2011 Joint AAPM/COMP Meeting
Peter Roberson
For more information about the American Association of Physicists in Medicine, visit aapm.org/

Targeted radionuclide therapy (TRT) is becoming an increasingly important tool for therapy of
some cancers including non-Hodgkins lymphoma, thyroid cancer, and neuroendocrine tumors.
The dose delivered to neoplasms and normal tissues, and thus the therapeutic response and
incidence of toxicities, depends on the anatomy and physiology of the patient. Therefore, optimal
treatment planning requires estimating the dose distribution for each patient. The dose
distribution is estimated in two steps: measurement of the activity distribution of a planning dose
at a series of time points using nuclear medicine imaging methods and calculating the dose to
organs or voxels in the patient. State of the art dose estimation requires, as an input, estimates of
the 3D activity distribution of the planning dose in the patient at each time point.

The 3D activity distribution of a radionuclide can be estimatedusing the tomographic nuclear
medicine imaging modalities SPECT and PET. Since uptake times of TRT agents are typically
on the order of several days, radionuclides such as In-111 or I-131 (for SPECT) or I-124 or Y-86
(for PET) having longer half-lives are used for treatment planning. The SPECT radionuclides
emit medium or high-energy photons, which can make quantification of the SPECT images more
difficult and requires compensating for the collimator-detector response. The PET radionuclides
have prompt gamma emissions that can result in false coincidences, once again complicating
quantification. For both modalitiesattenuation and scatter compensation are essential.
Compensation for partial volume effects is important for dose-estimation in small objects such as
tumors.

This lecture will review the challenges and recent advances in methods for quantifying 3D
activity distributions from SPECT and PET imaging using radionuclides relevant for TRT. It will
describe the impact of these advances in terms of metrics relevant to dose estimation and
describe the levels of accuracy and precision that are obtainable with state-of-the-art methods.






Learning Objectives:
1. Understand the applications and requirements of quantitative SPECT and PET for
targeted radionuclide therapy.
2. Understand the factors that limit the quantitative reliability of SPECT images and how to
compensate for them.
3. Understand the factors that limit the quantitative reliability of PET.

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