...he always scrambled from the car and pranced through your door with a smile on his face. In his view every visit to you was a party with friends.
Case: Kissy, a nine-year-old, female spayed Labrador Retriever presented to her local veterinarian for foul smelling breath. She had been previously healthy with the exception of a cranial cruciate rupture that had been successfully repaired when she was 7 years of age. Other than her foul breath she was clinically healthy. On examination, she was found to have moderate erythema of her gums diffusely, with moderate tartar. Based on a working diagnosis of stomatitis she was treated with a 14-day course of an antibiotic (clindamycin). Initially, she responded well with improvement of the odor from her mouth, but the gingival erythema did not improve. Over the next month, Katie’s red gums progressed and she developed an area on the external portion of her left lower lip, which was erythematous and ulcerated.
Given the progression of the lesion(s) the decision was made to biopsy the area on the lip. The biopsy revealed epitheliotropic lymphoma.
Epitheliotropic lymphoma is a form of lymphoma. However, instead of involvement of the lymph nodes, liver, spleen and bone marrow, dogs (and rarely cats) with this disease usually get a diffuse involvement of the skin, oral mucosa or both. On histopathology, malignant lymphocytes, usually, T-cells, infiltrate within the layers of the epithelium, either the skin or oral mucosa, which is why the disease usually starts as erythema and slowly progresses to plaques and nodules (Moore et al Vet Derm 2009). Eventually, the disease can spread to lymph nodes and to the peripheral blood. It is also known as mycosis fungoides.
The majority of dogs with epitheliotropic lymphoma have a diffuse form of the disease and chemotherapy using CCNU (lomustine) has been shown to result in responses in some dogs, but long-term control of the disease is uncommon with control times of 2 t o6 months published (Risbon et al, JVIM 2006, Williams et al JVIM 2006).
However, a subpopulation of these dogs will have involvement of the oral mucosa only. In these cases, radiation therapy may also be of benefit as it may be effective against the local tumor. Radiation therapy for this disease can involve either a definitive protocol involving 15 to 20 treatments delivered daily, or palliative radiation involving between 3 and 6 treatments over a three week period. Typically the radiation will need to be directed to large portions of the mouth, so side effects can be significant. However, these side effects can be managed well with anti-inflammatory and pain medications. Palliative protocols usually result in much less significant side effects.
Tumor control with radiation is not well defined yet and there are no true reports of the effectiveness that have been published.
However, local tumor control following radiation is very common with long-term tumor control in some dogs.
Kissy was treated with a course of palliative radiation involving 6 treatments delivered twice a week to the oral cavity and the local lymph nodes. Moderate erythema developed within the radiation field but resolved within 2 weeks of the end of treatment. Her owners decided not to pursue chemotherapy based on concerns about her quality of life.
The area of erythema and ulceration resolved following radiation and has been in remission for approximately 6 months.
Submitted by Dr. John Farrelly DVM, MS,
ACVIM (Oncology), ACVR (Radiation Oncology)
Radiation Oncologist/Medical Oncologist at The Veterinary Cancer Center
Moore PF, Affolter VK, Graham PS, Hirt B. Canine epitheliotropic cutaneous T-cell lymphoma: an investigation of T-cell receptor immunophenotype, lesion topography and molecular clonality. Vet Dermatol. 2009 Oct;20(5-6):569-76.
Risbon RE, de Lorimier LP, Skorupski K, Burgess KE, Bergman PJ, Carreras J, Hahn K, Leblanc A, Turek M, Impellizeri J, Fred R 3rd, Wojcieszyn JW, Drobatz K, Clifford CA. Response of canine cutaneous epitheliotropic lymphoma to lomustine (CCNU): a retrospective study of 46 cases (1999-2004). J Vet Intern Med. 2006 Nov-Dec;20(6):1389-97.
Williams LE, Rassnick KM, Power HT, Lana SE, Morrison-Collister KE, Hansen K, Johnson JL. CCNU in the treatment of canine epitheliotropic lymphoma. J Vet Intern Med. 2006 Jan-Feb;20(1):136-43.
