Rotating-shield brachytherapy for cervical cancer

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.

The population percentile allowance method for determining systematic spatial error tolerances for temporary intensity modulated brachytherapy

BACKGROUND

Multiple approaches are under development for delivering temporary intensity modulated brachytherapy (IMBT) using partially shielded applicators wherein the delivered dose distributions are sensitive to spatial uncertainties in both the applicator position and shield orientation, rather than only applicator position as with conventional high-dose-rate brachytherapy (HDR-BT). Sensitivity analyses to spatial uncertainties have been reported as components of publications on these emerging technologies, however, a generalized framework for the rigorous determination of the spatial uncertainty tolerances of dose-volume parameters is needed.

PURPOSE

To derive and present the population percentile allowance (PPA) method, a generalized mathematical and statistical framework to evaluate the tolerance of temporary IMBT approaches to spatial uncertainties in applicator position and shield orientation.

METHODS

A mathematical formalism describing geometric applicator position and shield orientation shifts was derived that supports straight and curved applicators and applies to serial and helical rotating shield brachytherapy (RSBT) and direction modulated brachytherapy (DMBT). The PPA method entails defining the percentage of a patient population receiving a given therapy that is, allowed to receive dose-volume errors in the target volume and specified organs at risk of a defined percentage or less, then determining what combinations of applicator position and shield orientation systematic errors would be expected to produce that outcome in the population. The PPA method was applied to the use case of multi-shield helical 169 Yb-based RSBT for cervical cancer, with 45° and 180° shield emission angles. A total of 37 cervical cancer patients were considered in the population, with average (± 1 standard deviation) HR-CTV volumes of 79 cm3  ± 37 cm3 and optimized baseline treatment plans (no spatial uncertainties applied) created for each patient to meet dose-volume requirements of 85 GyEQD2 (equivalent uniform dose in 2 Gy fraction), with D2cc tolerance doses of 90 GyEQD2 , 75 GyEQD2 , and 75 GyEQD2 for bladder, rectum, and sigmoid colon, respectively.

RESULTS

For the PPA requirement that 90% of cervical cancer patients receiving multi-shield helical RSBT could have a maximum dose-volume uncertainty of 10% for high-risk clinical target volume (HR-CTV) D90 (minimum dose to hottest 90%) and bladder, rectum, and sigmoid colon D2cc (minimum dose to hottest 2 cm3 ), the tolerance systematic applicator position and shield orientation uncertainties were approximately ± 1.0 mm and ± 4.25°, respectively. For ± 1.5 mm and ± 5° systematic applicator position and shield orientation tolerances, 90% of the patients considered would have a maximum dose-volume uncertainty of 12.8% or less.

CONCLUSION

The PPA method was formalized to determine the temporary IMBT spatial uncertainty tolerances that would be expected to result in an allowed percentage of a population of patients receiving relative dose-volume errors above a defined percentage. Multi-shield, helical 169 Yb-based RSBT for cervical cancer was evaluated and tolerances determined, which, if applied on each treatment fraction, would represent an extreme situation. The PPA method is applicable to a variety of temporary IMBT approaches and can be used to rigorously determine the design parameters for the delivery systems such as mechanical driver motor accuracy, shield angle backlash, applicator rotation, and applicator fixation stability.

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.

Integration of rotatable tandem applicator to conventional ovoid applicator toward complete framework of intensity modulated brachytherapy (IMBT) for cervical cancer

A new tandem applicator with tungsten shield for Ir-192 radiation source used in intra-cavitary brachytherapy (ICBT) enabled intensity modulated brachytherapy (IMBT) in cervical cancer treatment through fluence-modulation by rotating shield. Our previous work employed group-wise and element-wise sparsity constraints for plan optimization of tandem applicator to minimizes the number of activated angles and source dwell points for delivery efficiency. It, however, did not incorporate the ovoid applicators into the optimizing process, which is generally used to prevent cancer recurrence. To integrate ovoid applicators to the new tandem applicator, this work proposed a comprehensive framework that modifies 1) dose deposition matrix for inverse planning, and 2) plan optimizing algorithm. The dose deposition matrix was newly formulated by the Monte-Carlo simulated dose distribution for 10 positions of ovoid applicators, followed by combining those with tandem-associated dose deposition matrix. The plan optimizing algorithm decomposed entire elements into tandem and ovoid applicators, which were governed by different constraints adaptive to specified plan objectives. The integrated framework was compared against conventional ICBT, and IMBT with tandem only for three patients with asymmetric dose distributions. Integrated IMBT framework resulted in the most optimal plans. Including fluence-modulation by rotating-shield outperformed conventional ICBT in dose sparing to critical organs. Adopting ovoid applicators to the optimization yielded more conformal dose distribution around inferior, laterally expanded region of target volume. The resulting plans reduced D_5cc and D_2cc by 30.9% and 27.8% for critical organs over conventional ICBT, and by 20.6% and 21.5% for target volume over IMBT with tandem only.

