What are the differential diagnoses for a right iliac fossa (RIF) mass?

Differential Diagnoses for Right Iliac Fossa Mass

In patients over 50 years, CT abdomen/pelvis with IV contrast is mandatory before any surgical intervention to identify the underlying pathology, as this age group has a significantly different disease spectrum than younger patients, with malignancy and diverticular disease being common causes. 1

Age-Stratified Diagnostic Approach

Patients Over 50 Years

CT scanning should be mandatory unless contraindicated in patients aged over 50 years presenting with RIF mass or peritonism, as this population has a 97% sensitivity for identifying pathology and prevents unexpected operative findings. 1

The most common diagnoses in this age group include:

• Appendicitis (27%) - remains common but less frequent than in younger patients 1
• Colonic neoplasia (15%) - cecal carcinoma is a critical diagnosis that must be identified preoperatively 1, 2
• Acute colonic diverticulitis (10%) - particularly right-sided diverticulitis which can mimic appendicitis 1
• Non-specific abdominal pain (15%) 1

All Age Groups

Based on a large single-center analysis, the frequency distribution of RIF masses is: 2

• Appendicular mass (45.3%) - the most common cause overall
• Appendicular abscess (17.5%) - represents complicated appendicitis
• Ileocecal tuberculosis (12.9%) - important in endemic regions
• Cecum carcinoma (7.4%) - increases with age
• Ovarian tumors (6.4%) - in women of reproductive age
• Parietal lipoma (4.6%)
• Retroperitoneal tumors, parietal abscesses, ileocecal lymphadenopathy (1.8% each) 2

Pre-Menopausal Women

In women of reproductive age, ultrasound is the first-line imaging modality but has significant limitations for appendicitis diagnosis. 3

Key diagnostic considerations include:

• Gynecological pathology - ovarian cysts, tubo-ovarian abscess, ectopic pregnancy, broad ligament leiomyoma 4, 3
• Appendicitis - ultrasound identifies only 19% of cases (5 of 26 confirmed cases), making negative ultrasound unreliable for excluding appendicitis 3
• Broad ligament leiomyoma - rare but can present as RIF mass with compression symptoms 4

Critical pitfall: In pre-menopausal women with strong clinical suspicion of appendicitis, proceed directly to diagnostic laparoscopy rather than relying on negative ultrasound findings, as 68% of patients with negative or non-specific ultrasound had appendicitis at surgery. 3

Imaging Algorithm

Initial Imaging Selection

Ultrasound is the first-line imaging modality in younger patients and women of reproductive age because it avoids radiation exposure, correctly identifies the organ of origin in 97% of cases when positive, and can guide further management. 5

However, ultrasound has important limitations:

• No abnormality detected in 24% of cases with actual pathology 5
• Poor sensitivity for appendicitis (21-95.7% range), with significant variation by body habitus 6
• False diagnosis rates increase dramatically in obese patients: 34.4% in obese males vs 6.2% in non-obese males 6

CT Indications

CT abdomen/pelvis with IV contrast achieves 85.7-100% sensitivity and 94.8-100% specificity for acute abdominal pathology and should be used when: 7

• Patient age >50 years (mandatory) 1
• Ultrasound equivocal or negative with high clinical suspicion 3
• Obesity limiting ultrasound visualization 6
• Suspected malignancy or complicated pathology 1

MRI Considerations

MRI demonstrates 96% sensitivity and 96% specificity for appendicitis and is particularly valuable in pregnant patients, though average scan time is 14 minutes. 6 No significant difference exists between contrast-enhanced and non-contrast protocols for appendicitis detection. 6

Clinical Scoring Systems

The Adult Appendicitis Score (AAS) outperforms other clinical scores with area under ROC curve of 0.882 vs 0.790 for Alvarado score, stratifying patients into high-risk (score ≥16,93% specificity) and low-risk (score <11, only 4% have appendicitis) groups. 6

For women, AAS with cutoff ≤8 achieves 63.1% specificity with 3.7% failure rate; for men, AIR score with cutoff ≤2 achieves 24.7% specificity with 2.4% failure rate. 6

Specific Pathology Considerations

Appendiceal Pathology

• Fecal loading sign on plain radiography has 97% sensitivity and 85% specificity for appendicitis, though cross-sectional imaging has largely replaced this. 6
• Complicated vs uncomplicated appendicitis can be predicted using temperature, CRP, free fluid on ultrasound, and appendiceal diameter, with 95% accuracy for identifying uncomplicated cases. 6

Inflammatory Bowel Disease

When the appendix appears normal but cecum and terminal ileum are inflamed during surgery, obtain multiple biopsies as this pattern suggests Crohn's disease requiring different long-term management. 8

Malignancy Red Flags

In patients >50 years with RIF mass, maintain high suspicion for cecal carcinoma (7.4% of cases) and ensure preoperative tissue diagnosis when possible, as unexpected malignancy at surgery requires modification of the operative approach. 1, 2