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Multilobular tumors of bone have been known in the past by many names, including multilobular osteochondrosarcoma (MLO), chondroma rodens and multilobular osteoma.  These tumors are relatively uncommon and no breed or sex predilection has been described.  They occur on flat bones ad most commonly arise from the skull of dogs.  Radiographically, they have a characteristic appearance, sometimes referred to as 'popcorn ball' due to he well circumscribed, lobular and granular features.  Histologically, they are typically composed of multiple lobules, each centered on a core of cartilaginous or bony matrix that is surrounded by a thin layer of spindle cells. 

Dogs with multilobular tumors of bone often present with a non-painful, often well-defined bony mass on the skull.  Histories may reveal a long course and a relatively slow, asymptomatic growth.  When the tumor occurs near the TMJ (temperomandibular joint), pain can be associated with opening the jaw.  The biological behavior is generally considered to be more favorable than osteosarcoma, and appears to be grade dependent.  Dernell, et al. reported 39 cases of multilobular tumor of bone, in this report, 47% of the dogs had local tumor reoccurrence following resection at a median time interval of 800 days.  Fifty-six percent developed metastasis after treatment, with a median time to metastasis of 542 days, and an overall survival time of 800 days.  Even in patients that do experience metastasis, death due to metastasis can be delayed.  Dog’s diagnosed radiographically with lung metastasis have remained asymptomatic for up to one year or more.  

Although these tumors often appear well circumscribed clinically, radiographically, and histologically, surgical management can be a challenge due to the fact that the tumors arise very commonly from the skull bones. Obtaining microscopically tumor-free margins can be difficult.  CT is critical in determining extent of disease and for surgical planning.

Puppet Danner, an 8 year old spayed female mixed breed dog, presented for a slow growing mass on her head.  The original mass had been removed the year before, and had come back and continued to grow.  Due to the growth on the top of her head, she was no longer able to close her eyelids and her lower eyelids were being pulled up to make sight difficult.  A CT scan was performed, revealing a typical granular appearance of a multilobular tumor of bone.  Although large, the tumor was fairly well demarcated and did not appear to be invading through the cranium; therefore the decision was made to go to surgery.

The tumor had to be lifted from the bone, but was able to be removed due to its well circumscribed nature.  Recovery was uneventful, and vision was immediately restored.  Due to the low grade nature of Puppet’s tumor, adjunctive chemotherapy or radiation therapy was not advised.  As of 6 months post operatively, regrowth had not been observed.
This is Puppet at her 2 month post recheck!

The role of chemotherapy and radiation therapy for higher grade multilobular tumors of bone is not well defined.  Local tumor excision appears to result in long-term control for lower grade tumors.  We do recommend adjunct radiation therapy for incompletely excised higher grade tumors to potentially slow the rate of reoccurrence.  For incompletely excised lower-grade tumors, reoccurrence can take months to years.  For inoperable tumors, radiation therapy can provide palliation.