Inverse treatment planning for an electronic brachytherapy system delivering anisotropic radiation therapy

An inverse radiation treatment planning algorithm for Sensus Healthcare's Sculptura^TM electronic brachytherapy system has been designed. The algorithm makes use of simulated annealing to optimize the conformation number (CN) of the treatment plan. The highly anisotropic dose distributions produced by the Sculptura^TM x-ray source empower the inverse treatment planning algorithm to achieve highly conformal treatment plans for a wide range of prescribed planning target volumes. Over a set of 10 datasets the algorithm achieved an average CN of 0.79 ± 0.08 and an average gamma passing rate of 0.90 ± 0.10 at 5%/5 mm. A regularization term that encouraged short treatment plans was used, and it was found that the total treatment time could be reduced by 20% with only a nominal reduction in the CN and gamma passing rate. It was also found that downsampling the voxelized volume (from 320³ to 64³ voxels) prior to optimization resulted in a 150× speedup in the optimization time (from 2 + minutes to < 1 s) without affecting the quality of the treatment plan.

Intensity modulated Ir-192 brachytherapy using high-Z 3D printed applicators

Gynecologic cancers are often asymmetric, yet current Ir-192 brachytherapy techniques provide only limited radial modulation of the dose. The shielded solutions investigated here solve this by providing the ability to modulate between highly asymmetric and radially symmetric dose distributions at a given location. To find applicator designs that can modulate between full dose and less than 50% dose, at the dimensions of the urethra, a 2D calculation algorithm was developed to narrow down the search space. Two shielding design types were then further investigated using Monte Carlo and Boltzmann-solver dose calculation algorithms. 3D printing techniques using ISO 10993 certified biocompatible plastics and 3D printable tungsten-loaded plastics were tested. It was also found that shadowing effects set by the shape of the shielding cannot be easily modulated out, hence careful design is required. The shielded applicator designs investigated here, allow for reduction of the dose by over 50% at 5 mm from the applicator surface in desired regions, while also allowing radially symmetric dose with isodose line deviations less than 0.5 mm from circular. The shielding designs were also chosen with treatment delivery time in mind. Treatment times for these shielded designs were found to be less than 1.4 times longer than a 6-channel unshielded cylinder for the equivalent fully symmetric dose distribution. The 2D calculation methods developed here provide a simple way to rapidly evaluate shielding designs, while the 3D printing techniques also allow for devices with novel shapes to be rapidly prototyped. Both TOPAS Monte Carlo and Acuros BV calculations show that significant dose shaping and organ at risk sparing can be achieved without significantly compromising the plan in regions that require the full dose.

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 GyEQD2 (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 .

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.

Fast dose optimization for rotating shield brachytherapy

PURPOSE

To provide a fast computational method, based on the proximal graph solver (POGS) - A convex optimization solver using the alternating direction method of multipliers (ADMM), for calculating an optimal treatment plan in rotating shield brachytherapy (RSBT). RSBT treatment planning has more degrees of freedom than conventional high-dose-rate brachytherapy due to the addition of emission direction, and this necessitates a fast optimization technique to enable clinical usage.

METHODS

The multi-helix RSBT (H-RSBT) delivery technique was investigated for five representative cervical cancer patients. Treatment plans were generated for all patients using the POGS method and the commercially available solver IBM ILOG CPLEX. The rectum, bladder, sigmoid colon, high-risk clinical target volume (HR-CTV), and HR-CTV boundary were the structures included in our optimization, which applied an asymmetric dose-volume optimization with smoothness control. Dose calculation resolution was 1 × 1 × 3 mm³ for all cases. The H-RSBT applicator had 6 helices, with 33.3 mm of translation along the applicator per helical rotation and 1.7 mm spacing between dwell positions, yielding 17.5° emission angle spacing per 5 mm along the applicator.

