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Keywords:

  • radioimmunotherapy;
  • iodine-131;
  • dosimetry;
  • imaging;
  • geometric mean;
  • scatter correction

Abstract

BACKGROUND

Activity in regions of interest can be measured using serial whole-body scintigraphic images to estimate the dose received by a patient after therapeutic injections. As scatter and attenuation introduce biases in quantitative measurements, these phenomena need to be corrected to allow accurate determination of tracer concentration.

METHODS

The feasibility of iodine-131 whole-body imaging in list mode was studied over an extended spectrum (0–750 keV) in order to compare four scatter correction methods by the geometric mean approach (20%, Dual Energy Window, Triple Energy Window, and Spectral Factor Analysis methods). All data were corrected for attenuation using a Transmission Attenuation Correction prototype from Sopha Médical Vision international. The half-life of an iodine-131 standard source was calculated from scatter-corrected anterior views. Whole-body activities, using the Day 0, Hour 1, image as a reference (calibration from an administered dose) and an external calibration source (calibration from an imaged known-activity source), were calculated for three patients undergoing a radioimmunotherapy trial in order to assess the reliability of quantification by the geometric mean approach.

RESULTS

Patient studies confirmed the clinical feasibility of this type of acquisition. As expected, all methods allowed determination of an accurate half-life for the calibration source. A slight impact of scatter correction was observed in quantification with calibration from an administered dose. For quantification with calibration from an imaged known-activity source, whole-body activity was overestimated by +100% to +200% with the 20% window, depending on the size of the patient, whereas errors were about +50% with scatter correction. However, the influence of patient morphology was less marked when a scatter correction method was used.

CONCLUSIONS

When the geometric mean approach is used together with a sophisticated transmission acquisition device for quantification with calibration from an administered dose, the 20% energy window appears to be adequate. However, for quantification with calibration from an imaged known-activity source, accurate activity estimates cannot be obtained even when scatter correction is used to compensate for the influence of patient morphology. Cancer 2002;94:1224–30. © 2002 American Cancer Society.

DOI 10.1002/cncr.10289