MSK and Oncology

Discussion

68-year-old male with history of right nephrectomy due to renal carcinoma 10 years ago and removal of lymph nodes. The patient had a partial resection of the right psoas muscle due to metastasis in the following years.

Increased conspicuity of contrast uptake and better demarcation of the right psoas nodular lesion on lower keV monoenergetic images.

Coronal and axial images

Conventional coronal and axial CT images with and without contrast showed an enhancing mass in right residual psoas muscle. The mass had increased in size compared to previous examinations. Virtual non-contrast images were comparable to true non-contrast images. The mass was better delineated on virtual monoenergetic images at 45 keV. Iodine density and Z effective images showed iodine uptake quantitatively compared to unaffected parts of the psoas muscle. The patient underwent resection of the mass, and pathology results were compatible with metastasis of chromophobe renal cell carcinoma. The follow-up images 6 months later did not show contrast uptake or attenuation difference in the surgery location; so the small soft tissue density was regarded as a postoperative change, and follow-up evaluation was decided.

The attenuation difference between the mass with increased iodine uptake and unaffected segments of psoas muscle was increased at virtual monoenergetic images at 45 keV. Increased conspicuity of contrast uptake of a mass on virtual monoenergetic images at lower keV may improve diagnostic performance. Iodine density and Z effective images could also demonstrate the iodine uptake quantitatively.

Conventional CT with contrast, coronal image at 120 kVp: enhancing mass in right psoas muscle (arrow).

Virtual monoenergetic coronal image at 45 keV: better demarcation of the enhancing mass in right psoas muscle (arrow).

True non-contrast CT, axial image: right psoas muscle mass (37.9 HU, arrow).

Virtual non-contrast CT, axial image: right psoas muscle mass density (38.9 HU, arrow) was comparable to true non-contrast images (37.9 HU).

Virtual non-contrast image: shows right psoas muscle mass (arrow). Images were comparable to true non-contrast images.

Virtual monoenergetic image at 45 keV: better demarcation of right psoas muscle (arrow).

Iodine density image: increased iodine uptake in right psoas muscle mass (white arrow, 2.41 mg/ml) compared to unaffected medial portion of the muscle (blue arrow, 0.19 mg/ml).

Z effective image: contrast uptaking right psoas muscle mass color coded in blue (white arrow) had higher effective atomic number (8.56) compared to unaffected medial portion of psoas muscle (7.40) color coded in yellow (blue arrow).

Follow-up (6 months later) conventional CT with contrast, axial image: soft tissue density at the location of surgery (arrow) without contrast uptake regarded as postoperative changes rather than residue or recurrence.

Follow-up iodine density image: iodine uptake of the soft tissue density (arrow) was measured at 0.27 mg/ml (below threshold of 0.5 mg/ml).

Z effective image: soft tissue density at the location of surgery with similar effective atomic number (7.44, white arrow) to contralateral psoas muscle (7.46, blue arrow) both color coded in yellow.