Quantitative CT assessment of a novel direction-modulated brachytherapy tandem applicator

A Novel Workflow with a Customizable 3D Printed Vaginal Template and a Direction Modulated Brachytherapy (DMBT) Tandem Applicator for Adaptive Interstitial Brachytherapy of the Cervix

A novel clinical workflow utilizing a direction modulated brachytherapy (DMBT) tandem applicator in combination with a patient-specific, 3D printed vaginal needle-track template for an advanced image-guided adaptive interstitial brachytherapy of the cervix. The proposed workflow has three main steps: (1) pre-treatment MRI, (2) an initial optimization of the needle positions based on the DMBT tandem positioning and patient anatomy, and a subsequent inverse optimization using the combined DMBT tandem and needles, and (3) rapid 3D printing. We retrospectively re-planned five patient cases for two scenarios; one plan with the DMBT tandem (T) and ovoids (O) with the original needle (ND) positions (DMBT + O + ND) and another with the DMBT T&O and spatially reoptimized needles (OptN) positions (DMBT + O + OptN). All retrospectively reoptimized plans have been compared to the original plan (OP) as well. The accuracy of 3D printing was verified through the image registration between the planning CT and the CT of the 3D-printed template. The average difference in D_2cc for the bladder, rectum, and sigmoid between the OPs and DMBT + O + OptNs were -8.03 ± 4.04%, -18.67 ± 5.07%, and -26.53 ± 4.85%, respectively. In addition, these average differences between the DMBT + O + ND and DMBT + O + OptNs were -2.55 ± 1.87%, -10.70 ± 3.45%, and -22.03 ± 6.01%, respectively. The benefits could be significant for the patients in terms of target coverage and normal tissue sparing and increase the optimality over free-hand needle positioning.

Metal artifact reduction in cervix brachytherapy with titanium applicators using dual-energy CT through virtual monoenergetic images and an iterative algorithm: A phantom study

PURPOSE

To evaluate an iterative metal-artifact reduction (iMAR) algorithm, dual-energy CT (DECT) through virtual monoenergetic images (VMI), and a combination of iMAR and DECT for reducing metal artifact severity (AS) induced by Fletcher titanium applicators used in cervix brachytherapy, the efficacy of which are hitherto unreported.

METHODS AND MATERIALS

120 kV_p single-energy CT (SECT) (Siemens) of BEBIG tandem applicators, varying in shape (straight or curved) and diameter (3.5 mm or 5 mm) in a custom-made water-filled phantom, and their DECT images obtained from extrapolation of 80 kVp and 140 kVp, were reconstructed using four methods: DECT through VMI±iMAR, and SECT±iMAR. The DECT images were reconstructed monoenergetically at 70, 150, and 190 keV. AS was evaluated using measured values and statistical analysis.

RESULTS

iMAR, DECT, and combined DECT and iMAR reduced AS (p < 0.05). DECT had a lower AS than SECT, even without iMAR (p < 0.025). SECT+iMAR was more effective than DECT-iMAR with VMI at 70 and 190 keV (p < 0.05), whereas showing no statistically significant difference at 150 keV. With DECT and iMAR combined, AS was reduced more effectively compared to the SECT+iMAR or DECT alone. It also reduced the mean interobserver uncertainty by 0.2 mm.

CONCLUSIONS

These findings indicate that iMAR reduces the AS caused by Fletcher titanium applicators for both SECT and DECT, a combination of iMAR and DECT is superior to either strategy alone, and at low energies, DECT+iMAR also produces similar artifact reduction. These practical strategies promise more accurate source-position and structure definitions in CT-based gynecological brachytherapy treatment planning.

Contemporary image-guided cervical cancer brachytherapy: Consensus imaging recommendations from the Society of Abdominal Radiology and the American Brachytherapy Society

PURPOSE

To present recommendations for the use of imaging for evaluation and procedural guidance of brachytherapy for cervical cancer patients.

METHODS

An expert panel comprised of members of the Society of Abdominal Radiology Uterine and Ovarian Cancer Disease Focused Panel and the American Brachytherapy Society jointly assessed the existing literature and provide data-driven guidance on imaging protocol development, interpretation, and reporting.

