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Advances in Radiotherapy
& Nuclear Medicine Software impact in Ho dosimetry
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170 ± 68 Gy, 125 ± 44 Gy, and 83 ± 26 Gy, significantly maximum activity level at the time of imaging, which was
higher than the non-CF-corrected values. estimated to be 727.5 MBq. This allows for the calculation
of the minimum time interval needed to wait before post-
4. Discussion treatment SPECT/CT acquisition. For this specific patient

Dose estimation in Hermia was considerably cohort, this time interval could last up to 3 days. However,
diminished due to the general used CF, which was these values can vary significantly depending on the
assumed to be valid for all patients regardless of the administered activity. Therefore, Hermia’s dose estimation
activity administered and the time of image acquisition problems could have been reduced if more time had been
after therapy. Q-Suite’s data were obtained calculating a given for the administered activity to decay.
patient-specific CF, which did not depend on the response Figure 4 illustrates how sensitivity and the CF decrease
of the gamma camera. Instead of relying on count-based due to dead time effects. The next step was to use this
sensitivity, the software assumes that the total administered curve not only to correct the initial dose maps but also
activity is correctly distributed according to the geometric to enable the calculation of an accurate and reliable dose
pattern of the microspheres visible in the image. The CF is map without the need to wait for days to perform the
then adjusted to match this assumed distribution with the SPECT/CT acquisition, considering hospital logistical
known injected activity. challenges.
Undoubtedly, the primary difference in the absorbed It is essential to note that the correction method was
dose calculation using these two software tools lies in the not applicable to all patients, as the fitting curve remained
considered CF. This factor is essential as it enables the 1
conversion of the count rate to activity concentration, valid only for activity values below , which in this
facilitating accurate activity and dose quantification. system corresponds to approximately 4 GBq. At this
The clinical implications of underestimating the threshold, the count rate reaches its maximum; beyond it,
absorbed dose, particularly in the tumor region, are as activity continues to increase, the count rate paradoxically
far-reaching. In the context of 166 Ho-TARE, treatment begins to decrease due to dead-time effects. This introduces
planning decisions often rely on achieving a minimum ambiguity, as a single count rate value could correspond to
threshold absorbed dose to the tumor—commonly two different activity levels. As a result, the relationship
150 Gy—to justify proceeding with therapy. If voxel-based between count rate and activity is no longer one-to-one,
dosimetry significantly underestimates the true dose, as and the function loses its injectivity. Since the CF
observed with non-corrected Hermia data, patients may calculation depends on a strictly increasing (i.e., positive
be incorrectly deemed ineligible for treatment or wrongly derivative) relationship between count rate and activity,
classified as having received subtherapeutic dosing. This the correction method becomes unreliable beyond this
misclassification not only affects immediate clinical point. To ensure accuracy, the correction is only applied
decisions but may also compromise outcome prediction, within the range where the response curve remains
response assessment, and long-term follow-up strategies. monotonic and unambiguous.
Conversely, underestimation of liver doses might result As a result, for patients whose SPECT/CT acquisitions
in a false sense of safety, potentially exposing patients were performed with activities near this threshold, the
to unexpected hepatotoxicity. Therefore, accurate dose correction could not be applied. In these cases, the corrected
quantification is not merely an academic or technical CFs would yield exceptionally low values, resulting in
concern—it is a critical component of patient safety and unrealistically high-dose values within the dose maps. This
treatment efficacy in nuclear medicine therapy. is why only six patients are included in Figure 5.
Thus, determining the maximum activity value of each As previously noted, the differences between the
individual system becomes crucial primarily because the CF-corrected Hermia’s mean doses and Q-Suite results
gamma camera’s response is affected by the dead time were not statistically significant. Therefore, further research
effect and given the high-count rate after treatment. This is imperative to thoroughly understand this challenge.
ensures the optimal timing for image acquisition following This investigation should aim to determine whether the
the administration of the microspheres. proposed explanation for the underestimated dose maps
At low activity levels, where dead time effects are in Hermia Voxel Dosimetry is the primary concern and
negligible, a clear linear relationship between Ho activity to evaluate the accuracy and reliability of the correction
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and count rate is observed (Figure 4). Within this range, method employed. In this context, standardizing the
the camera’s sensitivity and the CF can be considered parameters used for image conversion and processing is
constant. The end of the linear regime corresponds to the essential to ensure accuracy and software-independent

Volume 3 Issue 3 (2025) 61 doi: 10.36922/ARNM025220023