What is the most appropriate initial investigation for a patient presenting with a painless abdominal mass?

CT Abdomen is the Most Appropriate Investigation

For a patient presenting with a painless abdominal mass discovered on routine examination, CT abdomen and pelvis with IV contrast is the most appropriate initial investigation to characterize the mass, determine its organ of origin, assess for malignancy, and guide definitive management 1, 2.

Why CT is Superior to Other Options

CT Provides Comprehensive Diagnostic Information

• CT detects the mass origin, size, and relationship to surrounding structures with high accuracy, altering diagnosis in 49-54% of patients with abdominal pathology and changing management plans in 42% of cases 1, 2
• CT with IV contrast is essential for characterizing tissue enhancement patterns that distinguish benign from malignant lesions and for evaluating vascular involvement 1
• Multiplanar reformations available with modern CT technology improve diagnostic confidence without requiring additional imaging 1

Why NOT the Other Options

Abdominal X-ray (Option C) is inadequate because conventional radiography has severely limited diagnostic value for abdominal masses, with sensitivity of only 49% even for bowel obstruction, and provides no tissue characterization 1, 3, 2. Plain films cannot determine the organ of origin, assess for malignancy, or guide surgical planning 1, 2.

Biopsy (Option A) is premature without imaging first because you cannot safely perform a biopsy without knowing the mass location, vascularity, relationship to vital structures, and whether it represents a surgical emergency like contained rupture or abscess 1. Imaging must precede tissue diagnosis 4.

MRI abdomen (Option B) is not first-line because while MRI provides excellent soft tissue characterization, it has longer acquisition times, limited availability, higher cost, and cannot adequately assess calcifications that may be diagnostically important 1, 2. MRI is reserved for specific scenarios where CT is contraindicated or inconclusive 2.

Specific CT Protocol Recommendations

• Use CT abdomen and pelvis with IV contrast as the single-phase examination is typically sufficient for diagnosis without requiring pre-contrast or delayed phases 1
• Include the pelvis in the scan range because masses may extend into the pelvis or arise from pelvic organs, and this was critical in 75.5% of cases in one surgical series 1, 4
• Oral contrast is generally unnecessary and delays diagnosis without improving diagnostic accuracy in 96.6% of cases 5

Critical Clinical Pearls

Red Flags Requiring Urgent CT

• Any concern for contained rupture (especially if considering abdominal aortic aneurysm) requires immediate CT without and with contrast to detect the "crescent sign" and other signs of imminent rupture 1
• Pulsatile masses mandate urgent CT angiography (CTA) to evaluate for aneurysm, with measurements using outer-to-outer wall diameter perpendicular to the aortic long axis 1

What CT Will Reveal

• Organ of origin was specified by CT in 89.3% of cases where ultrasound was inconclusive 4
• Malignant vs benign features through enhancement patterns, invasion of adjacent structures, and presence of metastases 6
• Surgical planning details including vascular anatomy, resectability, and optimal surgical approach 1, 4

Common Pitfalls to Avoid

• Do not obtain plain radiographs first as they will not change management and only delay definitive diagnosis 1, 3, 2
• Do not start with ultrasound for a palpable abdominal mass unless there is specific concern for AAA screening, as ultrasound specified the diagnosis in only 75.5% of cases and required CT for confirmation in most patients 7, 4
• Do not proceed to biopsy without cross-sectional imaging as this risks complications and may be unnecessary if imaging reveals a clearly resectable lesion 4

When Ultrasound Has a Role

Ultrasound is appropriate only for:

• Suspected abdominal aortic aneurysm screening in asymptomatic patients, where dedicated aortic ultrasound with leading-edge to leading-edge AP diameter measurements is adequate 1
• Resource-limited settings where CT is unavailable, though 52.8% of patients still required CT for definitive diagnosis 4

In all other scenarios of painless abdominal mass, proceed directly to CT abdomen and pelvis with IV contrast 1, 2.