Psoas muscle metastatic disease mimicking a psoas abscess on imaging

Abstract

Here, we report a case of malignant psoas syndrome presented to us during the second peak of the COVID-19 pandemic. Our patient had a medical history of hypertension, recently diagnosed with left iliac deep vein thrombosis and previous breast and endometrial cancers. She presented with exquisite pain and a fixed flexion deformity of the left hip. A rim-enhancing lesion was seen within the left psoas muscle and was initially deemed to be a psoas abscess. This failed to respond to medical management and attempts at drainage. Subsequent further imaging revealed the mass was of a malignant nature; histology revealing a probable carcinomatous origin. Following diagnosis, palliative input was obtained and, unfortunately, our patient passed away in a hospice shortly after discharge. We discuss the aetiology, radiological findings and potential treatments of this condition and learning points to prompt clinicians to consider this diagnosis in those with a personal history of cancer.

Background

Intramuscular metastasis is a rare occurrence and is often associated with advanced disease.^1–3 When it does occur, the iliopsoas muscle is most commonly involved.² Pain is a common presenting feature and there are numerous ways this can be managed to improve the quality of life of affected patients.^4–11 However, adequate treatment requires a firm diagnosis, and muscular metastasis can adopt a range of appearances on imaging therefore making this a difficult task as they may mimic the more common psoas abscess or haematoma.^{1 2 12} We believe that clinicians should be aware of the possibility of malignancy in such patients, especially those with a personal history of cancer, and investigate thoroughly to allow accurate diagnosis and effective treatment.

Case presentation

We present the case of a woman in her 70s with a history of hypertension, recent left iliac deep vein thrombosis (DVT), grade 1b endometrioid adenocarcinoma 7 years ago and a previous grade 1 infiltrating ductal carcinoma 17 years ago. Her endometrioid adenocarcinoma was treated with hysterectomy and bilateral salpingo-oopherectomy followed by vaginal vault brachytherapy; she was under annual follow-up with no clinical signs of recurrence and hence no additional imaging performed. Her ductal carcinoma was treated with wide local excision and axillary clearance; she was discharged at 5 years postoperatively with no signs of recurrence and had normal mammograms up until her last 3 years. Our patient presented to the emergency department during the second peak of the COVID-19 pandemic with left hip and thigh pain unresponsive to analgesia (paracetamol, codeine, morphine) and difficulty mobilising with an inability to bear weight on the left side. She reported weight loss of 3–4 kg over the past 2 months alongside increasing lethargy. There was no preceding trauma, and she described her pain as sharp with intermittent burning sensations. On examination, she could not perform an active straight leg raise on the left side and held the left hip in flexion with tender and restricted left hip movements. Cardiorespiratory examination showed no abnormality. Radiographs showed no fracture, dislocation or bony destruction. Blood results on admission revealed mildly raised white cell count (WCC) at 10.8, neutrophils 7.6 and C creative protein (CRP) 70; U+E/LFT were unremarkable. A CT scan was performed reporting a 7×7 cm multiseptated collection within the left psoas muscle belly with a hypodense centre and enhancing peripheral rim, likely representing a left psoas abscess (figure 1). Previous MRI of the pelvis 7 years ago showed no abnormality of the psoas muscle. She was treated initially with intravenous antibiotics as per protocol and underwent three unsuccessful attempts at interventional radiology (IR) drainage. Repeat CT 10 days later showed minimal changes (4–8 mm increase in size) from the previous scan. The mass was deemed inaccessible to IR; therefore, she was discharged home to complete 4 weeks of oral antibiotics with a repeat CT scan in 4 weeks. She re-presented 5 days after discharge with unbearable pain. Blood tests were relatively unchanged: WCC 11.1, neutrophils 7.7 and CRP 76, and she was admitted for pain control. She went home 10 days later only to then be readmitted the day after discharge for the third time due to unrelenting pain. Repeat CT scan was performed showing interval increase in the size of the abscess to 85×71×75 mm. Further attempt at IR drainage yielded 8 mL of clotted blood, and the working diagnosis changed to one of a psoas haematoma. During her inpatient stay, she was found to have new-onset left upper limb weakness in addition to her left lower limb weakness previously attributed to pain from the abscess/haematoma. CT of the head was performed to investigate a possible stroke. However, two regions of low density, one in the right frontal and the other in the right parietal regions suggestive of metastasis, were seen; these were confirmed as metastasis by MRI (figure 2). MRI of the lumbar spine was performed to investigate the psoas lesion as a potential malignancy. This showed an aggressive soft tissue lesion causing compression of both L4 and L5 nerve roots, suggestive of malignancy. A solitary pulmonary nodule was also seen and further investigated with CT confirming a solitary nodule with no thoracic lymphadenopathy. Pregabalin 75 mg two times per day was introduced at this point for neuropathic pain control. A biopsy (figure 3) revealed mostly necrotic tissue that stained positive with cytokeratins AE1/AE3. Cancer of unknown primary MDT deemed that this could have been considered a carcinoma. Unfortunately, our patient deteriorated rapidly following this diagnosis and was discharge to hospice care where she passed away shortly after discharge.

