Improving MRI-based dosimetry for holmium-166 transarterial radioembolization using a nonrigid image registration for voxelwise Δ R 2 ∗ $\Delta R_2^*$ calculation

Spatial distribution of fractionally administered holmium microspheres in non-tumorous human liver tissue: how livers survive transarterial radioembolisation

BACKGROUND

Relatively high mean absorbed doses to the non-tumorous liver tissue (NTLT) are generally well tolerated in transarterial radioembolisation (TARE), potentially due to a heterogeneous dose distribution. This study investigates the macroscopic and microscopic distribution of fractionally administered TARE holmium microspheres in NTLT using an experimental setup of ex vivo perfused human donor livers under magnetic resonance imaging (MRI), and validates these findings through a comparison with MRI data from TARE-treated patients.

RESULTS

MRI-based dose maps of the TARE-treated ex vivo livers and patients revealed a heterogeneous dose distribution pattern throughout the NTLT (heterogeneity index (HI) range 2.96-10.11). Microscopic analysis confirmed this, as a wide variation in the percentage of tissue within 2.1 mm of microspheres (5.4%-84.3%) was observed. Microspheres administered in consecutive fractions decreased the heterogeneity, which was observed macroscopically by a decreased HI, and microscopically by the formation of new microsphere clusters. However, this HI decrease appeared finite, and new clusters formed near existing clusters, maintaining the overall distribution pattern.

CONCLUSIONS

TARE induces a heterogeneous dose distribution pattern in human NTLT. This heterogeneous dose distribution pattern persists across additional microsphere fractions, leaving parts of the NTLT unexposed to lethal doses of ionising radiation. Combined with the regenerative capacity of the liver, this may explain why relatively high mean absorbed doses to the NTLT are generally well tolerated in TARE.

REGISTRATION

For validation purposes, clinical data from patients who participated in a previous study (ClinicalTrials.gov, identifier NCT04269499, registered on February 13, 2020) was analysed in the current study.

Development and validation of an innovative administration system to facilitate controlled holmium-166 microsphere administration during TARE

BACKGROUND

To develop and validate a novel administration device for holmium-166 transarterial radioembolisation (TARE) with the purpose of facilitating controlled fractional microsphere administration for a more flexible and image-guided TARE procedure.

METHODS

A Controlled Administration Device (CAD) was developed using MR-conditional materials. The CAD contains a rotating syringe to keep the microspheres in suspension during administration. Different rotational speeds were tested ex vivo to optimise the homogeneity of microsphere fractions administered from the device. The technical performance, accuracy, and safety was validated in three patients in a clinical TARE setting by administering a standard clinical dose in 5 fractions (identifier: NCT05183776). MRI-based dosimetry was used to validate the homogeneity of the given fractions in vivo, and serious adverse device event ((S)A(D)E) reporting was performed to assess safety of the CAD.

RESULTS

A rotational speed of 30 rpm resulted in the most homogeneous microsphere fractions with a relative mean deviation of 1.1% (range: -9.1-8.0%). The first and last fraction showed the largest deviation with a mean of -26% (std. 16%) and 7% (std. 13%). respectively. In the three patient cases the homogeneity of the microsphere fractions was confirmed given that MRI-based dosimetry showed near linear increase of mean absorbed target liver dose over the given fractions with R2 values of 0.98, 0.97 and 0.99. No (S)A(D)E's could be contributed to the use of the CAD.

CONCLUSIONS

The newly developed CAD facilitates safe and accurate fractional microsphere administration during TARE, and can be used for multiple applications in the current and future workflows of TARE.

Quantitative CT imaging and radiation-absorbed dose estimations of 166Ho microspheres: paving the way for clinical application

BACKGROUND

Microbrachytherapy enables high local tumor doses sparing surrounding tissues by intratumoral injection of radioactive holmium-166 microspheres (166Ho-MS). Magnetic resonance imaging (MRI) cannot properly detect high local Ho-MS concentrations and single-photon emission computed tomography has insufficient resolution. Computed tomography (CT) is quicker and cheaper with high resolution and previously enabled Ho quantification. We aimed to optimize Ho quantification on CT and to implement corresponding dosimetry.