History: An 8-month-old male intact Rottweiler presented to his rDVM for evaluation of acute onset of right hind limb (RHL) lameness following a traumatic blow while playing with another dog. Radiographs of the affected limb were normal. The problem was managed conservatively with controlled activity and carprofen, 75mg PO BID for 2 weeks. The lameness progressed to metacarpal knuckling and dragging of the leg. The patient was referred to a neurologist for further evaluation. A neurologic exam localized a lesion to the T3-L3 region with lateralization to the right side. The remainder of the exam was unremarkable. Initial staging tests were done: CBC and chemistry were within normal limits. A lumbar CSF tap was done and results showed a normocellular sample with a mildly elevated protein level (71mg/dL). No other cytological abnormalities were detected. A myelogram was done next and revealed a contrast filing defect at L1-L2 with a classic “golf-tee” sign consistent with an intradural-extramedullary mass localized to the right (image 1 and 2). A right-sided hemilaminectomy (L1-L2) was done and the intradural mass was removed piecemeal. Histopathology revealed a primitive complex neoplasm comprised of both epithelial and mesenchymal components organized into tubular and glomeruloid structures. There were 5 mitoses/HPF within the mesenchymal component. The mass was noted to be invading the nerve root and white matter and margins were incomplete (images 3-4). Immunohistochemistry was done with a panel of markers and was negative for chromogranin, neurofilament, neuron-specific enolase and glial fibrillary acidic protein. However, the samples were positive for cytokeratin and Wilm’s gene product (WT1) confirming the diagnosis of an incompletely excised nephroblastoma.
Two weeks after neurosurgery, the patient was referred to the oncology service to discuss prognosis and treatment options. The patient presented ambulatory x 4 but exhibiting marked difficulty in rising and a frequent “bunny hopping” gait. There was also significant muscle atrophy in the hind end (R>L). Neurology exam revealed mild bilateral hindlimb ataxia and paraparesis (R>L) with moderate scuffing of the nails of the RHL. Conscious proprioception was absent on the RHL and delayed on the LHL. But strong withdrawls remained intact. No ataxia or paresis was noted in the thoracic limbs and conscious proprioception in thoracic limbs was also normal. Panniculus and perineal reflexes were intact and spinal palpation was unremarkable. Further staging diagnostics included thoracic radiographs and abdominal ultrasound which were both within normal limits, as well as lateral thoracolumbar radiographs and CT and MRI scans for treatment planning (images 5 and 6).
The MRI showed mild soft tissue swelling with the presence of an autogenous fat graft as well as cord compression and invasion by a hyperintense lesion.
Treatment options were discussed as being limited to radiation therapy and a guarded prognosis was given. The owners elected to proceed with definitive radiation therapy. A CT scan was obtained for treatment planning (image 7). The plan was to administer 16×3 Gy fractions to a total of 48Gy via parallel, opposed treatment portals . Prednisone was started at 20mg PO SID. Treatment proceeded uneventfully with relatively stable clinical signs. By 6 weeks post treatment, the patient presented for a recheck and was reportedly showing markedly improved strength in his hind end. He was still showing difficulty in rising with a mild bunny hopping gait but the muscle atrophy was resolved and the CPs were decreased but intact on the RHL and normal on the LHL He was weaned off of the prednisone at this time. At a 3 month recheck, the patient continued to show improved strength. He was rising with more ease, no longer exhibiting nail scuffing and was able to jump up on his hindlegs with completely normal hindlimb CPs. The patient continued to do very well up to a year post radiation therapy when he was lost to follow-up.
Spinal nephroblastoma in dogs is a very rare disease. Most reports in the veterinary literature are of patients euthanized after imaging or shortly after surgical exploration. Surgery alone is reported to result in survival times of 71 days 1. One report of cytoreductive surgery and radiation reported survival times up to 560 days 2. In people, Wilm’s tumor is the most common primary malignant renal tumor of childhood. They are histologically diverse with blastalemal, epithelial and stromal components (not all present in each case). Prognosis is dependent on stage and histologic differentiation (anaplastic being worse). In dogs, nephroblastomas can affect spinal or renal tissues. The spinal tumors have a debatable histogenesis in dogs (neural rests vs ectopic renal blastema), however, histology, immunohistochemistry and a virtually uniform spinal involvement at T10-L2 argue more strongly for a renal origin of these tumors. Metastasis is reported to occur in the spine, bone marrow, lung and liver 3.
