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Background
The retromolar trigone is a small yet crucial anatomic crossroad and source of multidirectional tumor spread, due to its spatial relationship with multiple adjacent deep neck structures. The extent of deep tumour spread from retromolar trigone tumors is however often underestimated on clinical examination, and cross sectional imaging is therefore critical in delineating the extent of tumor involvement of the deep neck structures for disease staging and optimal treatment planning. Anatomy of this region is complex, especially to the unfamiliar and inexperienced reader. It is important to understand the multiple potential pathways of tumor spread, in order to avoid missing out on critical structures that may be involved by tumor. This would ensure a more complete tumor clearance both with surgery and radiotherapy. In this exhibit, we illustrate the spectrum of possible pathways of tumor spread to and from the retromolar trigone through nicely annotated images of real life cases, highlighting key anatomic structures and concepts in a concise yet easy to understand manner.

Educational Goals / Teaching Points
At the end of reviewing the exhibit, the reader should understand and identify the retromolar trigone as well as its adjacent key anatomic structures on CT/MRI imaging, understand that tumors of the retromolar trigone may be primary or may result from regional extension from other nearby sites such as the base of tongue or tonsil, understand and identify the multiple potential pathways of cancer spread from the retromolar trigone, identify osseous involvement by tumor, identify regional cervical lymphatic spread/nodal disease and identify key structures that can be involved in perineural spread by oral cavity or oropharyngeal cancers.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
This exhibit will describe a systematic step by step approach to evaluate the deep neck structures when the retromolar region is involved with tumors. There will be illustration of the multiple potential pathways of cancer spread to and from the retromolar trigone on CT/MRI imaging, including lateral spread from the retromolar trigone into the buccal space and buccinator muscle, medial spread to the masticator space and lateral tongue edge, superomedial spread along the pterygomandibular raphe to the soft palate, tonsillar fossa, and oronasopharynx, and caudal spread into the floor of the mouth. Key anatomic structures that are involved or spared by tumor will be clearly annotated in order to help the reader orientate, understand the complexity of this region, and ultimately accurately stage cancers.

Conclusion
The retromolar trigone is a complex yet critical anatomic crossroad for multidirectional tumor spread. Understanding and being able to identify the key adjacent anatomic structures that can be involved by tumor spread is of vital importance to optimal treatment planning and patient care.