1. Muscular tumor spread along the posterior belly of the digastric muscle A unique route of extranodal oropharyngeal squamous cell carcinoma disease extension Xin Wu MD, Christine Glastonbury MBBS University of California, San Francisco eP 319

2. Disclosures The authors have nothing to disclose.

3. Background In oral cavity and oropharyngeal squamous cell cancers, level I and II cervical nodes are typically the first sites of nodal metastasis. The posterior belly of the digastric muscle separates these lymphatic levels and lies in close proximity to the jugulodigastric nodal group, potentially serving as a route of extranodal disease spread. (The entire digastric muscle is highlighted in red at right).

4. The posterior half of the digastric (blue) arises from mastoid notch, medial to the mastoid process The muscle belly courses between the submandibular space and carotid space, thus coursing between submandibular and jugular chain lymph nodes (purple), enlarged in this case. Normal course and MRI appearance of the posterior half of the digastric muscle Please press FORWARD to start the animation, and BACK to review the animation.

5. Purpose Extranodal spread is a poor prognostic factor in head and neck cancers and is a reason why many patients require adjuvant therapy. In one study, extranodal spread doubled the likelihood of local recurrence and distant metastases. Poorer overall survival has also been shown. If there is suspected extranodal spread involving a muscle (such as the sternocleidomastoid), current recommendation is to include the entire muscle when patient undergoes intensity modulated radiotherapy (IMRT) in order to treat potential disease spread along the muscle. We describe the imaging findings of perimuscular metastasis along the posterior belly of the digastric muscle, which forms a unique pattern of extranodal disease spread in H&N squamous cell carcinomas.

6. Methods HIPAA compliant, retrospective search conducted through the institutional teaching files, identifying pathologically proven head and neck squamous cell carcinoma cases with imaging findings of tumor involvement of the posterior belly of the digastric muscle A total of four cases were reviewed and their clinical and imaging findings synthesized

7. Results: Perimuscular tumor spread developing over time: Illustrative case 1 64 year old man with history of p16+ T2N1M0 squamous cell carcinoma of the left tonsil, confirmed by tonsillectomy 7 years ago, followed by neoadjuvant chemotherapy and chemo-radiation. Disease recurred in the anterior tonsillar pillar 4 years after diagnosis, which was resected. This was followed by left oral tongue recurrence 2 years later, s/p resection, along with with left levels I-III lymph node dissections which demonstrated possible nodal disease in two lymph nodes, although evaluation was limited as the nodes have been cauterized. Less than a year later, had biopsy-proven recurrence in a left neck level II lymph node that was discovered on surveillance imaging

8. Preoperative imaging prior to resection of oral tongue recurrence Pre-C Post-C Pre- and post-contrast images prior to resection of the oral tongue recurrence demonstrate a normal appearing posterior belly of the left digastric muscle coursing in close proximity to an area of ill defined, enhancing soft tissue in the left masticator space, where there was a prior resection.

9. Surveillance imaging leading up to biopsy, confirming metastatic disease A T2 hyper-intense, enhancing mass is now centered within the operative bed The mass encases the left internal carotid artery, extending along the posterior belly of the digastric, eroding the left mastoid tip. A T2 hyper-intense, enhancing mass is now centered within the operative bed The mass encases the left internal carotid artery, extending along the posterior belly of the digastric, eroding the left mastoid tip. Ax T2 Cor Post-C

10. A more subtle presentation of posterior belly involvement: Illustrative Case 2 67 year old man with T4bN2cM0, p16 positive right tonsillar squamous cell carcinoma status post treatment with immunotherapy (cetuximab) and external beam radiation, with persistent/recurrent disease by imaging 7 months later, confirmed on biopsy. Presented with right ear fullness/difficulty hearing, had a right mastoid effusion.

11. Axial T2, pre- and post- contrast T1 weighted images demonstrate subtle enlargement, increased T2 signal, and enhancement of the right posterior belly of the digastric muscle when compared to the left.

12. Perimuscular involvement as peripheral muscle enhancement: Illustrative Case 3 55 year old male presented with a left parotid/neck mass. Initial imaging with neck CT demonstrated a necrotic left level II mass inferior to the left parotid gland Patient was eventually found to have a palatine tonsil lesion on physical examination which was considered to be the primary site of disease.

13. The left posterior belly of the digastric demonstrates abnormal T2 signal hyperintensity and peripheral enhancement as it courses superior to the necrotic mass Ax T2 Ax Post-C Please press FORWARD to start the animation, and BACK to review the animation.

14. Results/Discussion We identified four cases with findings suggestive of perimuscular involvement of the posterior belly of the digastric muscle. One patient had a history of tongue base primary while three others had a history of palatine tonsil primary. All patients had concurrent ipsilateral level II nodal metastatic disease in association with findings suggestive of perimuscular spread. In one case, abnormal perimuscular enhancement around the posterior belly of the digastric was in direct contiguity with a necrotic nodal mass.

15. Study Limitations Although fine needle aspiration of the left neck mass in Case 1 along the course of the posterior left digastric muscle demonstrated malignancy, we do not have gross pathological proof of perimuscular spread, and this is primarily an imaging observation. Small number of observed cases, although as with other subtle patterns of tumor spread (such as perineural tumor), this may be an under-recognized disease behavior.

16. Conclusion Findings supporting perimuscular involvement of the posterior belly of the digastric muscle: Asymmetric thickening of the muscle borders on T1 precontrast imaging Perimuscular or intramuscular T2 hyperintensity Ill-defined internal or peripheral muscular enhancement.

17. Conclusion Extranodal extension of SCCa is a poor prognostic factor, and a high level of suspicion should be maintained while evaluating H&N cancer patients with nodal disease. By its proximity to level I and level II lymph nodes, which are common first sites of metastatic disease, the posterior belly of the digastric muscle can serve as an unusual and unique route of tumor spread, and vigilance should be maintained in its evaluation in order to detect subtle signs of malignant involvement. Direct muscle involvement by nodal disease requires including the entire muscle in the treatment plan for intensity modulated radiotherapy.

18. References Trotta, B. M., et al. (2011). "Oral Cavity and Oropharyngeal Squamous Cell Cancer: Key Imaging Findings for Staging and Treatment Planning." RadioGraphics 31(2): 339-354 Merlotti A, Alterio D, Vigna-Taglianti R, et al. Technical guidelines for head and neck cancer IMRT on behalf of the Italian association of radiation oncology - head and neck working group. Radiation Oncology (London, England). 2014;9:264. doi:10.1186/s13014-014- 0264-9. Diagrammatic images of the cervical lymph node levels and digastric muscle: Som PM, Curtin HD, Mancuso AA. AJR Am J Roentgenol. 2000 Mar;174(3):837-44. Imaging-based nodal classification for evaluation of neck metastatic adenopathy. Plate 385, Henry Gray (1918) Anatomy of the Human Body, via Wikimedia Commons