METHODS

Two scanners were calibrated for Ho detection using phantoms and multiple settings. Quantification was evaluated in five phantoms and seven canine patients using subtraction and thresholding including influences of the target tissue, injected amounts, acquisition parameters, and quantification volumes. Radiation-absorbed dose estimation was implemented using a three-dimensional 166Ho specific dose point kernel generated with Monte Carlo simulations.

RESULTS

CT calibration showed a near-perfect linear relation between radiodensity (HU) and Ho concentrations for all conditions, with differences between scanners. Ho detection during calibration was higher using lower tube voltages, soft-tissue kernels, and without a scanner detection limit. The most accurate Ho recovery in phantoms was 102 ± 11% using a threshold of mean tissue HU + (2 × standard deviation) and in patients 98 ± 31% using a 100 HU threshold. Thresholding allowed better recovery with less variation and dependency on the volume of interest compared to the subtraction of a single HU reference value. Corresponding doses and histograms were successfully generated.

CONCLUSION

CT quantification and dosimetry of 166Ho should be considered for further clinical application with on-site validation using radioactive measurements and intra-operative Ho-MS and dose visualizations.

RELEVANCE STATEMENT

Image-guided holmium-166 microbrachytherapy currently lacks reliable quantification and dosimetry on CT to ensure treatment safety and efficacy, while it is the only imaging modality capable of quantifying high in vivo holmium concentrations.

KEY POINTS

Local injection of 166Ho-MS enables high local tumor doses while sparing surrounding tissue. CT enables imaging-based quantification and radiation-absorbed dose estimation of concentrated Ho in vivo, essential for treatment safety and efficacy. Two different CT scanners and multiple acquisition and reconstruction parameters showed near-perfect linearity between radiodensity and Ho concentration. The most accurate Ho recoveries on CT were 102 ± 11% in five phantoms and 98 ± 31% in seven canine patients using thresholding methods. Dose estimations and volume histograms were successfully implemented for clinical application using a dose point kernel based on Monte Carlo simulations.

Adjuvant holmium-166 radioembolization after radiofrequency ablation in early-stage hepatocellular carcinoma patients: a dose-finding study (HORA EST HCC trial)

PURPOSE

The aim of this study was to investigate the biodistribution of (super-)selective trans-arterial radioembolization (TARE) with holmium-166 microspheres (166Ho-MS), when administered as adjuvant therapy after RFA of HCC 2-5 cm. The objective was to establish a treatment volume absorbed dose that results in an absorbed dose of ≥ 120 Gy on the hyperemic zone around the ablation necrosis (i.e., target volume).

METHODS

In this multicenter, prospective dose-escalation study in BCLC early stage HCC patients with lesions 2-5 cm, RFA was followed by (super-)selective infusion of 166Ho-MS on day 5-10 after RFA. Dose distribution within the treatment volume was based on SPECT-CT. Cohorts of up to 10 patients were treated with an incremental dose (60 Gy, 90 Gy, 120 Gy) of 166Ho-MS to the treatment volume. The primary endpoint was to obtain a target volume dose of ≥ 120 Gy in 9/10 patients within a cohort.

RESULTS

Twelve patients were treated (male 10; median age, 66.5 years (IQR, [64.3-71.7])) with a median tumor diameter of 2.7 cm (IQR, [2.1-4.0]). At a treatment volume absorbed dose of 90 Gy, the primary endpoint was met with a median absorbed target volume dose of 138 Gy (IQR, [127-145]). No local recurrences were found within 1-year follow-up.

CONCLUSION

Adjuvant (super-)selective infusion of 166Ho-MS after RFA for the treatment of HCC can be administered safely at a dose of 90 Gy to the treatment volume while reaching a dose of ≥ 120 Gy to the target volume and may be a favorable adjuvant therapy for HCC lesions 2-5 cm.

TRIAL REGISTRATION

Clinicaltrials.gov NCT03437382 . (registered: 19-02-2018).