Comparison between the diagnostic accuracy of F-18 FDG PET/CT scan and Tc-99m MDP bone scintigraphy for detection of distant metastasis in extra-ocular retinoblastoma - a single centre initial experience

Introduction: The most common intra-ocular malignancy of childhood is retinoblastoma. Extra-ocular dissemination, albeit rare, is the most important cause of poor prognosis and death in these children, particularly in the developing countries. The most common sites of metastasis include central nervous system, bone and lymph nodes. The systemic work-up for suspected metastatic disease includes brain and orbital MRI with or without contrast, lumbar puncture for cerebrospinal fluid cytology, bone marrow aspiration/biopsy and bone scintigraphy. Detection of distant metastasis, is important, not only to prognosticate the patient, but also to determine the appropriate dose and schedule of the chemotherapeutic regime. To the best of our knowledge, the incremental role of F-18 FDG PET/CT scan, if any, over conventional Tc-99m MDP bone scintigraphy has not been studied in the available literature.

Methods: Patients of extra-ocular retinoblastoma underwent both Tc-99m MDP bone scintigraphy and F-18 FDG PET/CT scan either as a part of baseline staging or to rule out suspected recurrence. The scans were reviewed by two expert nuclear medicine physicians.

Results: A total of 8 patients ( aged between 1-4 years, 5 male and 3 female ) of extra-ocular retinoblastoma were evaluated in the study from May 2017 to December 2021. 5 patients were included in the retrospective arm and 3 patients in the prospective arm of the study respectively. Out of 8 patients, 4 patients underwent Tc-99m MDP bone scintigraphy and F-18 FDG PET/CT scans post primary treatment completion in view of suspected recurrence. The rest 4 underwent Tc-99m MDP bone scintigraphy and F-18 FDG PET/CT scan for baseline staging. Both Tc-99m MDP bone scintigraphy and F-18 FDG PET/CT scans were negative in 3 out of 8 patients. However, bone marrow aspiration and biopsy was positive in 1 of the above 3 patients. Tc-99m MDP bone scintigraphy and F-18 FDG PET/CT scans were positive for localised bone involvement in 2 of the 8 patients with concordant results. However, in 2 patients, Tc-99m MDP bone scintigraphy was negative while F-18 FDG PET/CT scan detected bony metastasis, which was further biopsy proven. In addition to the bony metastasis, additional intra-cranial and distant lymph nodal metastasis were also determined by the F-18 FDG PET/CT scan. In 1 patient, Tc-99m MDP bone scintigraphy identified a solitary bone lesion. F-18 FDG PET/CT scan performed in the same patient, identified involvement of multiple other bones, including intra-spinal extension, visceral and distant lymph nodal involvement which was missed by the Tc-99m MDP bone scintigraphy.

Conclusions: F-18 FDG PET/CT scan appears to demonstrate superior sensitivity over conventional Tc-99m MDP bone scintigraphy for detection of osseous metastasis in extra-ocular retinoblastoma. In addition, extra-osseous sites of metastasis can also be picked up by F-18 FDG PET/CT scan. Although, the results of this preliminary study seem promising, furthermore studies are required to validate the same.