RESULTS

Image-guidance during applicator implantation reduces rates of uterine perforation by the tandem. Postimplant images may be acquired with radiography, computed tomography (CT), or magnetic resonance imaging (MRI), and CT or MRI are preferred due to a decrease in severe complications. Pre-brachytherapy T2-weighted MRI may be used as a reference for contouring the high-risk clinical target volume (HR-CTV) when CT is used for treatment planning. Reference CT and MRI protocols are provided for reference.

CONCLUSIONS

Image-guided brachytherapy in locally advanced cervical cancer is essential for optimal patient management. Various imaging modalities, including orthogonal radiographs, ultrasound, computed tomography, and magnetic resonance imaging, remain integral to the successful execution of image-guided brachytherapy.

Emerging technologies in brachytherapy

Brachytherapy is a mature treatment modality. The literature is abundant in terms of review articles and comprehensive books on the latest established as well as evolving clinical practices. The intent of this article is to part ways and look beyond the current state-of-the-art and review emerging technologies that are noteworthy and perhaps may drive the future innovations in the field. There are plenty of candidate topics that deserve a deeper look, of course, but with practical limits in this communicative platform, we explore four topics that perhaps is worthwhile to review in detail at this time. First, intensity modulated brachytherapy (IMBT) is reviewed. The IMBT takes advantage ofanisotropicradiation profile generated through intelligent high-density shielding designs incorporated onto sources and applicators such to achieve high quality plans. Second, emerging applications of 3D printing (i.e. additive manufacturing) in brachytherapy are reviewed. With the advent of 3D printing, interest in this technology in brachytherapy has been immense and translation swift due to their potential to tailor applicators and treatments customizable to each individual patient. This is followed by, in third, innovations in treatment planning concerning catheter placement and dwell times where new modelling approaches, solution algorithms, and technological advances are reviewed. And, fourth and lastly, applications of a new machine learning technique, called deep learning, which has the potential to improve and automate all aspects of brachytherapy workflow, are reviewed. We do not expect that all ideas and innovations reviewed in this article will ultimately reach clinic but, nonetheless, this review provides a decent glimpse of what is to come. It would be exciting to monitor as IMBT, 3D printing, novel optimization algorithms, and deep learning technologies evolve over time and translate into pilot testing and sensibly phased clinical trials, and ultimately make a difference for cancer patients. Today's fancy is tomorrow's reality. The future is bright for brachytherapy.

A review of brachytherapy physical phantoms developed over the last 20 years: clinical purpose and future requirements

Within the brachytherapy community, many phantoms are constructed in-house, and less commercial development is observed as compared to the field of external beam. Computational or virtual phantom design has seen considerable growth; however, physical phantoms are beneficial for brachytherapy, in which quality is dependent on physical processes, such as accuracy of source placement. Focusing on the design of physical phantoms, this review paper presents a summary of brachytherapy specific phantoms in published journal articles over the last twenty years (January 1, 2000 – December 31, 2019). The papers were analyzed and tabulated by their primary clinical purpose, which was deduced from their associated publications.

A substantial body of work has been published on phantom designs from the brachytherapy community, but a standardized method of reporting technical aspects of the phantoms is lacking. In-house phantom development demonstrates an increasing interest in magnetic resonance (MR) tissue mimicking materials, which is not yet reflected in commercial phantoms available for brachytherapy. The evaluation of phantom design provides insight into the way, in which brachytherapy practice has changed over time, and demonstrates the customised and broad nature of treatments offered.

A novel minimally invasive dynamic-shield, intensity-modulated brachytherapy system for the treatment of cervical cancer

PURPOSE

To present a novel, MRI-compatible dynamicshield intensity modulated brachytherapy (IMBT) applicator and delivery system using ¹⁹² Ir, ⁷⁵ Se, and ¹⁶⁹ Yb radioisotopes for the treatment of locally advanced cervical cancer. Needle-free IMBT is a promising technique for improving target coverage and organs at risk (OAR) sparing.

METHODS AND MATERIALS

The IMBT delivery system dynamically controls the rotation of a novel tungsten shield placed inside an MRI-compatible, 6-mm wide intrauterine tandem. Using 36 cervical cancer cases, conventional intracavitary brachytherapy (IC-BT) and intracavitary/interstitial brachytherapy (IC/IS-BT) (10Ci ¹⁹² Ir) plans were compared to IMBT (10Ci ¹⁹² Ir; 11.5Ci ⁷⁵ Se; 44Ci ¹⁶⁹ Yb). All plans were generated using the Geant4-based Monte Carlo dose calculation engine, RapidBrachyMC. Treatment plans were optimized then normalized to the same high-risk clinical target volume (HR-CTV) D₉₀ and the D_2cc for bladder, rectum, and sigmoid in the research brachytherapy planning system, RapidBrachyMCTPS. Plans were renormalized until either of the three OAR reached dose limits to calculate the maximum achievable HR-CTV D₉₀ and D₉₈ .