RESULTS

For each patient, HR-CTV D₉₀ , HR-CTV D₁₀₀ , rectum D_2cc , sigmoid D_2cc , and bladder D_2cc matched within 1% for CPLEX and POGS methods. Also, similar EQD2 values between CPLEX and POGS methods were obtained. POGS was around 18 times faster than CPLEX. For all patients, total optimization times were 32.1-65.4 s for CPLEX and 2.1-3.9 s for POGS.

CONCLUSIONS

POGS reduced treatment plan optimization time approximately 18 times for RSBT with similar HR-CTV D₉₀ , organ at risk (OAR) D_2cc values, and EQD2 values compared to CPLEX, which is significant progress toward clinical translation of RSBT.

Multihelix rotating shield brachytherapy for cervical cancer

PURPOSE

To present a novel brachytherapy technique, called multihelix rotating shield brachytherapy (H-RSBT), for the precise angular and linear positioning of a partial shield in a curved applicator. H-RSBT mechanically enables the dose delivery using only linear translational motion of the radiation source/shield combination. The previously proposed approach of serial rotating shield brachytherapy (S-RSBT), in which the partial shield is rotated to several angular positions at each source dwell position [W. Yang et al., "Rotating-shield brachytherapy for cervical cancer," Phys. Med. Biol. 58, 3931-3941 (2013)], is mechanically challenging to implement in a curved applicator, and H-RSBT is proposed as a feasible solution.

METHODS

A Henschke-type applicator, designed for an electronic brachytherapy source (Xoft Axxent™) and a 0.5 mm thick tungsten partial shield with 180° or 45° azimuthal emission angles and 116° asymmetric zenith angle, is proposed. The interior wall of the applicator contains six evenly spaced helical keyways that rigidly define the emission direction of the partial radiation shield as a function of depth in the applicator. The shield contains three uniformly distributed protruding keys on its exterior wall and is attached to the source such that it rotates freely, thus longitudinal translational motion of the source is transferred to rotational motion of the shield. S-RSBT and H-RSBT treatment plans with 180° and 45° azimuthal emission angles were generated for five cervical cancer patients with a diverse range of high-risk target volume (HR-CTV) shapes and applicator positions. For each patient, the total number of emission angles was held nearly constant for S-RSBT and H-RSBT by using dwell positions separated by 5 and 1.7 mm, respectively, and emission directions separated by 22.5° and 60°, respectively. Treatment delivery time and tumor coverage (D90 of HR-CTV) were the two metrics used as the basis for evaluation and comparison. For all the generated treatment plans, the D90 of the HR-CTV in units of equivalent dose in 2 Gy fractions (EQD2) was escalated until the D2cc (minimum dose to hottest 2 cm3) tolerance of either the bladder (90 Gy3), rectum (75 Gy3), or sigmoid colon (75 Gy3) was reached.

RESULTS

Treatment time changed for H-RSBT versus S-RSBT by -7.62% to 1.17% with an average change of -2.8%, thus H-RSBT treatments times tended to be shorter than for S-RSBT. The HR-CTV D90 also changed by -2.7% to 2.38% with an average of -0.65%.

CONCLUSIONS

H-RSBT is a mechanically feasible delivery technique for use in the curved applicators needed for cervical cancer brachytherapy. S-RSBT and H-RSBT were clinically equivalent for all patients considered, with the H-RSBT technique tending to require less time for delivery.

Paddle-based rotating-shield brachytherapy

PURPOSE

The authors present a novel paddle-based rotating-shield brachytherapy (P-RSBT) method, whose radiation-attenuating shields are formed with a multileaf collimator (MLC), consisting of retractable paddles, to achieve intensity modulation in high-dose-rate brachytherapy.