1: Spinal cord nephroblastoma in dogs: 11 cases (1985-2007). Brewer DM. et al. J Am Vet Med Assoc. 2011 Mar 1;238(5):618-24.
2: Canine spinal nephroblastoma: long-term outcomes associated with treatment of 10 cases (1996-2009). Liebel FX. Vet Surg. 2011 Feb;40(2):244-52.
3: Possible Intraspinal Metastasis of a Canine Spinal Cord Nephroblastoma. Terrel SP. et al. Veterinary Pathology January 2000 vol. 37 no. 1 94-97.
Image 1: “golf tee” filling defect
Image 2: “golf tee” filling defect
Image 3: Tubular structures
Image 4: Glomeruloid structure
Image 5: CT image post laminectomy, pre-RT.
Image 6: Post laminectomy MRI; white line denotes position of fat graft and region of cord invasion.
Image 7: RT planning (bilateral opposed fields)
Prostatic tumors are rare in dogs. The majority of canine prostatic tumors are malignant carcinomas, accounting for 98% of prostatic tumors, and include transitional cell carcinoma (TCC), adenocarcinoma, squamous cell carcinoma, and undifferentiated carcinoma. Prostatic tumors have an insidious onset and most are advanced at the time of diagnosis with 70%-80% of dogs having evidence of metastasis. The management of non-metastatic prostatic tumors is challenging because the majority of these dogs present as a result of secondary urethral obstruction or urinary tract infection.
Management options include non-steroidal anti-inflammatory drugs alone or in combination with chemotherapy, tube cystostomy to bypass the urinary obstruction, transurethral resection or ultrasound-guided endoscopic diode laser ablation of the intraurethral component of the prostatic tumor, urethral stents for dogs with secondary urinary obstruction, partial prostatectomy, and radiation therapy. Other than radiation therapy, particularly intensity-modulated and image-guided radiation therapy (IMRT) (see Nolan et al, J Vet Intern Med 26:987-995, 2012), all of these techniques are palliative. Theoretically, for dogs with non-metastatic prostatic carcinomas, control of the local tumor followed by chemotherapy should provide the best chance for resolution of the clinical signs associated with the local prostatic tumor as well as long-term tumor control.
Total prostatectomy has traditionally not recommended for the management of dogs with prostatic tumors because of the high risk of morbidity and no improvement in local tumor control or survival times compared to less aggressive techniques. However, much of this research was performed over 30 years ago when surgery was performed on more advanced cases and when owner tolerance of outcomes such as urinary incontinence was less than it is now. In addition, the inevitability of urinary incontinence following total prostatectomy has been questioned following the publication and personal experience of outcomes following inadvertent total prostatectomy in cryptorchid dogs where urinary incontinence is exceedingly rare. As a result, some surgical oncologists are re-exploring total prostatectomy as an option for the management of dogs with localized, non-metastatic prostatic carcinomas, particularly where IMRT is either not available or declined by the owners.
Dr. Ralph Henderson has extensive experience with total prostatectomy in dogs. Based on his advice, I have been offering total prostatectomy for dogs with non-metastatic prostatic carcinomas confined to the prostate with no extension into the pre- or post-prostatic urethra. Contrast-enhanced CT scans are recommended preoperatively to determine the extent of the local prostatic tumor, as well as for abdominal and thoracic staging. Total prostatectomy is performed through a ventral midline celiotomy. In most cases, a pubic symphyseal osteotomy (Figure 1) or pubic flap is required to access the prostate and post-prostatic urethra for resection and anastomosis. The prostate is dissected free from adjacent tissue with care, especially dorsally where the capsule is closely associated with neurovascular structures important for maintaining continence. The prostate is resected with 1cm margins cranially and caudally (Figure 2) and the pre- and post-prostatic urethra is anastomosed with either a simple interrupted or simple continuous suture pattern (Figure 3). Urinary incontinence is less likely postoperatively with a delicate dissection technique and by preserving the urethral sphincter and urinary bladder neck.
