The Influence of Homogeneous and Non-homogeneous 131I and 123I SPECT Activity Distributions on Patient-Specific Three-dimensional Dosimetry.

Neuroblastoma is the second most common solid tumour in young children and outcomes for patients with high-risk neuroblastoma remain poor. Functional and morphological imaging modalities have complimentary roles at various stages of disease assessment. Metaiodobenzylguanidine (MIBG) scintigraphy remains the leading functional modality in neuroblastoma evaluation because of its high
sensitivity, specificity, diagnostic accuracy, and prognostic value. I-123-MIBG scans have superior image quality and deliver a lower radiation dose to the patient than I-131-MIBG scans. However due to the longer half-life of I-131 in comparison to I-123 it may be questioned as to if the biological wash-out is
predicted accurately when using I-123-MIBG. Furthermore, the cost and availability of I-123 in many centres in Africa are problematic and it will be ideal if I-131 could be used for both the diagnostic and the therapeutic studies. If I-131 is used for the diagnostic studies with the purpose of dose planning for
therapeutic studies it is important to know what the I-131 quantification accuracy is in comparison to I-123 studies. The question also arises to what extend the dosimetry parameters will be influenced by a possible difference in quantification using I-131 vs I-123. Non-uniform activity distributions will lead to
corresponding non-uniformities in tissue-absorbed dose. The implications of non-uniform dose distributions can be significant and might contribute to treatment failure.

The aim of this study will be to determine the accuracy of tumour activity quantification as well as absorbed dose calculations for uniform vs non-uniform activity distribution for I-123 and I-131 by using patient-specific 3D methods for the absorbed dose calculation.