METHODS

Five cervical cancer patients using an intrauterine tandem applicator were considered to assess the potential benefit of the P-RSBT method. The P-RSBT source used was a 50 kV electronic brachytherapy source (Xoft Axxent™). The paddles can be retracted independently to form multiple emission windows around the source for radiation delivery. The MLC was assumed to be rotatable. P-RSBT treatment plans were generated using the asymmetric dose-volume optimization with smoothness control method [Liu et al., Med. Phys. 41(11), 111709 (11pp.) (2014)] with a delivery time constraint, different paddle sizes, and different rotation strides. The number of treatment fractions (fx) was assumed to be five. As brachytherapy is delivered as a boost for cervical cancer, the dose distribution for each case includes the dose from external beam radiotherapy as well, which is 45 Gy in 25 fx. The high-risk clinical target volume (HR-CTV) doses were escalated until the minimum dose to the hottest 2 cm(3) (D(2cm(3)) of either the rectum, sigmoid colon, or bladder reached their tolerance doses of 75, 75, and 90 Gy3, respectively, expressed as equivalent doses in 2 Gy fractions (EQD2 with α/β = 3 Gy).

RESULTS

P-RSBT outperformed the two other RSBT delivery techniques, single-shield RSBT (S-RSBT) and dynamic-shield RSBT (D-RSBT), with a properly selected paddle size. If the paddle size was angled at 60°, the average D90 increases for the delivery plans by P-RSBT on the five cases, compared to S-RSBT, were 2.2, 8.3, 12.6, 11.9, and 9.1 Gy10, respectively, with delivery times of 10, 15, 20, 25, and 30 min/fx. The increases in HR-CTV D90, compared to D-RSBT, were 16.6, 12.9, 7.2, 3.7, and 1.7 Gy10, respectively. P-RSBT HR-CTV D90-values were insensitive to the paddle size for paddles angled at less than 60°. Increasing the paddle angle from 5° to 60° resulted in only a 0.6 Gy10 decrease in HR-CTV D90 on average for five cases when the delivery times were set to 15 min/fx. The HR-CTV D90 decreased to 2.5 and 11.9 Gy10 with paddle angles of 90° and 120°, respectively.

CONCLUSIONS

P-RSBT produces treatment plans that are dosimetrically and temporally superior to those of S-RSBT and D-RSBT, although P-RSBT systems may be more mechanically challenging to develop than S-RSBT or D-RSBT. A P-RSBT implementation with 4-6 shield paddles would be sufficient to outperform S-RSBT and D-RSBT if delivery times are constrained to less than 15 min/fx.

Direction Modulated Brachytherapy for Treatment of Cervical Cancer. II: Comparative Planning Study With Intracavitary and Intracavitary-Interstitial Techniques

PURPOSE

To perform a comprehensive comparative planning study evaluating the utility of the proposed direction modulated brachytherapy (DMBT) tandem applicator against standard applicators, in the setting of image guided adaptive brachytherapy of cervical cancer.

METHODS AND MATERIALS

A detailed conceptual article was published in 2014. The proposed DMBT tandem applicator has 6 peripheral grooves of 1.3-mm width, along a 5.4-mm-thick nonmagnetic tungsten alloy rod of density 18.0 g/cm(3), capable of generating directional dose profiles. We performed a comparative planning study with 45 cervical cancer patients enrolled consecutively in the prospective observational EMBRACE study. In all patients, MRI-based planning was performed while utilizing various tandem-ring (27 patients) and tandem-ring-needles (18 patients) applicators, in accordance with the Groupe Européen de Curiethérapie-European Society for Radiotherapy and Oncology recommendations. For unbiased comparisons, all cases were replanned with an in-house-developed inverse optimization code while enforcing a uniform set of constraints that are reflective of the clinical practice. All plans were normalized to the same high-risk clinical target volume D90 values achieved in the original clinical plans.

RESULTS

In general, if the standard tandem was replaced with the DMBT tandem while maintaining all other planning conditions the same, there was consistent improvement in the plan quality. For example, among the 18 tandem-ring-needles cases, the average D2cm(3) reductions achieved were -2.48% ± 11.03%, -4.45% ± 5.24%, and -5.66% ± 6.43% for the bladder, rectum, and sigmoid, respectively. An opportunity may also exist in avoiding use of needles altogether for when the total number of needles required is small (approximately 2 to 3 needles or less), if DMBT tandem is used.

CONCLUSIONS

Integrating the novel DMBT tandem onto both intracavitary and intracavitary-interstitial applicator assembly enabled consistent improvement in the sparing of the OARs, over a standard "single-channel" tandem, though individual variations in benefit were considerable. Although at an early stage of development, the DMBT concept design is demonstrated to be useful and pragmatic for potential clinical translation.