I have now done five total prostatectomies, four for prostatic TCCs and one for a dog with congenital vascular ectasia of the prostate. Three of these dogs required a pubic osteotomy procedure for total prostatectomy whereas total prostatectomy was able to be performed via a caudal ventral midline celiotomy alone in two dogs. One dog had urinary leakage from the anastomosis site and required revision. One dog had a histologically incomplete excision but did not develop local recurrence. One dog was incontinent prior to surgery and continued to be incontinent following surgery. The remaining four dogs were either continent immediately (n=2) or were initially incontinent but regained continence over a 1-3 month period (n=2). The dog with prostatic vascular ectasia is alive, disease-free, and continent 9 years postoperatively. For the four dogs with TCC, three were treated with chemotherapy (mitoxantrone and a non-steroidal anti-inflammatory drug) and chemotherapy was declined for one dog. The one dog treated with surgery alone was euthanatized at 9 months because of suspected local tumor recurrence. Of the three dogs treated with total prostatectomy and chemotherapy, one was lost to follow-up at 12 months (and was disease-free at the time), and two were euthanatized because of lung metastasis 10 months and 13 months postoperatively.
The advantages of total prostatectomy include better local tumor control than any other technique (and hence the resolution of clinical signs commonly associated with non-metastatic prostatic tumors), less expensive than either palliative urethral stenting or curative-intent IMRT, more readily available than IMRT, and a relatively low risk of postoperative complications. The incidence of urinary incontinence following total prostatectomy is similar to urethral stenting (20%-25%). Furthermore, there is the theoretical advantage that chemotherapy should be more effective following adequate local tumor control, but larger case studies are required to investigate whether there is a survival benefit over less aggressive management strategies such as chemotherapy alone. While total prostatectomy is not for every owner, and it should only be offered as a treatment alternative for dogs with localized and non-metastatic prostatic tumors, it does present another option for a select subset of dogs with prostatic carcinomas.
ACVS Founding Fellow in Surgical Oncology
Alta Vista Animal Hospital, Ottawa, Ontario, Canada
Stereotactic radiation (SRT) and intensity modulated radiation therapy (IMRT) are becoming more readily available for animals. In a survey of veterinary radiation facilities performed in 2010 (Farrelly et al VRUS, 2014) approximately 1/3 of facilities had IMRT capability, SRT capability or both, and this number has been growing rapidly.
Prior to the advent of these technologies, almost any solid tumor could be treated with radiation. The clinical problem has been that, although radiation could theoretically be used to treat any tumor, most tumors are surrounded by normal tissues that can be adversely affected by the radiation. This results in significant side effects in these areas and severely limits the dose that can be safely given to the tumor. Many radiation oncologists and pet owners might decide not to pursue radiation because the side effects of treatment can result in severe negative effects on quality of life.
Probably the best example of this is nasal tumors in dogs. Given the location of these tumors most early studies showed no significant benefit to surgery. (Theon et al JAVMA 1993) so radiation therapy has been the focus of local therapy for these tumors. However, side effects from conventional radiation can be severe for dogs with nasal tumors. Short term side effects such as conjunctivitis, oral mucositis, dermatitis etc. can cause a severe impact on quality of life during the treatment. Also, late effects, such as damage to the eyes, can lead to significant long-term health issues and even euthanasia in some cases (Pinard et al Can Vet J 2012).
Figure 1 Acute conjunctivitis (left) and chronic cataract (right) from radiation to the eye for treatment of a nasal tumor
IMRT and SRT will allow the dose of radiation to be targeted to the tumor while minimizing the dose to the local normal tissues.
Figure 2 Dose distribution for treatment of a nasal tumor in a dog.
The nasal cavity (red color wash) is receiving greater than 98% of the prescribed dose while the lenses receive less than 20-30% of the dose.
Being able to target the radiation better to the tumor makes it easier to deliver radiation to tumors without severe side effects.
