Superscan on Methylene Diphosphonate Skeletal Scintigraphy in Prostatic Adenocarcinoma: A Common Finding but Rare Etiology.

Excessive skeletal radioisotope uptake in relation to soft tissues along with absent or faint activity in the genitourinary tract on skeletal scintigraphy is known as a "superscan." Prostate cancer is the most common cause of superscan in skeletal scintigraphy due to diffuse skeletal metastases. However, prostate cancer may cause secondary renal osteodystrophy leading to metabolic superscan also. Differentiating between these two entities are important as treatment options are different. We, hereby, describe a case of prostatic adenocarcinoma presented with metabolic superscan on methylene diphosphonate skeletal scintigraphy and demonstrate the utility of single emission computed tomography-computed tomography in differentiating between two entities.

A 57-year-old male patient diagnosed with prostatic adenocarcinoma having Gleason score 5 (2 + 3) and serum prostate-specific antigen (PSA) level 23 ng/ml presented to the Department of nuclear medicine for 99m-techniteum methylene diphosphonate (99m-Tc MDP) skeletal scintigraphy as a part of metastatic workup. 25 mCi of 99m-Tc MDP was injected and skeletal scintigraphy was performed 3 h later which revealed diffusely increased tracer uptake involving both axial and appendicular skeleton with nonvisualization of bilateral kidneys [Figure 1]. In view of history of prostatic adenocarcinoma, a provisional diagnosis of metastatic superscan was made. However, in view of low Gleason score and serum PSA level single emission computed tomography-computed tomography (SPECT-CT) of the pelvic bone was performed for further confirmation. SPECT-CT of the pelvis surprisingly did not reveal any sclerotic lesion [Figure 2]. On further evaluation, it was noted that patient is suffering from chronic renal failure with serum creatinine level was 8 mg/dl. Hence, in the absence of any demonstrable abnormality on SPECT-CT and presence of chronic renal failure differential diagnosis of diffuse marrow metastasis versus metabolic superscan due to chronic renal failure was made. So for further confirmation, bone marrow biopsy and serum parathyroid hormone (PTH) with calcium estimation were advised. Bone marrow biopsy was negative for metastasis. However, blood examination revealed increased serum PTH (972 ng/L) and decreased serum calcium (7.5 mg/dl) confirming diagnosis of metabolic superscan.

Figure 1

Planar skeletal scintigraphy revealed diffusely increased tracer uptake involving both axial and appendicular skeleton with non-visualization of bilateral kidneys

Figure 2

Single emission computed tomography-computed tomography of the pelvis did not reveal any sclerotic lesion

A superscan is defined as a skeletal scintigraphy which demonstrates markedly increased skeletal uptake relative to soft tissue and is usually with absent or faint genitourinary tract activity.[12] Superscans have been described mostly in metastatic and metabolic bone diseases, and less commonly in myeloproliferative disorders and miscellaneous conditions.[345] Among the metastatic causes of superscan prostatic malignancy is the most common cause.[5] However, long-standing prostatic malignancy may resulting in chronic renal failure which is the most common cause of metabolic super scan.[6] Differentiating between metastatic superscan and metabolic superscan is important because treatment modalities are different for two entities. The patterns of uptake between the metastatic and metabolic superscans are different. A metastatic superscan often demonstrates diffuse heterogeneous uptake in the axial skeleton and visualized tracer accumulation in the urinary bladder in spite of absent renal activity, but increased uptake on a metabolic superscan affects both axial and appendicular bones as well as the skull, mandible, and sternum uniformly.[6] However, it is sometimes difficult to differentiate between two entities based on planar scintigraphy. Hence, SPECT-CT should be performed in doubtful cases such as prostatic adenocarcinoma with low Gleason score, low serum PSA level and high serum creatinine level to differentiate between metastatic superscan and metabolic superscan. We, hereby, described a unique case of prostatic adenocarcinoma presented with metabolic superscan and highlighted the importance of SPECT-CT in correctly diagnosing underlying etiology of the superscan.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.