Untreated expansion of cystic craniopharyngiomas can have significant consequences from mass effect including worsening headache, vision loss, and obstructive hydrocephalus. Conventional surgical treatments include attempted resection, typically via craniotomy. Less invasive aspiration of cystic contents only provides a temporary solution, with cysts tending to refill and expand after a single drainage.Reference Blackburn, Doughty and Plowman 1 The beneficial effects of intracavitary irradiation for treatment of cystic craniopharyngiomas as a means to abolish the secretory capability of the cyst’s epithelial lining, while sparing nearby critical brain structures, has been previously reported.Reference Julow 2 - Reference Vanhauwaert, Hallaert, Baert, Van Roost, Okito and Caemaert 4
90Yttrium colloid is considered a suitable isotope for treatment of cystic craniopharyngiomas because of its short half-life and pure β emission.Reference Blackburn, Doughty and Plowman 1 , Reference Julow 2 Intracavitary irradiation with stereotactically implanted 90yttrium colloid has been shown to be effective in long-term shrinkage of the cystic portion of recurrent craniopharyngiomas. A retrospective review of 78 patients over a 36-year period showed sustained cyst reduction in 63% of patients following 90yttrium colloid treatment, with 33% of cysts disappearing completely.Reference Julow 2 The administered dose is calculated based on delivering 200 Gy to the inner wall of the cyst.Reference Blackburn, Doughty and Plowman 1 , Reference Backlund, Johansson and Sarby 5 Although radiation delivery into the cyst using a stereotactically guided needle had been previously outlined in Pollack et al,Reference Pollock, Lunsford, Kondziolka, Levine and Flickinger 3 imaging techniques to demonstrate the ongoing intracystic radiation have not been previously described. Although Bremsstrahlung single-photon emission computed tomography/computed tomography (CT) can be used to image the activity distribution after administration of 90yttrium colloid, positron emission tomography (PET)-CT has become the preferred modality in the brain because of higher spatial and energy resolution .Reference Zade, Rangarajan and Purandare 6 In this report, we demonstrate the use of PET-CT imaging, fused to anatomic CT and magnetic resonance imaging (MRI), as a means of documenting the actual distribution of 90yttrium colloid delivered to the cystic craniopharyngioma.
METHODS
As part of a Health Canada Phase III clinical trial (NCT02081768), informed consent was obtained from patients treated with 90yttrium colloid for a suprasellar cystic craniopharyngioma (surgical methods described elsewhereReference Pollock, Lunsford, Kondziolka, Levine and Flickinger 3 ) to undergo PET-CT 24 to 48 hours postoperatively. PET-CT was performed on a Discovery STE16 camera (General Electric Medical Systems, Milwaukee, WI), to delineate the distribution of the 90yttrium colloid within the cyst. Although 90yttrium is primarily a beta emitter and does not directly emit radiation amenable to imaging, it decays to 89zirconium, which emits a positron/electron pair, making it possible to image the radioisotope with a PET-CT scanner. Images were coregistered with MRI done on the same day. PET imaging consisted of one bed position (15 cm) centered on the sella, in 3 dimensions, 20 minutes, Vue point algorithm, 128×128 matrix. Given the lack of anatomic reference on the PET scan, a low-dose CT scan was performed at the same time with a helical scan (0.8 seconds), 3.75-mm slice thickness, 140 kV, 95 mA, and 512×512 matrix. The reconstructed 90yttrium colloid PET voxels have a spatial resolution of approximately 12 mm full width half maximum; the spatial correspondence between the PET and CT scans is less than 5 mm, the approximate size of one PET voxel. The CT provided anatomical landmarks and was used to facilitate attenuation correction of the PET data. PET/CT and MRI scans were coregistered using Statistical Parametric Mapping software (SPM5, Wellcome Trust Centre for Neuroimaging) and fused images displayed using Rview (version 9.073, Colin Studholme).
RESULTS
The images shown in Figure 1 are from a patient treated with 90yttrium colloid for a suprasellar cystic craniopharyngioma as part of the trial. PET imaging data have been fused to both anatomical CT (top) and MRI (middle) scans.
FIGURE 1 PET/CT and PET/MRI fusions are presented for visualization of 90Yttrium colloid within a suprasellar cystic craniopharyngioma. The distribution and accumulation of radiopharmaceutical within the multilobulated cystic craniopharyngioma is visualized by the positron-emitting radioisotope properties of 90Yttrium colloid. Based on the 5.7cc volume of the cystic component of the craniopharyngioma, 50.0MBq of 90Yttrium colloid was injected into the cyst via a stereotactic transcranial approach. Top row: PET imaging data fused to CT (axial, coronal, and sagittal images). Middle row: PET imaging data fused to MRI. Bottom row: Corresponding MRI images.
CONCLUSIONS
We have used PET-CT imaging to characterize the distribution of 90yttrium colloid delivered to a cystic craniopharyngioma. Image fusion to anatomical imaging (CT and/or MRI) confirms successful injection into the target site, assesses the distribution of 90yttrium colloid within all of the cyst, and may provide a means to evaluate leakage of 90yttrium colloid. Further studies will determine whether PET/CT/MRI proves useful in predicting short- and long-term treatment responses.
ACKNOWLEDGMENTS
We acknowledge Ron Hill for his help with processing imaging data and Murray Hong, Jonathan Bower, Nichole Smith, and Christina Kelly for their help in preparations for the surgical procedure.
Disclosures
The authors do not have anything to disclose.
