Dosimetric performance of Strut-Adjusted Volume Implant: A new single-entry multicatheter breast brachytherapy applicator

Validation of the TOPAS Monte Carlo toolkit for HDR brachytherapy simulations

PURPOSE

The goal of this work is to validate the user-friendly Geant4-based Monte Carlo toolkit TOol for PArticle Simulation (TOPAS) for brachytherapy applications.

METHODS AND MATERIALS

Brachytherapy simulations performed with TOPAS were systematically compared with published TG-186 reference data. The photon emission energy spectrum, the air-kerma strength, and the dose-rate constant of the model-based dose calculation algorithm (MBDCA)-WG generic Ir-192 source were extracted. For dose calculations, a track-length estimator was implemented. The four Joint AAPM/ESTRO/ABG MBDCA-WG test cases were evaluated through histograms of the local and global dose difference volumes. A prostate, a palliative lung, and a breast case were simulated. For each case, the dose ratio map, the histogram of the global dose difference volume, and cumulative dose-volume histograms were calculated.

RESULTS

The air-kerma strength was (9.772 ± 0.001) × 10-8 U Bq-1 (within 0.3% of the reference value). The dose-rate constant was 1.1107 ± 0.0005 cGy h-1 U-1 (within 0.01% of the reference value). For all cases, at least 96.9% of voxels had a local dose difference within [-1%, 1%] and at least 99.9% of voxels had a global dose difference within [-0.1%, 0.1%]. The implemented track-length estimator scorer was more efficient than the default analog dose scorer by a factor of 237. For all clinical cases, at least 97.5% of voxels had a global dose difference within [-1%, 1%]. Dose-volume histograms were consistent with the reference data.

CONCLUSIONS

TOPAS was validated for high-dose-rate brachytherapy simulations following the TG-186 recommended approach for MBDCAs. Built on top of Geant4, TOPAS provides broad access to a state-of-the-art Monte Carlo code for brachytherapy simulations.

Inter-fractional variations in the dosimetric parameters of accelerated partial breast irradiation using a strut-adjusted volume implant

The aim of the study was to evaluate inter-fractional dosimetric variations for high-dose rate breast brachytherapy using a strut-adjusted volume implant (SAVI). For the nine patients included, dosimetric constraints for treatment were as follows: for the planning target volume for evaluation (PTV_Eval), the volume receiving 90, 150 and 200% of the prescribed dose (V90%,150%,200%) should be >90%, ≤50 cm3 and ≤20 cm3, respectively; the dose covering 1 cm3 (D1cc) of the organs at risk should be ≤110% of the prescribed dose; and the air volume should be ≤10% of PTV_Eval. Differences in V90%,150%,200%, D1cc and air volume ($\Delta V$ and $\Delta D$) as inter-fractional dosimetric variations and SAVI displacements were measured with pretreatment and planning computed tomography (CT) images. Inter-fractional dosimetric variations were analyzed for correlations with the SAVI displacements. The patients were divided into two groups based on the distance of the SAVI from the surface skin to assess the relationship between the insertion position of the SAVI and dosimetric parameters. The median ΔV90%,150%,200% for the PTV_Eval in all patients was -0.3%, 0.2 cm3 and 0.2 cm3, respectively. The median (range) ΔD1cc for the chest wall and surface skin was -0.8% (-18.9 to 9.4%) and 0.3% (-7.6 to 5.3%), respectively. SAVI displacement did not correlate with inter-fractional dosimetric variations. In conclusion, the dose constraints were satisfied in most cases. However, there were inter-fractional dosimetric changes due to SAVI displacement.

Evolution of brachytherapy treatment planning to deterministic radiation transport for calculation of cardiac dose

Brachytherapy was among the first methods of radiotherapy and has steadily continued to evolve. Here we present a brief review of the progression of dose calculation methods in brachytherapy to the current state-of-the art computerized methods for heterogeneity correction. We further review the origin and development of the BrachyVision (Varian Medical Systems, Inc., Palo Alto, CA) treatment planning system and evaluate dosimetric results from 12 patients implanted with the strut-assisted volumetric implant (SAVI) applicator (Cianna Medical, Aliso Viejo, CA) for accelerated partial breast irradiation (APBI). Dosimetric results from plans calculated using homogenous and heterogeneous algorithms have been compared to investigate the impact of heterogeneity corrections. Our study showed large percent difference between mean cardiac doses 11.8 ± 6.2% (p = 0.0007) calculated with and without heterogeneity corrections. Our findings are consistent with those of others, indicating an overestimation of the distal dose to organs-at-risk by traditional methods, especially at interfaces between air and tissue.

