Radioembolization and the Dynamic Role of (90)Y PET/CT

Abstract

Before the advent of tomographic imaging, it was postulated that decay of (90) Y to the 0(+) excited state of (90)Zr may result in emission of a positron-electron pair. While the branching ratio for pair-production is small (~32 × 10(-6)), PET has been successfully used to image (90) Y in numerous recent patients and phantom studies. (90) Y PET imaging has been performed on a variety of PET/CT systems, with and without time-of-flight (TOF) and/or resolution recovery capabilities as well as on both bismuth-germanate and lutetium yttrium orthosilicate (LYSO)-based scanners. On all systems, resolution and contrast superior to bremsstrahlung SPECT has been reported. The intrinsic radioactivity present in LYSO-based PET scanners is a potential limitation associated with accurate quantification of (90) Y. However, intrinsic radioactivity has been shown to have a negligible effect at the high activity concentrations common in (90) Y radioembolization. Accurate quantification is possible on a variety of PET scanner models, with or without TOF, although TOF improves accuracy at lower activity concentrations. Quantitative (90) Y PET images can be transformed into 3-dimensional (3D) maps of absorbed dose based on the premise that the (90) Y activity distribution does not change after infusion. This transformation has been accomplished in several ways, although the most common is with the use of 3D dose-point-kernel convolution. From a clinical standpoint, (90) Y PET provides a superior post-infusion evaluation of treatment technical success owing to its improved resolution. Absorbed dose maps generated from quantitative PET data can be used to predict treatment efficacy and manage patient follow-up. For patients who receive multiple treatments, this information can also be used to provide patient-specific treatment-planning for successive therapies, potentially improving response. The broad utilization of (90) Y PET has the potential to provide a wealth of dose-response information, which may lead to development of improved radioembolization treatment-planning models in the future.

Keywords: 90 Y PET; post-treatment imaging; quantitative imaging; radioembolization; radioembolization dosimetry.

Figure 1

Transaxial profile of a ⁹⁰ Y line source in water with a FWHM of 3.1 mm.

Figure 2

Yttrium-90 PET/CT following radioembolization. Note the non-uniformity present in the hepatic parenchyma (arrow).

Figure 3

(A) Pre-treatment contrast enhanced CT imaging of metastatic cholangiocarcinoma in the left hepatic lobe. Areas of increased vascularity (arrow) are seen surrounding areas of necrosis. (B) Left hepatic angiogram during infusion of ⁹⁰ Y into left lobe. (C) Bremsstrahlung SPECT following radioembolization demonstrates diffuse activity in the region of the left lobe hepatic neoplasm. (D) ⁹⁰ Y PET/CT demonstrates more detailed information with multifocal areas of maximum activity corresponding to the peripheral viable tumor on pre-treatment imaging.

Figure 4

(A) ¹⁸FDG PET/CT showing large focal lesion from metastatic cholangiocarcinoma prior to radioembolization (arrow). This was the only conspicuous lesion in this patient’s left lobe. (B) ⁹⁰ Y PET/CT following radioembolization. Poor tumor uptake can be seen (T/N <1) with the majority of the dosage distributing to non-target areas of the left lobe.

Figure 5

(A) ⁹⁰ Y PET of a patient with HCC. Scan performed on Siemens BioGraph mCT Flow with TOF and RR. A continuous bed speed of 0.2 mm/s, 1 iteration, and 21 subsets were used for reconstruction. The contour (red) was performed by an automatic segmentation tool with the lower threshold set to an absorbed dose of 100 Gy. (B) ⁹⁰ Y PET of a patient with HCC. Scan performed on Siemens BioGraph mCT Flow with TOF, RR, and optimal respiratory gating (HD-Chest). A continuous bed speed of 0.2 mm/s, 1 iteration, and 21 subsets were used for reconstruction. The contour (red) was performed by an automatic segmentation tool with the lower threshold set to an absorbed dose of 100 Gy.