Figure 1.

Coronal view from CT of the abdomen and pelvis; 7×7 cm multiseptated collection within the left psoas muscle belly with a hypodense centre and enhancing peripheral rim. Appearances were thought to be suggestive of a psoas abscess.

Figure 2.

MRI of the brain showing a hypodense area in the right frontal lobe. Neuroradiology reported this was suggestive of malignancy rather than an abscess.

Figure 3.

Transverse view from a CT-guided punch biopsy.

Differential diagnosis

Based on the initial clinical presentation, raised inflammatory markers and CT imaging findings, our initial differential diagnosis was that of the more common psoas abscess, which following failed attempts at drainage, we planned to manage medically. Following readmission and rescan showing interval enlargement, a repeat drainage attempt gave 8 mL clotted blood. Given the history of recent DVT, ongoing therapeutic anticoagulation and failure to improve with antibiotics, the working diagnosis changed to a psoas haematoma. Only then following the development of upper limb neurological signs and MRI of the lesion was this seen to be a malignant mass within the psoas.

Outcome and follow-up

After the discovery that this was in fact a malignant mass, palliative care input was requested. Shortly following this, our patient was discharged home where she unfortunately passed around a month later.

Discussion

We report a skeletal muscle metastatic carcinoma of unclear primary mimicking a psoas abscess on initial imaging. One could speculate that this was due to occult residual disease from our patient’s previous malignancies. Our case showed features of the rare malignant psoas syndrome (MPS). MPS was first described in 1990 when Stevens and Gonet reported four patients with advanced cancer who developed proximal lumbosacral plexopathy and painful flexion of the ipsilateral hip with positive psoas stretch test due to malignant involvement of the psoas major muscle.¹³ MPS is thought to be associated with neoplastic infiltration of the psoas muscle directly or extrinsically via the adjacent para-aortic and paracaval lymph nodes that invade the psoas or lumbosacral plexus.¹³ Both nociceptive and neuropathic pain present due to involvement of the psoas muscle itself and the lumbosacral plexus. This, combined with the fact that MPS is usually associated with advanced cancer, means that the symptoms of MPS are difficult to control.⁴ Female genital tract malignancies cause around 27% of MPS cases and represent the most common primary malignancy.⁴ Numerous cases in the literature have reported gynaecological cancers causing malignant involvement of the psoas muscle, the most common of which being endometrial malignancies.^{4 14–17} Also of note is that these malignant deposits can be seen to mimic psoas abscesses radiologically.^{14 16} Endometrial cancer can metastasise in various ways, for example, direct local extension, transtubal passage, lymphatic drainage and haematogenous dissemination. Usual sites of metastasis include the cervix, parametria, pelvic and para-aortic lymph nodes and lungs, with less common sites being the liver, bones and brain.^{15 18} When the lungs are involved, the most common pattern is one of multiple nodules (72%), followed by solitary nodules (18%).¹⁹ Although apparently successfully treated in our patient, endometrial cancer has been seen to have a recurrence rate of around 13%, with 68%–100% of these recurrences occuring within 3 years.²⁰ Interestingly, although very rare, breast cancer has been reported to metastasise to skeletal muscle resulting in CT appearances of a rim-enhancing mass with central hypoattenuation,²¹ similar to those seen in our case.

