Myxofibrosarcoma Recurrence

Serial contrast-enhanced axial images of the forearm in a patient with recurrent myxofibrosarcoma (MFS). Arrows indicate slowly enlarging, tail-like recurrence along the superficial fascia in the resection bed.

Myxofibrosarcoma (MFS, previously myxoid malignant fibrous histiocytoma) is an intermediate-grade soft-tissue sarcoma with fibroblastic and myxoid components. Patients are typically in the sixth to eighth decades of life.

These lesions can range from round and well-defined (like most soft tissue sarcomas) to predominantly infiltrating, with tails extending along fascial planes. They have high T2 signal intensity related to their myxoid content and heterogeneous enhancement that can be feather-like (typically seen with myxoid neoplasms).

Despite best efforts at acquiring negative margins (made difficult by infiltrating tails), MFS has a propensity for repeated local recurrence, with rates of up to 79%. Recurrence can be tricky to detect. As seen in the image above, tail-like recurrence can be easily dismissed as being related to inflammation or trauma. Our group has found no association between the appearance of MFS at presentation (well-defined vs. tail-like) and the pattern of recurrence, so it's important to watch out for tails regardless of the initial tumor presentation.

In summary, the key to early detection of recurrent MFS is

1. Know that you're dealing with MFS (look at the path report, and know that some pathologists may still call these "myxoid MFH").
2. Be aware of propensity for tail-like recurrence.
3. Make sure to compare to multiple studies dating back to post-operative baseline to increase sensitivity for detection of these slowly growing lesions.

References

Daniels C , Wang WL , Madewell JE, Wei W, Amini B. Pattern of Recurrence of Myxofibrosarcoma is not Associated with Pattern at Presentation or Rate of Delayed Diagnosis. Iran J Radiol. 2017 ;14(1):e13469.

Pleomorphic adenoma

Pleomorphic adenoma (also known as benign mixed tumor) is the most common tumor of the major salivary glands, and the most common benign tumor of the parotid glands (~75% of all benign salivary gland tumors). Prior head and neck irradiation is a risk factor. They are typically solitary, slowly growing, and asymptomatic. They are typically diagnosed after palpation by the patient or incidentally on imaging studies. Surgical resection is advised due to risk of malignant transformation.

On MRI, pleomorphic adenomas, have polylobulated margins a rim of low T2 signal corresponding to a fibrous capsule. They have heterogeneous low-to-intermediate signal intensity on T1-weighted images. High T2 signal and avid, solid enhancement are considered relatively specific features, especially when present in a younger patient (< 57 years). Dynamic contrast-enhanced (DCE) MRI shows gradual enhancement. On diffusion-weighted imaging (DWI), pleomorphic adenomas tend to have very high ADC values; however, DWI is not able to differentiate between benign and malignant parotid gland tumors.

On ultrasound, pleomorphic adenomas are typically hypoechogenic. They can have mild to moderate uptake on FDG PET.

Differential considerations include:

• Warthin tumor: 10–15% are bilateral. Can have proteinaceous cystic components with high T1 signal ranging from a few millimeters to 1–2 cm. Solid components have rapid enhancement and washout.
• Adenoid cystic carcinoma: Small, low-grade lesions can be mistaken for pleomorphic adenomas. Variable signal intensity on T2-weighted depending on type. Low-grade tumors have high T2 signal in the solid parts. Large tumors can have cystic areas of hemorrhagic necrosis.
• Myoepithelial adenoma: Also tend to have very high ADC values
• Basal cell adenoma: More commonly in the superficial lobe of the parotid gland. Tend to be round and well-circumscribed tumors. Have heterogeneous enhancement on CT.
• Carcinoma in pre-existing pleomorphic adenoma (carcinoma ex pleomorphic adenoma): Typically less well-circumscribed than benign pleomorphic adenoma. Tends to occur after 10–15 years of an existing pleomorphic adenoma, with sudden rapid growth (3–6 months) in patients in the sixth-to-eighth decades of life.
• Lymphoma: Tend to be multiple. Can have well-defined and lobulated margins. Tend to have low T2 signal and slight enhancement.
• Sarcoid: Can be placed in any differential, including this one.

References

• Christe A, Waldherr C, Hallett R, Zbaeren P, Thoeny H. MR imaging of parotid tumors: typical lesion characteristics in MR imaging improve discrimination between benign and malignant disease. AJNR Am J Neuroradiol. 2011 Aug;32(7):1202-7.
• Habermann CR, Arndt C, Graessner J, Diestel L, Petersen KU, Reitmeier F, Ussmueller JO, Adam G, Jaehne M. Diffusion-weighted echo-planar MR imaging of primary parotid gland tumors: is a prediction of different histologic subtypes possible? AJNR Am J Neuroradiol. 2009 Mar;30(3):591-6.
• Heaton CM, Chazen JL, van Zante A, Glastonbury CM, Kezirian EJ, Eisele DW. Pleomorphic adenoma of the major salivary glands: diagnostic utility of FNAB and MRI. Laryngoscope. 2013 Dec;123(12):3056-60.
• Kato H, Kanematsu M, Watanabe H, Kajita K, Mizuta K, Aoki M, Okuaki T. Perfusion imaging of parotid gland tumours: usefulness of arterial spin labeling for differentiating Warthin's tumours. Eur Radiol. 2015 Nov;25(11):3247-54.
• Kato H, Kanematsu M, Mizuta K, Ito Y, Hirose Y. Carcinoma ex pleomorphic adenoma of the parotid gland: radiologic-pathologic correlation with MR imaging including diffusion-weighted imaging. AJNR Am J Neuroradiol. 2008 May;29(5):865-7.
• Okahara M, Kiyosue H, Hori Y, Matsumoto A, Mori H, Yokoyama S. Parotid tumors: MR imaging with pathological correlation. Eur Radiol. 2003 Dec;13 Suppl 4:L25-33.

Osseous Pseudoprogression after Spine Stereotactic Radiosurgery

One of the issues we run into when assessing response to radiation therapy is pseudoprogression: Enlargement of the area of abnormality that is not truly progression. This phenomenon is best known in brain lesions following gamma knife therapy, but has also been seen in lung lesions after stereotactic body radiotherapy (SBRT).

In the spine, we can see pseudoprogression after spine stereotactic radiosurgery (SSRS) in bone lesions (osseous pseudoprogression, or OPP), as well as in the epidural soft-tissue components of bone lesions.

The take-away messages are:

• We have so far only seen OPP after single-fraction SSRS. This is likely due to the higher biological dose of single-fraction therapy.
• If you see an enlarging bone lesion on MRI performed within 3-6 months after single-fraction SSRS, you can't be confident that it represents true progression, because about 1/3 of these enlarging bone lesions will represent OPP.
• The only finding on conventional MRI that has been shown to be associated with OPP is tumor growth confined to the 80% iso-dose line and the slope of enlargement (earlier time to tumor enlargement).

References

• Amini B, Beaman CB, Madewell JE, Allen PK, Rhines LD, Tatsui CE, Tannir NM, Li J, Brown PD, Ghia AJ. Osseous Pseudoprogression in Vertebral Bodies Treated with Stereotactic Radiosurgery: A Secondary Analysis of Prospective Phase I/II Clinical Trials. AJNR Am J Neuroradiol. 2016 Feb;37(2):387-92.
• Bahig H, Simard D, Létourneau L, Wong P, Roberge D, Filion E, Donath D, Sahgal A, Masucci L. A Study of Pseudoprogression After Spine Stereotactic Body Radiation Therapy. Int J Radiat Oncol Biol Phys. 2016 Nov 15;96(4):848-856.