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Advances in Radiotherapy
& Nuclear Medicine Software impact in Ho dosimetry
166

C(A) = α × A × e –ταA To correct the Hermia dose map, a systematic approach

Where C is the measured count rate, A is the activity at was employed based on the gamma camera response curve
the time of acquisition, α represents the detector’s linear and its derivative, which represents the system’s sensitivity
response coefficient, and τ denotes the detector dead time. to activity. The underlying assumption was that the camera’s
sensitivity is directly proportional to the CF, such that:
The fit was enforced to pass through the origin to
determine the linear response coefficient and estimate the CF = k × s
new
maximum activity level by identifying where a 20% count Here, s denotes the sensitivity, defined as the derivative
loss occurs, as illustrated in Figure 4. of the count rate with respect to activity, and k is a
5
The curve fitting analysis demonstrated a linear proportionality constant. This constant was determined
response coefficient of 8.1 ± 0.2 cps/MBq and a dead time using the known CF of the Jaszczak phantom by evaluating
of 3.10 ± 0.03 × 10 s. The determination coefficient (R ) the derivative of the count rate at the phantom’s activity
−5
2
was 0.998. The calculated activity value for a 20% count level.
loss amounted to 727.5 MBq. dC
s e A 1 A
To determine the waiting period for the SPECT/ dA
CT acquisition after administering the treatment, the
administered activity was set between 2 and 6 GBq based New CFs were determined by multiplying the constant
on the patients’ sample. The waiting time varied between with the activity’s derivative at the imaging time. These
1.6 and 3.4 days. However, in practice, this waiting period updated CFs were then utilized to generate new SPECT/CT
was not followed. Instead, the SPECT/CT scans were always reconstructions, which were subsequently employed to
performed the next day for all patients, resulting in a time create new dose maps.
interval usually <24 h. Most patient’s activities at the time Thereafter, another statistical comparison was made
of imaging exceeded the previously determined maximum comparing the CF-corrected doses (when feasible) with
activity value of 727.5 MBq (ranging from 0.7 to 4.1 GBq). the Q-suite results (Figure 5).
Therefore, the previous calculated CF using a uniformly Applying the correction resulted in mean doses of
filled Jaszczak phantom with a 600 MBq activity, which was 42 ± 6 Gy for the liver and 196 ± 17 Gy for the tumor,
used for the patients’ SPECT/CT image reconstruction, closely matching the Q-Suite outcomes (p=0.69 for liver,
was invalid for higher activities in the therapeutic range. p=0.64 for tumor). The new D , D , and D values were
This was evident as the count rate no longer showed a 50 70 85
linear relationship with the activity, leading to a decline in
the sensitivity of the gamma camera, as indicated by the
changing slope in Figure 4.

Figure 4. Variation of measured count rate with 166 Ho microspheres Figure 5. Hermia Voxel Dosimetry and Q-Suite mean liver doses (top
activity. The solid line represents the linear model, while the dashed graph) and mean tumor doses (bottom graph) for each patient along
blue line represents the paralyzable dead time model. The activity value with the administered activity, including the Hermia correction results.
correspondent to 20% count loss is 727.5 MBq (dashed red line). Patients no. 2 and 13 had two target lesions.

Volume 3 Issue 3 (2025) 60 doi: 10.36922/ARNM025220023