Trilateral retinoblastoma: A rare tumor involving the “third eye”

Clinical History

A 2 year-old girl initially presented with esotropia of the left eye. A left intraocular tumour was found on ophthalmologic exam. A brain MRI was performed and showed bilateral intraocular masses with calcifications and a pineal mass.

Imaging Findings

Multiplanar MRI of the brain and orbits with and without gadolinium was performed. The gradient images (Figure 1) show susceptibility artefacts within the bilateral intraocular masses indicating the presence of calcification. These heterogeneous masses are located in the posterior chamber without evidence of optic nerve involvement or extraorbital extension (Figures 2 and 3). A third lesion involving the "third eye" in the pineal region also shows a lobulated and heterogeneous mass without local invasion or evidence of CSF dissemination (Figure 4 and 5).

Discussion

Retinoblastoma is the most common intraocular malignancy of childhood with 95% of cases developing before age 5. All cases of bilateral retinoblastoma are caused by a germline mutation of the RB1 gene [1, 2]. Approximately 5% of these children also develop an additional midline neuroectodermal tumour, more commonly (75%) in the pineal region [2-4]. Since the pineal gland in some animals has vestigial retinal elements, it is sometimes referred to as the "central or third eye". Also, these midline tumours are similar to retinoblastoma as they both arise from primitive neuroectodermal tissue and hence described by some as trilateral retinoblastoma (TRb) when they occur together. Classic trilateral retinoblastoma is associated with poor prognosis having a 5-year survival rate of only 22% [2]. However, a case series of children with bilateral retinoblastoma who developed benign pineal neoplasms suggest that a benign variant of trilateral retinoblastoma exists and that it might have a better prognosis than classic TRb [5]. Over the last decade, survival has increased substantially with earlier detection coupled with advances in chemotherapy [2, 3, 6].

Magnetic Resonance Imaging (MRI) is the preferred modality for staging retinoblastoma prior to treatment. The retinal mass can show variable enhancement depending on the size of the tumor, associated calcifications and necrotic changes. When conservative management is contemplated, it is important to assess the optic nerve and the presence of extraocular tumor extension [7].

MRI findings of pineal involvement will show an enlarged gland with irregular enhancement and necrotic or cystic changes [5]. Although less common than germ cell tumours, primary pineal parenchymal tumors are mostly represented by pineocytomas and pineoblastomas. Pineoblastomas are more common in children and tend to be more aggressive with associated local invasion and often metastasis though the CSF. Pineocytomas on the other hand are usually slow growing and often non-invasive but can also metastasize.

This patient was found to have bilateral retinoblastoma and a pineal neoplasm on MRI compatible with trilateral retinoblastoma. She subsequently underwent enucleation of the left eye and laser resection of the right retinal tumour to spare the right eye, as well as removal of the pineal mass, followed by chemotherapy. Final histopathology of the pineal tumor demonstrated a benign pineocytoma. She is now 8 years old with 20/25 vision of her right eye.

Differential Diagnosis List

Final Diagnosis

Trilateral retinoblastoma

URL:	https://www.eurorad.org/case/13455
DOI:	10.1594/EURORAD/CASE.13455
ISSN:	1563-4086