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Advances in Radiotherapy
& Nuclear Medicine Software impact in Ho dosimetry
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results. Such standardization is crucial for achieving These findings reinforce that precise dosimetric
reproducible outcomes. assessment in radionuclide therapy requires more than
The findings of this study highlight the urgent advanced software—it depends critically on the calibration
need for standardization in post-therapeutic imaging strategy, the acquisition protocol, and the interaction
protocols, particularly in therapies involving high-activity between administered activity and camera performance.
radionuclides such as Ho. One of the central observations Clinical decisions based on inaccurate dose estimates
166
was that camera dead time effects—and the resulting may lead to suboptimal treatment or undue risk to the
166
underestimation of absorbed doses—were strongly patient. Therefore, centers performing Ho-TARE must
associated with the timing of SPECT/CT acquisition implement calibration protocols that account for system
relative to the administered activity. To mitigate this, it non-linearities, especially when imaging is performed
may be beneficial for institutions to define upper activity shortly after high-activity administrations.
thresholds beyond which early imaging should be Future work should aim at establishing standardized
deferred. Based on our gamma camera characterization, calibration procedures and acquisition schedules tailored
we identified a 20% count loss threshold corresponding to administered activity levels, ideally supported by
to approximately 727.5 MBq; activity levels above this phantom studies and multi-center validation.
point require either delayed acquisition or compensation
through patient-specific calibration. To effectively translate our findings into clinical
practice, the implementation of patient-specific calibration
In clinical settings where immediate imaging is protocols must follow a structured, reproducible, and
operationally favored, a standardized protocol could involve technically feasible pathway. The process begins with the
stratifying acquisition timing based on the administered thorough characterization of the gamma camera’s response
activity. For example, therapies exceeding 3.5 GBq could to 166 Ho over a wide range of activities, including the
be scheduled for imaging ≥36 h post-injection, while lower identification of non-linear behavior due to dead time.
activity cases may be imaged within 24 h. In parallel, the Static SPECT acquisitions of a decaying source allow the
development of dynamic calibration workflows—either construction of a count rate versus activity curve, which
through automated software modules or phantom-based can be fitted using a paralyzable detector model. This
response modeling—could allow real-time adaptation of model yields both the linear response coefficient and the
the CF according to actual imaging conditions. system’s dead time, enabling the definition of an upper
Furthermore, establishing multi-center guidelines that activity threshold—such as the 20% count loss level
include camera-specific dead time characterization and (~727 MBq in our system)—beyond which quantification
standard dose reconstruction settings would enhance without correction becomes unreliable.
reproducibility and facilitate inter-center comparisons. In the clinical setting, once the actual activity at the time
Incorporating such protocols into European or of SPECT/CT acquisition is estimated for each patient, the
international nuclear medicine practice guidelines (e.g. by corresponding sensitivity can be derived from the slope of
the EANM or SNMMI) would support harmonized the response curve at that point. A patient-specific CF is then
adoption. Ultimately, personalized dosimetry will only calculated and applied during image reconstruction and
reach its full potential if supported by equally personalized
and standardized imaging workflows. dose mapping. For clinical feasibility, this workflow should
be integrated into software platforms as an automated or
5. Conclusion semi-automated module and supported by clear standard
operating procedures, staff training, and periodic phantom
This study demonstrates that significant discrepancies in validation. Such integration ensures consistency and safety,
post-therapeutic dosimetry using 166 Ho-TARE can arise even when logistical constraints favor early post-therapy
from differences in calibration methodology and the imaging. Furthermore, harmonization across centers
timing of image acquisition. When using a fixed CF derived would facilitate inter-institutional comparability and
from phantom measurements at low activity, voxel-based improve the reliability of multicenter dosimetric studies.
dosimetry performed with Hermia software systematically
underestimated liver and tumor absorbed doses due to Acknowledgments
dead time effects and underestimation/saturation of the
gamma camera. By applying a patient-specific CF derived None.
from the system’s response curve, these underestimations Funding
were corrected, resulting in close alignment with
Q-Suite-generated dose maps. None.

Volume 3 Issue 3 (2025) 62 doi: 10.36922/ARNM025220023