Bone tumors, such as osteosarcoma, are usually best treated with surgical removal followed by chemotherapy. When surgery is not an option due to the location of the tumor (e.g. some axial tumors) or the ability of the patient to tolerate an amputation, stereotactic radiation may be beneficial. In one study 11 dogs were treated with stereotactic radiation (plus or minus chemotherapy) for osteosarcoma involving the distal limbs. These dogs had a median survival of 363 days and most of the dogs had excellent function of their limbs (Farese, et al JAVMA 2004).
This type of treatment may be an alternative to amputation in dogs where a lot of the bone has not been destroyed yet.
Treatment of choice for adrenal tumors is typically surgical removal. However, when there is a significant invasion of the tumor into the nearby vessels, surgery can sometimes be dangerous. Given the relatively stable location of the adrenal gland stereotactic radiation may be used to treat these tumors. At the 2014 ACVR Scientific Meeting, an abstract was presented on this type of treatment for 12 dogs (Malfassi et al ACVR Abstract 2014). All dogs were treated with 3 large doses of radiation. Ten dogs had partial responses and 87% of dogs were alive with a mean follow-up of 21 months. While this data is still early, this may prove to be a possible treatment option for dogs with invasive adrenal tumors.
Brain tumors are typically best treated with surgery or surgery plus radiation, with reported survival times of 12-24 months (Rohrer-Bley et al JVIM 2005;Kyerleber et al VCO 2013). However, some brain tumors are in locations where surgery is not possible. For these, definitive radiation may give the best chance for long-term tumor control. However, in cases where definitive radiation is not an option, stereotactic radiation may be an option. In one study 51 dogs were treated with stereotactic radiation with a median survival of 399 days. In an abstract from the 2014 ACVR Meeting 69 dogs were treated with stereotactic radiation and their outcome was compared to dogs that had surgery or palliative care alone. Dogs that had radiation had survival times ranging from 22-26 months compared to less than 19 months for dogs with surgery alone (Malfassi et al ACVR Abstract 2014).
Nasal tumors in dogs
Nasal tumors are some of the most difficult and most frustrating tumors to treat in dogs. Nasal tumors are very locally invasive and given their location they are often right up against the eyes, the brain the skin and the oral mucosa. Traditional conformal radiation therapy has shown to be effective. However, dogs treated this way often have significant short-term side effects involving their eyes, and mouth which results in significant discomfort. Tumor control is good, with average survival times of approximately 12-18 months. However, long-term tumor control is not common as most dogs end up dying from their nasal tumor.
Stereotactic radiation may be a reasonable alternative for treating nasal tumors as the treatment causes much fewer side effects and only requires usually 3 anesthesias. One study has been published so far with 19 dogs that were treated with three large doses. Median survival time was 399 days. This is similar to definitive radiation but with much fewer side effects short term (Glasser et al JAAHA 2014).
Now that we have the ability to focus radiation to almost any area, pretty much any solid tumor can be safely treated with radiation.
Dr. John Farrelly DVM, MS, ACVIM (Oncology), ACVR (Radiation Oncology)
Radiation Oncologist/Medical Oncologist at The Veterinary Cancer Center
The Veterinary CyberKnife Cancer Center in Malvern, PA offers CyberKnife radiation therapy treatments for veterinary patients. CyberKnife is a form of stereotactic radiation therapy which allows pain-free, non-invasive precision tumor treatment with fewer visits to the hospital. For questions regarding cases that may benefit from this treatment, please contact Dr. Siobhan Haney (radiation oncologist) at Hope Veterinary Specialists 610-296-2099.
Farrelly J, McEntee MC. A survey of veterinary radiation facilities in 2010. Vet Radiol Ultrasound. 2014 Nov-Dec;55(6):638-43. http://onlinelibrary.wiley.com/doi/10.1111/vru.12161/full
Théon AP, Madewell BR, Harb MF, Dungworth DL. Megavoltage irradiation of neoplasms of the nasal and paranasal cavities in 77 dogs. J Am Vet Med Assoc. 1993 May 1;202(9):1469-75.http://europepmc.org/abstract/med/8496103
Pinard CL, Mutsaers AJ, Mayer MN, Woods JP. Retrospective study and review of ocular radiation side effects following external-beam Cobalt-60 radiation therapy in 37 dogs and 12 cats. Can Vet J. 2012 Dec;53(12):1301-7. Review.
