Giles Walter Boland MD, Presenter: Principal, RCG HealthCare Consulting

Non-contrast CT has proven highly effective for characterizing adrenal incidentalomas as benign when attenuation values are < 10 HU. Usually no further test is needed. PET/CT is gaining acceptance as a useful test to evaluate metastatic disease in patients with cancer, but often only a low dose attenuation correction CT is performed. This study compares the attenuation measurements in proven adrenal adenomas in this group of patients between normal dose non-contrast CT and low-dose PET/CT.

35 patients with 39 proven adrenal adenomas (M:F, 19:16, age 28-84, mean age 59 years) underwent non-contrast 16 slice MDCT (regular CT dose: kVP 140 = 8 patients, kVp 120 = 27 patients) and low-dose attenuation correction CT as part of a PET/CT study (kVp = 120). mA values ranged from 180-560 (mean = 310) for regular CT and 40-122 (mean = 60) for low dose CT. Lesions were proven either by follow-up (32 patients) or biopsy (3 patients). Mean lesion size was 1.8 cm (range 1.2 – 3.6 cm). Three ROI HU measurements were made for each lesion.

Mean attenuation values using regular CT = 14. 5 HU (range -16 to 41 HU), and for low dose CT = 3.5 HU (range – 33 to 49 HU) (P < 0.05 difference). Mean noise standard deviation for regular CT = 18 (range 9-39) and for low-dose CT = 33 (range 18-85) (P < 0.05 difference). The HU differences were more pronounced for smaller lesions, (possible partial voluming affects). In 5/39 lesions where attenuation > 10 HU by regular CT, were < 10 HU when measured by low dose CT. These 5 lesions would have therefore have been incorrectly characterized as benign rather than indeterminate if attenuation measurements alone were used.

Caution is advised when attempting to characterize adrenal adenomas with low dose non-contrast PET/CT, particularly when lesions are small and/or measure < 10 HU. Normal dose non-contrast CT is recommended if characterization is based on HU measurements alone.

The attenuation values from the the attenuation correction CT from PET/CT is not recommended to aid adrenal lesion characterization.

Boland, G, Adrenal Adenoma Attenuation Values: Comparison of Low Dose (Attenuation Correction) PET/CT versus Normal Dose CT.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8016501.html