What is ALAD?
Aorta-Lesion-Attenuation-Difference (ALAD) is a novel imaging biomarker on contrast-enhanced CT scans that has shown potential to differentiate certain types of renal tumors in initial retrospective studies.1,2 ALAD provides a noninvasive technique for examining tumor biology by evaluating dynamic enhancement patterns in an attempt to distinguish pathologic subtypes. Initial results demonstrate that ALAD can accurately discriminate malignant and benign oncocytic renal tumors (chromophobe renal cell carcinoma [RCC] and oncocytoma), which may aid in management of patients with indeterminate pathology on renal mass biopsy. Furthermore, ALAD may help reduce the number of needle biopsies or treatment in certain patients.
How to calculate ALAD
ALAD is calculated by subtracting the attenuation (in Hounsfield units) of a solid renal mass from the abdominal aorta on the same image slice on a contrast-enhanced CT scan in the nephrographic phase, defined as homogeneous enhancement of the renal cortex and medulla without excretion into the collecting system. To measure ALAD, the largest region of interest (ROI) possible should be placed in the enhancing portion of the renal mass while avoiding necrosis/scar/hemorrhage/calcification and the aorta while avoiding atherosclerotic change (see Figure below).
ALAD = aortic attenuation – lesion attenuation
A: 4.0 cm solid right renal mass with ALAD of 3 (135 HU – 132 HU). Note placement of the ROI within the lesion in the enhancing component of the tumor, avoiding measurement of the non-enhancing scar (arrow). Surgical pathology determined the tumor to be a benign renal oncocytoma (ALAD < 24).
B: 3.2 cm solid right renal mass with ALAD of 44 (142 HU – 98 HU). The largest possible ROI is placed in the aorta and the tumor, avoiding non-enhancing scar or necrosis. Surgical pathology showed the mass to be a clear cell RCC (ALAD > 24).
Interpretation of ALAD
Results from our team’s initial retrospective study show that a threshold ALAD value of 24 can differentiate chromophobe RCC from oncocytoma with a sensitivity of 100%, specificity of 86%, PPV of 75%, and NPV of 100% with AUROC of 0.98 (95% CI 0.91-1.00). A pre-treatment ALAD of > 24 was 93% predictive in discriminating RCC from oncocytoma or fat poor angiomyolipoma in our cohort.2
- Dhyani M, Grajo JR, Rodriguez D, Chen Z, Feldman A, Tambouret R, Gervais DA, Arellano RS, Hahn PF, Samir AE. “Aorta-Lesion-Attenuation-Difference (ALAD) on Contrast Enhanced CT: A Potential Imaging Biomarker for Differentiating Malignant from Benign Oncocytic Neoplasms.” Abdominal Radiology. 2017 Jun;42(6):1734-1743.
- Grajo JR, Terry RS, Ruoss J, Noennig BJ, Pavlinec JG, Bozorgmehri S, Crispen PL, Su LM. “Using Aorta-Lesion-Attenuation Difference (ALAD) on Preoperative Contrast-Enhanced CT Scan to Differentiate between Malignant and Benign Renal Tumors.” Urology: in press. Epub 2018 Dec 12.