Farese JP, Milner R, Thompson MS, et al. Stereotactic radiosurgery for treatment of osteosarcomas involving the distal portions of the limbs in dogs. J Am Vet Med Assoc. 2004 Nov 15;225(10):1567-72, 1548.
Malfassi L, Dolera M, Marcarini S, et al Adrenal tumors with vascular invasion: stereotactic hypofractionated volume modulated arc radiotherapy (VMAT) in 12 dogs. Proceeding of the ACVR 2014 Annual Scientific Meeting
Rohrer-Bley CR, Sumova A, Roos M, et al. Irradiation of brain tumors in dogs with neurologic disease. J Vet Intern Med. 2005 Nov-Dec;19(6):849-54. http://onlinelibrary.wiley.com/doi/10.1111/j.1939-1676.2005.tb02776.x/abstract
Keyerleber MA, McEntee MC, Farrelly J, et al. Three-dimensional conformal radiation therapy alone or in combination with surgery for treatment of canine intracranial meningiomas. Vet Comp Oncol. 2013 Jul 19. vco.12054. [Epub ahead of print] http://onlinelibrary.wiley.com/doi/10.1111/vco.12054/full
Malfassi L, Dolera M, Marcarini S, et al. Canine meningioma: comparison of palliative therapy , surgery and stereotactic radiosurgery. Proceeding of the ACVR 2014 Annual Scientific Meeting
Inflammatory myopathies can be debilitating autoimmune diseases in both dogs and humans characterized by focal or generalized muscle pain, sensitivity, and weakness– the inflammation can be generalized, throughout most of the striated muscles of the body, or they can be more localized and specific, such as with masticatory myositis. Initial diagnoses are often made on the basis of history, clinical signs, serum biochemical profiles and an anti-type 2M myofiber antibody test. Blood work is often normal or relatively non-specific, with serum creatinine kinase levels occasionally increased (more so with generalized myopathies) and an inflammatory leukogram on complete blood count. The gold standard for diagnosing inflammatory myopathies is a muscle biopsy, although this is an invasive procedure, and with the oftentimes patchy distribution of inflammation throughout the muscles, it does not always result in a representative sample. EMG, or electromyography, is highly sensitive for diseases compromising muscle function but lacks the specificity to distinguish amongst inflammatory, degenerative, neoplastic and/or infectious etiologies. Cross-sectional imaging in the form of both magnetic resonance imaging and computed tomography has been found to be both sensitive and specific for inflammatory myopathies, albeit that they cannot always differentiate between infectious and inflammatory causes of muscle disease. The following two cases detail the clinical and imaging findings in two patients with masticatory myositis, one of whom had an MRI performed and the other a CT scan.
“Brutus” Miller, a 6yr male castrated Rottweiler, presented with a several week history of progressive dysphagia. The initial physical exam revealed a normal range of motion of the jaw and normal temporal and masseter muscle mass through discomfort on palpation of those muscles. Routine bloodwork and radiographs of the skull were unremarkable. Initial differentials included inflammatory myositis, infectious myositis, or neoplasia. An anti-type 2M myofiber antibody test was sent out and later returned a positive result of 1:1000 (normal <1:100).
An MRI of the patient’s head was performed, with routine sequences including T1, T2, T2*, FLAIR and post-contrast T1-weighted Fat Sat images. The scan revealed ill-defined T2-hyperintensities throughout the temporal and masseter muscles primarily along the right side of the head. These areas were hyperintense on FLAIR images and iso- to mildly hyperintense on T1-weighted images. After administration of gadolinium IV, ill-defined, locally-extensive regions of contrast enhancement were observed throughout the temporal, masseter and medial pterygoid muscles on both sides of the head. The digastricus muscles were unaffected. After the MRI scan, an incisional biopsy was obtained from the right temporal muscle, using the MR images as a guide to obtain the most representative sample. The histopathology was consistent with masticatory myositis.