RESULTS

Compared to IC-BT, IMBT with either of the three radionuclides significantly improves the HR-CTV D₉₀ and D₉₈ by up to 5.2% ± 0.3% (P < 0.001) and 6.7% ± 0.5% (P < 0.001), respectively, with the largest dosimetric enhancement when using ¹⁶⁹ Yb followed by ⁷⁵ Se and then ¹⁹² Ir. Similarly, D_2cc for all OAR improved with IMBT by up to 7.7% ± 0.6% (P < 0.001). For IC/IS-BT cases, needle-free IMBT achieved clinically acceptable plans with ¹⁶⁹ Yb-based IMBT further improving HR-CTV D₉₈ by 1.5% ± 0.2% (P = 0.034) and decreasing sigmoid D_2cc by 1.9% ± 0.4% (P = 0.048). Delivery times for IMBT are increased by a factor of 1.7, 3.3, and 2.3 for ¹⁹² Ir, ⁷⁵ Se, and ¹⁶⁹ Yb, respectively, relative to conventional ¹⁹² Ir BT.

CONCLUSIONS

Dynamic shield IMBT provides a promising alternative to conventional IC- and IC/IS-BT techniques with significant dosimetric enhancements and even greater improvements with intermediate energy radionuclides. The ability to deliver a highly conformal, OAR-sparing dose without IS needles provides a simplified method for improving the therapeutic ratio less invasively and in a less resource intensive manner.

Needle-free cervical cancer treatment using helical multishield intracavitary rotating shield brachytherapy with the 169 Yb Isotope

PURPOSE

To assess the capability of an intracavitary ¹⁶⁹ Yb-based helical multishield rotating shield brachytherapy (RSBT) delivery system to treat cervical cancer. The proposed RSBT delivery system contains a pair of 1.25 mm thick platinum partial shields with 45° and 180° emission angles, which travel in a helical pattern within the applicator.

METHODS

A helically threaded tandem applicator with a 45° tandem curvature containing a helically threaded catheter was designed. A 0.6 mm diameter ¹⁶⁹ Yb source with a length of 10.5 mm was simulated. A 37-patient treatment planning study, based on Monte Carlo dose calculations using MCNP5, was conducted with high-risk clinical target volumes (HR-CTVs) of 41.2-192.8 cm³ (average ± standard deviation of 79.9 ± 35.8 cm³ ). All patients were assumed to receive 25 fractions of 1.8 Gy of external beam radiation therapy (EBRT) before receiving 5 fractions of high-dose-rate brachytherapy (HDR-BT). For each patient, ¹⁹² Ir-based intracavitary (IC) HDR-BT, ¹⁹² Ir-based intracavitary/interstitial (IC/IS) HDR-BT using a hybrid applicator with eight IS needles, and ¹⁶⁹ Yb-based RSBT plans were generated.

RESULTS

For the IC, IC/IS, and RSBT treatment plans, 38%, 84%, and 86% of the plans, respectively, met the planning goal of an HR-CTV D₉₀ (minimum dose to hottest 90%) of 85 Gy_EQD2 (α/β = 10 Gy). Median (25th percentile, 75th percentile) treatment times for IC, IC/IS, and RSBT were 11.71 (6.62, 15.40) min, 68.00 (45.02, 80.02) min, and 25.30 (13.87, 35.39) min, respectively. ¹⁹² Ir activities ranging from 159.1-370 GBq (4.3-10 Ci) and ¹⁶⁹ Yb activities ranging from 429.2-999 GBq (11.6-27 Ci) were used, which correspond to the same clinical ranges of dose rates at 1 cm off-source-axis in water. Extra needle insertion and planning time beyond that needed for intracavitary-only approaches was accounted for in the IC/IS treatment time calculations.

CONCLUSION

¹⁶⁹ Yb-based RSBT for cervical cancer met the HR-CTV D₉₀ goal of 85 Gy in a greater percentage of the patients considered than IC/IS (86% vs 84%, respectively) and can reduce overall treatment time relative to IC/IS. ¹⁶⁹ Yb-based RSBT could be used to replace IC/IS in instances where IC/IS treatment is not available, especially in instances when HR-CTV volumes are ≥30 cm³ .

