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Advances in Radiotherapy
& Nuclear Medicine Software impact in Ho dosimetry
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1. Introduction The transarterial radioembolization using
holmium-166 microspheres ( Ho-TARE), also known as
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Primary liver cancer is the sixth most diagnosed cancer selective internal radiation therapy, has been a promising
1
worldwide. Nevertheless, secondary liver malignancies approach for treating primary and secondary unresectable
are more common because the liver has a distinctive dual liver cancer, particularly in patients unresponsive to other
blood supply system from both the portal vein and the treatments. This procedure involves injecting radioactive
3
hepatic artery, which significantly increases the likelihood microspheres into specific branches of the hepatic artery
of metastatic deposits. 2
through a microcatheter usually placed in the femoral
Over the past decade, the emergence of personalized artery. The microspheres tend to selectively lodge in the
4
dosimetry has transformed the landscape of radionuclide capillary bed of tumors. As Ho is a beta emitter, it emits
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therapies, especially in the context of transarterial radiation that targets the tumor cells while minimizing
radioembolization (TARE). Traditional empirical damage to the healthy liver tissue. 5,6
approaches, in which fixed administered activities A small number of Ho-microspheres, with activities
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were used irrespective of patient-specific factors, are 7
increasingly being replaced by voxel-based techniques that ranging from 200 to 250 MBq, is used in the “scout phase.”
enable three-dimensional absorbed dose estimation at the A post-scout SPECT/computed tomography (SPECT/
organ and lesion levels. These methods allow for a more CT) scan allows the assessment of the microspheres’
accurate assessment of therapeutic efficacy and potential distribution within the liver and the exclusion of any extra-
toxicity, thereby supporting truly individualized treatment hepatic leakages, guiding the “treatment phase” planning.
planning. In the absence of clinical and dosimetric contraindications,
a higher therapeutic activity, usually ranging from 2 to
Among the isotopes used in TARE, yttrium-90 ( Y) 6 GBq, is administered to replicate the microspheres
90
has historically been the most widespread, due to its distribution from the scout. Subsequently, a post-therapy
established clinical protocols and availability. However, SPECT/CT scan is performed to assess the absorbed doses
holmium-166 ( Ho) offers distinct advantages, including in the whole-liver and the tumor(s).
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paramagnetic properties that enable magnetic resonance
imaging (MRI)-based imaging and a low-energy gamma The 166 Ho isotope is a high-energy beta-emitting
emission suitable for single-photon emission computed radioactive isotope (maximum energy: 1.85 MeV,
3
tomography (SPECT) quantification. These features abundance 49%) with a half-life of 26, 8 h. It also emits low-
make 166 Ho particularly well-suited for post-treatment energy gamma rays at 80.6 keV (abundance 6.6%), which
verification of microsphere distribution. Nevertheless, enable the visualization of microspheres’ distribution
its complex emission spectrum, shorter half-life, and through SPECT. 5,8,9 During a SPECT acquisition, the
higher initial count rates present unique challenges for detectors rotate around the patient, recording 2D
quantitative imaging. projection images of the 3D activity distribution from
various angles.
Accurate dose estimation is critical in clinical decision-
making, as underestimation may lead to insufficient tumor Conventional SPECT/CT scanner detectors have a limit
control, while overestimation could result in unnecessary on the photon count rate that they can record. If the count
toxicity to healthy liver tissue or adjacent organs. The rate exceeds this limit, the camera may not accurately
precision of voxel-based dosimetry is thus not merely an detect all photons, resulting in underestimation or/and
academic concern, but a key factor in determining eligibility saturation due to dead time (τ) effects. 10,11 Although the
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for therapy, predicting treatment outcomes, and improving abundance of the 80.6 keV Ho photopeak is only 6.6%,
patient safety. This underscores the importance of robust the count rate immediately after treatment is significantly
quantification protocols, appropriate calibration methods, high due to the substantial amount of Bremsstrahlung
-
and harmonized imaging workflows across institutions. resulting from β interactions within the patient along with
the presence of multiple gamma emissions in the MeV
While 90 Y has long been the standard in 166 5
radioembolization, Ho offers several unique advantages, range of the Ho spectrum.
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including simultaneous SPECT and MRI capabilities. The rise of radionuclide therapy, like Ho-TARE, has
However, these advantages are offset by greater sensitivity sparked a growing interest in quantitative SPECT/CT and
to acquisition parameters, particularly dead time effects subsequent dosimetry. 12,13 Therefore, the determination of
and energy spectrum complexity. Thus, the need for the calibration factor (CF), which is required to convert
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robust, validated dosimetric approaches tailored to Ho SPECT image counts into radioactivity concentration units
characteristics are of importance. (MBq/mL), plays a critical role in activity quantification. 14,15

Volume 3 Issue 3 (2025) 56 doi: 10.36922/ARNM025220023