantom and arbitrary patient data. The gamma evaluation results (with criteria of 3%/3 mm) were 98.1%, 99.1%, 99.2%, and 97.1% for energies of 6 MV, 10 MV, 6 MV FFF, and 10 MV FFF, respectively.

Conclusions

We constructed an MC‐based QA system for evaluating patient treatment plans and evaluated its feasibility in clinical practice. We observed robust agreement between dose calculations from our QA system and measurements for VMAT plans. Our QA system could be useful in other clinical settings, such as small‐field SRS procedures or analyses of secondary cancer risk, for which dose calculations using TPS are difficult to verify.