Prostate specific membrane antigen PET avidity in a granular cell tumour of the left supraspinatus muscle: a case report

Abstract Granular cell tumour is a rare, mostly benign, soft tissue, neuroectodermal tumour, most commonly seen in the skin and peripheral soft tissue. There are no publications to date of PSMA-PET avidity in a granular cell tumour. In this 60 year old male, staging PSMA-PET for a localized intermediate risk prostate cancer incidentally identified a PSMA-avid left supraspinatus lesion, which was subsequently biopsy-proven as a granular cell tumour. We present the first case of PSMA-avid granular cell tumour and add to the growing literature documenting PSMA-PET avidity in benign and malignant lesions apart from prostate cancer.


Case presentation
We present a case of a 60-year-old male with a prostate-specific membrane antigen PET (PSMA-PET) avid left supraspinatus mass, which was incidentally detected on staging PSMA-PET performed for a localized intermediate risk prostate cancer (T1cN0M0 Gleason 3 þ 4 ¼ 7 initial PSA 5.9 mg/L), with a maximum SUV 4.3 and measured 20 × 12 × 23 mm on the coregistered CT.No PSMA-avid lesion was seen in the prostate or elsewhere (Figure 1).The case was reviewed at the local urology multidisciplinary meeting which included radiology review by a dual-trained nuclear medicine physician and radiologist with an interest in PSMA-PET.The consensus recommendation was for further ultrasound and MRI to characterise the lesion, imageguided biopsy, and discussion at a tertiary sarcoma multidisciplinary meeting.
Targeted ultrasound of the left shoulder identified a 36 × 10 × 18 mm anechoic lesion with low internal vascularity, and was reported as indeterminate in nature (Figure 2).MRI of the left shoulder showed a 35 × 12 × 15 mm, ovoid, T1 hypo-intense, T2 hypo-intense lesion in the supraspinatus belly, with mild gadolinium enhancement (Figure 3).Ultrasound-guided core biopsy and histopathological analysis showed cells with abundant cytoplasm and cytoplasmic granularity, with positive immunoperoxidase staining for CD68 and S100, negative staining for pan cytokeratins: favoured to represent granular cell tumour (Figure 4).The case was reviewed at a tertiary sarcoma multidisciplinary meeting.The histopathology was confirmed as a benign granular cell tumour by a dedicated sarcoma pathologist.The patient was offered either complete excision, or repeat interval surveillance imaging in 6 months.
The patient has requested excision, and is pending surgical consultation.He is also currently undergoing definitive radiation therapy to his prostate cancer.

Discussion
To our knowledge, there are no published reports of PSMA-PET avidity in a granular cell tumour.We present the first case report of PSMA-PET avidity in a granular cell tumour.Granular cell tumour is a rare, mostly benign, soft tissue tumour with neuroectodermal differentiation, also known as an abrikossoff tumour or granular cell myoblastoma. 1t has a slight female predilection, a mean age at diagnosis at 45 years old, and is located predominantly in skin, peripheral soft tissues and alimentary tract (66% oesophagus), with rarer presentations in the breast, vulva, larynx and bronchus. 2istologically it is composed of plump cells with eosinophilic granular cytoplasm and is graded according to either the Fanburg-Smith or Nasser criteria. 1,3Approximately 93% of the reported cases are benign, 7% have uncertain malignant potential on histopathology and only 2.5% of all granular cell tumours are documented to have metastasized. 2n ultrasound, it is usually hypoechoic, and may variably display posterior acoustic shadowing, posterior enhancement and internal hypervascularity. 4It is usually isodense to muscle on CT 5 and non-glucose avid on FDG-PET, although malignant granular cell tumours have been reported to be strongly glucose-avid. 6On MRI it is usually T1 isointense, T2 iso or hypointense, and enhances with gadolinium contrast. 7PSMA-PET is increasingly used in staging, surveillance and treatment planning for prostate cancer as it can result in a change to management versus conventional imaging up to 50% of the time. 8PSMA-PET scans have been reported to demonstrate significant uptake in benign and malignant entities apart from prostate cancer. 9This case demonstrates the utility of obtaining histological confirmation of PSMA-PET avid lesions found in unusual locations for prostate cancer metastases.Prostate cancer typically metastasizes to abdomino-pelvic lymph nodes and bone, but rarely to other sites such as muscle, skin and peripheral soft tissues. 10In this case, if not for histologic confirmation, the patient may have been classified as stage IV in the presence of a presumed PSMA-avid distant metastasis, and thus denied curative   intent therapy to an otherwise localized prostate cancer, and also be subject to highly toxic life-long androgen deprivation therapy as the standard of care for metastatic prostate cancer.Additionally, with the increasing use of oligometastaticdirected stereotactic ablative radiotherapy in the oligometastatic setting, histologic confirmation is not always obtained prior to oligometastatic-directed ablative therapy.In this case, an unnecessary and potentially toxic treatment would have been avoided if a biopsy was obtained.By performing image-guided core biopsy, we were able to exclude metastatic involvement, confirm localised disease, and offer curative intent management.

Learning points
1) This case reminds the clinician that not all PSMA-PET avid lesions are malignant.2) Multidisciplinary input including a nuclear medicine physician with interest in PSMA-PET is crucial to provide guidance on the possible benign and malignant entities which are PSMA-PET avid.3) Obtaining histopathologic confirmation is especially recommended when the site of PSMA-avidity is in an unusual location for prostate cancer metastasis.4) Granular cell tumours are a mostly benign, but rarely malignant entity which may be PSMA-avid.

Conclusion
To our knowledge, this is the first case report of PSMA-PET avidity in a granular cell tumour.We add to the growing literature documenting PSMA-PET avidity in benign and malignant lesions apart from prostate cancer.