Efficiency of using the day-of-implant CT for planning of SAVI APBI

PURPOSE

The purpose of the study was to develop an optimized, efficient workflow for using the day-of-implant (DOI) CT for treatment planning of accelerated partial breast irradiation brachytherapy using the strut-adjusted volume implant (SAVI) device.

METHODS AND MATERIALS

For 62 consecutive SAVI patients, a DOI CT was acquired and used for treatment planning. A "verification" CT was acquired 24-72 h after implant and immediately before the first fraction, then registered to the DOI CT. If the DOI CT-based plan was no longer optimal, a replan was performed. An array of metrics describing the geometry of the device and its relative position in the patient from the DOI CTs for these patients was collected. These metrics from the DOI CT were evaluated to determine what features could predict for the need to replan before the first treatment fraction. Logistical regression analysis including χ² tests was used to determine if different factors correlated with replanning.

RESULTS

Twenty-two of 62 patients (35%) required replanning. Only the presence of splayed struts, where splay was toward the skin, and the use of a nine strut ("8-1") SAVI were significantly correlated (p < 0.05) with replanning. Within these individual populations, no additional factors showed a significant statistical correlation for requiring replanning.

CONCLUSIONS

For strut-based accelerated partial breast irradiation brachytherapy, it was feasible to treat with a plan based on the DOI CT for a majority (65%) of patients. Some factors correlate to needing replanning; recognizing these could be used to optimize treatment workflow for certain patients, increasing clinical efficiency while enhancing the quality of patient care.

Clinical implementation of a novel Double-Balloon single-entry breast brachytherapy applicator

PURPOSE

The purpose of the study was to describe the clinical utilization of a novel Double-Balloon applicator for accelerated partial breast irradiation (APBI).

METHODS AND MATERIALS

The Double-Balloon single-entry breast applicator contains a single central treatment catheter, as well as four peripheral catheters that can be differentially loaded to customize radiation dose coverage. An inner balloon is filled with up to 7-30 cm³ of saline to increase separation between the peripheral catheters, and an outer balloon is filled with up to 37-115 cm³ of saline to displace breast tissue from the peripheral catheters. Treatment planning objectives include coverage of the breast planning target volume to a minimum of V₉₀ > 90%, limiting dose heterogeneity such that V₂₀₀ < 10 cm³ and V₁₅₀ < 50 cm³, and limiting maximum dose to skin (<100% of prescription dose) and ribs (<145% of prescription dose).

RESULTS

High-dose-rate APBI was delivered to 11 women using this device (34 Gy in 10 twice daily fractions). The mean V₉₀ was 98.2% (range 94.2-99.4%). The mean skin D_max with the Double-Balloon applicator was 83.3% (range 75.6-99.5%). The mean breast V₂₀₀ was 5.8 cm³ (range 2.3-10.2 cm³), and the mean breast V₁₅₀ was 32.9 cm³ (range 25.0-41.7 cm³). Pretreatment quality assurance was performed using CT prior to each morning fraction and ultrasound prior to each afternoon fraction.

CONCLUSIONS

The Double-Balloon applicator can be easily introduced into a previously existing brachytherapy program. APBI plans created with this applicator achieve excellent planning target volume coverage, while limiting skin dose and maintaining breast V₂₀₀ < 10 cm³.

Validation of the Oncentra Brachy Advanced Collapsed cone Engine for a commercial (192)Ir source using heterogeneous geometries

PURPOSE

To validate the Advanced Collapsed cone Engine (ACE) dose calculation engine of Oncentra Brachy (OcB) treatment planning system using an (192)Ir source.

METHODS AND MATERIALS

Two levels of validation were performed, conformant to the model-based dose calculation algorithm commissioning guidelines of American Association of Physicists in Medicine TG-186 report. Level 1 uses all-water phantoms, and the validation is against TG-43 methodology. Level 2 uses real-patient cases, and the validation is against Monte Carlo (MC) simulations. For each case, the ACE and TG-43 calculations were performed in the OcB treatment planning system. ALGEBRA MC system was used to perform MC simulations.