“Hextall” Holloway, a 7yr male castrated Rottweiler, presented with a history of difficulty chewing and swelling around his right eye. The patient’s previous history had included an eosinophilic and lymphoplasmacytic gastritis diagnosed with endoscopy a year prior and successfully treated with prednisone and azathioprine. The initial physical exam showed mild exophthalmos of the right eye and pain on palpation of the temporal and masseter regions on both sides of the head. Routine bloodwork showed a mild increase in alkaline phosphatase of 337 (normal <212) and mild anemia of 36.3% (normal <37.3). A serum titer for 2M antibodies was negative at <1:100, although the patient had been on prednisone for seven days prior to the test being run.
A CT scan was performed on the patient’s head using routine series, including 1.25mm helical scan using a bone algorithm and a 1.25mm post-contrast helical scan using a detailed algorithm after administration of Iohexol IV. The pre-contrast scan demonstrated asymmetry of the masticatory muscles, with regional atrophy of the left temporal muscle, as well as ill-defined focal regions of hypoattenuating tissues within the temporal and masseter muscles on either side of the head. After administration of contrast, patchy and irregular areas of contrast enhancement were present throughout the temporal, masseter and medial pterygoid muscles bilaterally, although to a much greater extent throughout the left side. The degree of asymmetry of the contrast enhancement was unusual for this patient though in general it is not an atypical finding with masticatory myositis. The hypoattenuating areas on pre-contrast images likely corresponded to areas of necrosis and/or inflammatory infiltrate within the muscles, although this is not specifically backed up by the corresponding histopathology. The pattern and distribution of the patchy areas of contrast enhancement represent a classic appearance for masticatory myositis.
The two described cases demonstrate the utility of cross-sectional imaging in the diagnosis of masticatory myositis. The magnetic resonance characteristics of inflammatory myopathies, in general, are reliably consistent and have been well-described in the literature for both human and veterinary medicine. For masticatory myositis, in particular, it is often at the time of diagnosis– with the patients exhibiting signs ranging from stiffness and trouble walking to dropping food and trouble eating– that the MR characteristics are most apparent, paralleling the inflammatory infiltrates in the affected musculature. The imaging findings cannot necessarily distinguish an inflammatory process from neoplastic infiltration of the muscle– however, neoplastic lesions affecting the muscle tend to be more focal and better defined, with more of a random distribution. Similarly, infectious causes of muscle inflammation are typically more diffuse and do not have the characteristic distribution of masticatory myositis, in which only the muscles of mastication are involved.
The computed tomographic characteristics of masticatory myositis are well described, and they are just as sensitive and consistent, with almost identical patterns of contrast enhancement and variable muscle swelling and atrophy. Furthermore, computed tomography is typically less expensive and is a much faster imaging modality, resulting in less of a need for and expense due to anesthesia. Taking these factors into account, computed tomography may be a better imaging modality to pursue when there is a sufficient index of suspicion for masticatory myositis.
Submitted by Hadley Bagshaw VMD, DACVR (Radiology)
Platt S.R. et al. Magnetic Resonance Imaging in the Diagnosis of Canine Inflammatory Myopathies in Three Dogs. Vet Rad & US 2006; vol. 47(6): 532-537. http://onlinelibrary.wiley.com/doi/10.1111/j.1740-8261.2006.00181.x/full
Bishop T.M. et al. Imaging Diagnosis– Masticatory Muscle Myositis in a Young Dog. Vet Rad & US 2008; vol. 49(3): 270-272. http://onlinelibrary.wiley.com/doi/10.1111/j.1740-8261.2008.00364.x/abstract
Reiter AM, Schwarz T. Computed tomographic appearance of masticatory myositis in dogs: 7 cases (1999–2006). J Am Vet Med Assoc 2007; vol 231:924–930. http://avmajournals.avma.org/doi/abs/10.2460/javma.231.6.924
Evan J., et al Canine inflammatory myopathies: a clinicopathologic review of 200 cases. J Vet Intern Med. 2004; vol 18(5):679-91. http://onlinelibrary.wiley.com/doi/10.1111/j.1939-1676.2004.tb02606.x/abstract