Dynamic rotating-shield brachytherapy

PURPOSE

To present dynamic rotating shield brachytherapy (D-RSBT), a novel form of high-dose-rate brachytherapy (HDR-BT) with electronic brachytherapy source, where the radiation shield is capable of changing emission angles during the radiation delivery process.

METHODS

A D-RSBT system uses two layers of independently rotating tungsten alloy shields, each with a 180° azimuthal emission angle. The D-RSBT planning is separated into two stages: anchor plan optimization and optimal sequencing. In the anchor plan optimization, anchor plans are generated by maximizing the D90 for the high-risk clinical-tumor-volume (HR-CTV) assuming a fixed azimuthal emission angle of 11.25°. In the optimal sequencing, treatment plans that most closely approximate the anchor plans under the delivery-time constraint will be efficiently computed. Treatment plans for five cervical cancer patients were generated for D-RSBT, single-shield RSBT (S-RSBT), and (192)Ir-based intracavitary brachytherapy with supplementary interstitial brachytherapy (IS + ICBT) assuming five treatment fractions. External beam radiotherapy doses of 45 Gy in 25 fractions of 1.8 Gy each were accounted for. The high-risk clinical target volume (HR-CTV) doses were escalated such that the D2cc of the rectum, sigmoid colon, or bladder reached its tolerance equivalent dose in 2 Gy fractions (EQD2 with α∕β = 3 Gy) of 75 Gy, 75 Gy, or 90 Gy, respectively.

RESULTS

For the patients considered, IS + ICBT had an average total dwell time of 5.7 minutes∕fraction (min∕fx) assuming a 10 Ci(192)Ir source, and the average HR-CTV D90 was 78.9 Gy. In order to match the HR-CTV D90 of IS + ICBT, D-RSBT required an average of 10.1 min∕fx more delivery time, and S-RSBT required 6.7 min∕fx more. If an additional 20 min∕fx of delivery time is allowed beyond that of the IS + ICBT case, D-RSBT and S-RSBT increased the HR-CTV D90 above IS + ICBT by an average of 16.3 Gy and 9.1 Gy, respectively.

CONCLUSIONS

For cervical cancer patients, D-RSBT can boost HR-CTV D90 over IS + ICBT and S-RSBT without violating the tolerance doses to the bladder, rectum, or sigmoid. The D90 improvements from D-RSBT depend on the patient, the delivery time budget, and the applicator structure.

High resolution (3 Tesla) MRI-guided conformal brachytherapy for cervical cancer: consequences of different high-risk CTV sizes

Purpose

To evaluate conventional brachytherapy (BT) plans using dose-volume parameters and high resolution (3 Tesla) MRI datasets, and to quantify dosimetric benefits and limitations when MRI-guided, conformal BT (MRIG-CBT) plans are generated.

Material and methods

Fifty-five clinical high-dose-rate BT plans from 14 cervical cancer patients were retrospectively studied. All conventional plans were created using MRI with titanium tandem-and-ovoid applicator (T&O) for delivery. For each conventional plan, a MRIG-CBT plan was retrospectively generated using hybrid inverse optimization. Three categories of high risk (HR)-CTV were considered based on volume: non-bulky (< 2_{0 cc}), low-bulky (> 20 cc and < 40 cc) and bulky (≥ 40 cc). Dose-volume metrics of D₉₀ of HR-CTV and D_2cc and D_0.1cc of rectum, bladder, and sigmoid colon were analyzed.

Results

Tumor coverage (HR-CTV D₉₀) of the conventional plans was considerably affected by the HR-CTV size. Sixteen percent of the plans covered HR-CTV D₉₀ with the prescription dose within 5%. At least one OAR had D_2cc values over the GEC-ESTRO recommended limits in 52.7% of the conventional plans. MRIG-CBT plans showed improved target coverage for HR-CTV D₉₀ of 98 and 97% of the prescribed dose for non-bulky and low-bulky tumors, respectively. No MRIG-CBT plans surpassed the D_2cc limits of any OAR. Only small improvements (D₉₀ of 80%) were found for large targets (> 40 cc) when using T&O applicator approach.

Conclusions

MRIG-CBT plans displayed considerable improvement for tumor coverage and OAR sparing over conventional treatment. When the HR-CTV volume exceeded 40 cc, its improvements were diminished when using a conventional intracavitary applicator.