Dynamic Modulated Brachytherapy (DMBT) Balloon Applicator for Accelerated Partial Breast Irradiation

PURPOSE

To propose a novel high-dose-rate brachytherapy applicator for balloon-based dynamic modulated brachytherapy (DMBT) for accelerated partial breast irradiation (APBI) and to demonstrate its dosimetric advantage compared to the widely used Contura applicator.

METHODS AND MATERIALS

The DMBT balloon device consists of a fixed central channel enabling real-time, in vivo dosimetry and an outer motion-dynamic, adjustable-radius channel capable of moving to any angular position within the balloon. This design allows placement of dwell positions anywhere within the balloon volume, guaranteeing optimal placement and generation of the applicator and treatment plan, respectively. Thirteen clinical treatment plans for patients with early-stage breast cancer receiving APBI after lumpectomy using Contura were retrospectively obtained under institutional review board approval. New treatment plans were created by replacing the Contura with the DMBT device. DMBT plans were limited to 4 angular positions and an outer channel radius of 1.5 cm. The new plans were optimized to limit dose to ribs and skin while maintaining target coverage similar to that of the clinical plan.

RESULTS

Similar target coverage was obtained for the DMBT plans compared with clinical Contura plans. Across all patients the mean (standard deviation) reductions in D0.1 cc to the ribs and skin were 6.70% (6.28%) and 5.13% (6.54%), respectively. A threshold separation distance between the balloon surface and the organ at risk (OAR), below which dosimetric changes of greater than 5% were obtained, was observed to be 12 mm for ribs and skin. When both OARs were far from the balloon, DMBT plans were of similar quality to Contura plans, as expected.

CONCLUSIONS

This study demonstrates the superior ability of the APBI DMBT applicator to spare OARs while achieving target coverage comparable to current treatment plans, especially when in close proximity. The DMBT balloon may enable new modes of dynamic high-dose-rate treatment delivery and allow for ultrahypofractionated dose regimens to be safely used.

Efficient 169 Yb high-dose-rate brachytherapy source production using reactivation

PURPOSE

To present and quantify the effectiveness of a method for the efficient production of ¹⁶⁹ Yb high-dose-rate brachytherapy sources with 27 Ci activity upon clinical delivery, which have about the same dose rate in water at 1 cm from the source center as 10 Ci ¹⁹² Ir sources.

MATERIALS

A theoretical framework for ¹⁶⁹ Yb source activation and reactivation using thermal neutrons in a research reactor and ¹⁶⁸ Yb-Yb₂ O₃ precursor is derived and benchmarked against published data. The model is dependent primarily on precursor ¹⁶⁸ Yb enrichment percentage, active source volume of the active element, and average thermal neutron flux within the active source.

RESULTS

Efficiency gains in ¹⁶⁹ Yb source production are achievable through reactivation, and the gains increase with active source volume. For an average thermal neutron flux within the active source of 1 × 10¹⁴  n cm^-2  s^-1 , increasing the active source volume from 1 to 3 mm³ decreased reactor-days needed to generate one clinic-year of ¹⁶⁹ Yb from 256 days yr^-1 to 59 days yr^-1 , and 82%-enriched precursor dropped from 80 mg yr^-1 to 21 mg yr^-1 . A resource reduction of 74%-77% is predicted for an active source volume increase from 1 to 3 mm³ .

CONCLUSIONS

Dramatic cost savings are achievable in ¹⁶⁹ Yb source production costs through reactivation if active sources larger than 1 mm³ are used.

Modeling of the direction modulated brachytherapy tandem applicator using the Oncentra Brachy advanced collapsed cone engine

PURPOSE

The direction modulated brachytherapy (DMBT) magnetic resonance-compatible tandem applicator, made from a tungsten alloy rod, has six symmetric peripheral grooves, designed specifically to enhance intensity modulation capacity through achieving directional radiation dose profiles. In this work, the directional dose distributions of the DMBT tandem were modeled and calculated with the Oncentra Brachy advanced collapsed cone engine (ACE), which was validated against Monte Carlo (MC) calculations.