RESULTS

In Level 1, the ray effect depends on both accuracy mode and the number of dwell positions. The volume fraction with dose error ≥2% quickly reduces from 23% (13%) for a single dwell to 3% (2%) for eight dwell positions in the standard (high) accuracy mode. In Level 2, the 10% and higher isodose lines were observed overlapping between ACE (both standard and high-resolution modes) and MC. Major clinical indices (V100, V150, V200, D90, D50, and D2cc) were investigated and validated by MC. For example, among the Level 2 cases, the maximum deviation in V100 of ACE from MC is 2.75% but up to ~10% for TG-43. Similarly, the maximum deviation in D90 is 0.14 Gy between ACE and MC but up to 0.24 Gy for TG-43.

CONCLUSION

ACE demonstrated good agreement with MC in most clinically relevant regions in the cases tested. Departure from MC is significant for specific situations but limited to low-dose (<10% isodose) regions.

Comparative dosimetric findings using accelerated partial breast irradiation across five catheter subtypes

Purpose

Accelerated partial breast irradiation (APBI) with balloon and strut adjusted volume implants (SAVI) show promising results with excellent tumor control and minimal toxicity. Knowing the factors that contribute to a high skin dose, rib dose, and D₉₅ coverage may reduce toxicity, improve tumor control, and help properly predict patient outcomes following APBI.

Methods and materials

A retrospective analysis of 594 patients treated with brachytherapy based APBI at a single institution from May 2008 to September 2014 was grouped by applicator subtype. Patients were treated to a total of 34 Gy (3.4 Gy x 10 fractions over 5 days delivered BID) targeting a planning target volume (PTV) 1.0 cm beyond the lumpectomy cavity using a high dose rate source.

Results

SAVI devices had the lowest statistically significant values of D_maxSkin (81.00 ± 29.83), highest values of D₉₀ (101.50 ± 3.66), and D₉₅ (96.09 ± 4.55). SAVI-mini devices had the lowest statistically significant values of D_maxRib (77.66 ± 32.92) and smallest V₁₅₀ (18.01 ± 3.39). Multi-lumen balloons were able to obtain the smallest V₂₀₀ (5.89 ± 2.21). Strut-based applicators were more likely to achieve a D_maxSkin and a D_maxRib less than or equal to 100 %. The effect of PTV on V₁₅₀ showed a strong positive relationship (p < .001). PTV and D_maxSkin showed a weak negative relationship in multi-lumen applicators (p = .016) and SAVI-mini devices (p < .001). PTV and D_maxRib showed a weak negative relationship in multi-lumen applicators (p = .009), SAVI devices (p < .001), and SAVI-mini devices (p < .001).

Conclusion

PTV volume is strongly correlated with V₁₅₀ in all devices and V₂₀₀ in strut based devices. Larger PTV volumes result in greater V₁₅₀ and V₂₀₀, which could help predict potential risks for hotspots and resulting toxicities in these devices. PTV volume is also weakly negatively correlated with max skin dose and max rib dose, meaning that as the PTV volumes increase one can expect slightly smaller max skin and rib doses. Strut based applicators are significantly more effective in keeping skin and rib dose constraints under 125 and 100 % when compared to any balloon based applicator.

Recent developments and best practice in brachytherapy treatment planning

Brachytherapy has evolved over many decades, but more recently, there have been significant changes in the way that brachytherapy is used for different treatment sites. This has been due to the development of new, technologically advanced computer planning systems and treatment delivery techniques. Modern, three-dimensional (3D) imaging modalities have been incorporated into treatment planning methods, allowing full 3D dose distributions to be computed. Treatment techniques involving online planning have emerged, allowing dose distributions to be calculated and updated in real time based on the actual clinical situation. In the case of early stage breast cancer treatment, for example, electronic brachytherapy treatment techniques are being used in which the radiation dose is delivered during the same procedure as the surgery. There have also been significant advances in treatment applicator design, which allow the use of modern 3D imaging techniques for planning, and manufacturers have begun to implement new dose calculation algorithms that will correct for applicator shielding and tissue inhomogeneities. This article aims to review the recent developments and best practice in brachytherapy techniques and treatments. It will look at how imaging developments have been incorporated into current brachytherapy treatment and how these developments have played an integral role in the modern brachytherapy era. The planning requirements for different treatments sites are reviewed as well as the future developments of brachytherapy in radiobiology and treatment planning dose calculation.