Intramuscular metastasis is rare, with a prevalence of between 0.03% and 17.5% on postmortem,^1–3 and thus is often thought to be due to primary sarcoma.³ There are multiple postulated mechanisms as to why this may be the case, for example, muscle motion, lactic acid and pH-causing mechanical destruction and inhibition of angiogenesis.⁵ The iliopsoas muscle is the most commonly involved muscle in 27.5% of cases.² Radiologically, the most common presentation is a mass with homogeneous contrast enhancement followed by an abscess-like lesion in 27.7%–32.5% of cases.^{1 2} However, one case series showed 83% of skeletal muscle metastasis presenting as a rim-enhancing lesion with central hypoattenuation.¹² Due to this rarity, skeletal muscle metastases are often attributed to the more common psoas abscess or haematoma,²² as seen in our case, especially in a patient presenting with raised inflammatory markers and on therapeutic anticoagulation. However, in case of failure to improve with antibiotics, further actions should not be delayed. In particular, a CT-guided punch biopsy should be performed from the edge of the lesion to obtain histology as identification of the primary lesion will influence decision-making in regard to prognosis and future treatment. It is important to note that regardless of their appearance, skeletal muscle metastases are often a marker of advanced disease and are commonly seen with other concomitant metastases.^{12 21}

Management of the pain associated with MPS is very challenging, primarily due to the multiple mechanisms of pain due to both anatomical destruction, associated inflammation and spasm in the psoas muscle and also lumbosacral plexopathy leading to neuropathic pain.⁵ Numerous medications have been shown to be effective against the neuropathic element, for example, pregabalin, ketamine, lidocaine and clonidine.⁵ Our patient was managed with paracetamol, modified-release oxycodone with immediate release breakthrough as well as pregabalin to only minor success. Due to the advanced disease associated with MPS, pain control may be lost with standard opioid medication; in this situation, opioid switching to methadone has been shown to help regain control.^{5 6} Palliative radiotherapy can give significant improvements in pain scores and health-related quality of life^{4 7}; however, delays in access can often be an issue.⁵ This was the case with our patient who unfortunately passed before this could be offered. Despite its effectiveness, the nature of the condition causing a painful fixed flexion deformity at the hip can cause issues with performing effective radiotherapy as access is limited. Some centres have shown that epidural analgesia can be effective to relieve pain to allow patients to undergo effective radiotherapy in this case.⁸ Various, more invasive, methods have been successful to control pain associated with MPS such as psoas sheath catheters, spinal opioids and local anaesthetics and dorsal rhizotomy.^9–11

Learning points.

• Raised awareness of malignant psoas syndrome (MPS) and the potential for malignant psoas involvement in those presenting with a psoas mass, especially with a personal history of cancer and a combination of nociceptive and neuropathic sounding pain.

• Readers should consider a CT-guided biopsy and MRI to further characterise suspicious lesions in patients at risk of malignant disease to allow earlier diagnosis and therefore more effective treatment, especially in cases poorly responding to antibiotics.

• MPS and skeletal muscle metastasis are associated with advanced disease and poor outcomes; however, numerous treatment options are available to improve pain and quality of life and therefore early recognition is key.

Ethics statements

Patient consent for publication

Not required.