METHODS AND MATERIAL

The prototype 3D tandem applicator model was created for use in the Oncentra Brachy treatment planning system. The ¹⁹²Ir source was placed inside a DMBT tandem in one and six channels as a single dwell position (DP) per channel with the same index length, as well as 1 DP in a standard tandem. Dose distributions were calculated in a water medium by both ACE and MC and compared.

RESULTS

For 1DP/6DP inside the DMBT and 1DP inside the standard tandem, respectively, the mean dose differences were 3.5/3.3% and <2.8% with the range of 0.1%-6.5%/0.2%-5% and 0.1%-5%, between ACE and MC, respectively.

CONCLUSIONS

The DMBT tandem is successfully modeled in a commercial treatment planning system. The ACE algorithm is capable of accurately calculating highly directional dose distributions generated by a dense tungsten alloy contained within the DMBT tandem, with agreements achieved within <3.5%.

Direction modulated brachytherapy (DMBT) tandem applicator for cervical cancer treatment: Choosing the optimal shielding material

PURPOSE

To investigate the dose modulation capability of a novel MRI-compatible direction modulated brachytherapy (DMBT) tandem applicator design with various high-density shielding materials for brachytherapy treatment of cervical cancer. The shield materials that have been evaluated are tantalum (Ta), pure tungsten (W), gold (Au), rhenium (Re), osmium (Os), platinum (Pt), iridium (Ir), and W' tungsten alloy (95%W, 3.5%Ni, 1.5%Cu).

MATERIALS AND METHODS

The recently proposed six-channel DMBT tandem is composed of nonmagnetic tungsten alloy (W') rod with diameter of 5.4 mm and coated with 0.3-mm thick bio-safe plastic sheath. The tandem shielding material can, however, be individually replaced with various other shields to create directional radiation. Monte Carlo N-Particle (MCNP) code was used to calculate the three-dimensional (3D) dose distributions in a water phantom for an HDR 192 Ir (mHDR-v2) source inside each DMBT tandem with various shields and a plastic conventional tandem (Con.T). Then, the 3D dose distributions were imported into an in-house-coded inverse planning optimization algorithm to obtain optimal plans for 12 clinical cases chosen at random from the international RetroEMBRACE dataset involving conventional tandem and ring (Con.T&R) applicators. All plans generated by the DMBT tandem and ring (DMBT&R) with the tungsten alloy [DMBT(W')&R] were compared with the corresponding Con.T&R plans, to generate benchmark results. These benchmark results were then considered as reference plans for other shields performances. Plans were normalized to receive the same high-risk clinical target volume (CTVHR ) D90 . The D100 , D10 , and V100 for CTVHR , and D2cm3 for organs at risk (OARs) of bladder, sigmoid, and rectum were calculated and compared.

RESULTS

Transmission factor (TF), that is, the dose in the backside of the DMBT shield over that in the front opening, at a 5 cm distance, were 36.6%, 34.8%, 31.9%, 28.9%, 27.9%, 26.2%, 26.2%, and 25.5%, for Ta, W', W, Re, Au, Os, Pt, and Ir shields, respectively. On average, the CTVHR values for D100 , V100 , D10 were not significantly different across all DMBT&R shields and the Con.T&R plans (P > 0.219). For the D2cm3 , the benchmark results showed significant reductions (P < 0.03), that is, on average, -8.3% for bladder, -10.7% for rectum, and -10.1% for sigmoid, compared to the Con.T&R plans. However, the various shields showed little improvement from the tungsten alloy (W'), where on average, rectum (bladder) [sigmoid] D2cm3 were reduced by -1.32% (-0.85%) [-1.01%], -1.25% (-0.78%) [-0.91%], -1.22% (-0.75%) [-0.86%], -0.94% (-0.60%) [-0.70%], -0.84% (-0.51%) [-0.59%], and -0.38% (-0.24%) [-0.23%] for Ir, Pt, Os, Au, Re, and W shields, relative to the benchmark W' DMBT plans, respectively. These corresponding values for Ta increased by +0.28% (+0.08%) [+0.25%], respectively.

CONCLUSION

The Ir, Pt, Os, Au, Re, and W shielding materials, respectively, in descending order, lead to better OAR sparing than the DMBT(W')&R plans. However, the amount of improvement is limited and clinically insignificant. This finding suggests that the initial W' shield remains a suitable choice given the proven MR compatibility, for use in MR-guided adaptive brachytherapy of cervical cancer.