Response and toxicity prediction is essential to the implementation of Peptide
Receptor Radionuclide Therapy (PRRT) for neuroendocrine tumors. Radiolabelled
somatostatin analogue [177Lu-DOTA0
, Tyr3
] octreotate stands as a promising therapy
tool. Specific dosimetry is a crucial factor in patient treatment planning.
In our Institution, neuroendocrine tumor treatment, by radiopeptide infusion via
intrahepatic arterial catheterization, is a well established technique. Kidney protective
agents are also included in our protocol. The individualized patient dosimetry
calculations were based on planar and SPECT scintigraphy images. Counts were
determined in a region of interest (ROI) around the tumor, liver, kidneys and spleen.
In planar technique, the ROIs were drawn in both anterior and posterior images while
in SPECT counts measured per slice. For counts conversion to activities, calibration
factors were calculated. Planar and SPECT images of cylindrical water - filled
phantom, with five different known amounts of activity, were obtained. Corrections
for scatter attenuation, collimator efficiency and detector response were calculated.
Absorbed doses were calculated using MIRD formalism and S values were calculated
using the RADAR system.
The absorbed doses to organ per unity administered activity were comparable for
both planar and SPECT techniques. On average, the absorbed dose was in tumor [4-
40] mGy/ MBq, in kidneys[0.25-1.05] mGy/ MBq, in the spleen [0.3-2.1] mGy/ MBq
and in the liver [0.05-0.34] mGy/ MBq.
In order to deliver higher dose to tumor and avoid kidneys and red marrow toxicity,
accurate individualized dosimetry is obligatory. Furthermore, the results
quantitatively confirm the therapeutic efficacy of transhepatic administration and
introduce 177Lu labeled peptide as an ideal for peptide receptor radiotherapy.