What is the best initial step for assessing a 2 cm movable mass on the left flank, considering a history of benign tumor and family history of similar conditions?

Ultrasound is the Best Initial Imaging Study for a Flank Mass

Ultrasound should be the first imaging modality for evaluating the 2 cm movable, painful mass on the patient's left flank, considering his history of benign tumors and family history of similar conditions. 1

Rationale for Ultrasound as First-Line Imaging

Ultrasound is recommended as the initial imaging study for this patient for several key reasons:

1. Location-specific guidelines: The ACR Appropriateness Criteria specifically identifies the flank as an area where radiographs have limitations and may be unrewarding, particularly for small, non-mineralized masses 1

2. Superficial nature: The mass is palpable and appears to be superficial, making it ideal for ultrasound evaluation which has proven most useful for small superficial lesions 1

3. High diagnostic accuracy: For superficial soft tissue masses, ultrasound has demonstrated sensitivity of 94.1% and specificity of 99.7% 1

4. UK guidelines support: The British Sarcoma Group guidelines recommend urgent direct access ultrasound scan to assess for soft tissue sarcoma in adults with an unexplained lump that is increasing in size 1

5. Efficient triage tool: Ultrasound provides an effective initial triage tool that can help distinguish benign from malignant characteristics 1, 2

What Ultrasound Can Determine

An ultrasound of this flank mass will help determine:

• Solid versus cystic nature of the mass
• Vascularity patterns
• Relationship to adjacent structures
• Size and depth measurements
• Margin characteristics (well-defined vs. irregular)
• Internal echogenicity (homogeneous vs. heterogeneous)

Clinical Significance of Patient History

The patient's history raises several important considerations:

• History of benign tumor on left eyebrow in childhood
• Family history (mother) of benign spine tumors
• Recent onset (2 months)
• Pain on deep palpation but otherwise asymptomatic
• Movable mass on examination

These features suggest a possible genetic predisposition to benign tumors, though the painful nature of the mass warrants thorough evaluation.

Next Steps After Ultrasound

Based on ultrasound findings, the diagnostic pathway would proceed as follows:

1. If ultrasound shows benign characteristics (well-defined margins, homogeneous echogenicity, no concerning vascularity):

   • Clinical follow-up may be appropriate
   • Consider biopsy if the patient has anxiety or if there are other clinical concerns

2. If ultrasound shows suspicious features (irregular margins, heterogeneous echogenicity, increased vascularity):

   • Proceed to MRI for further characterization
   • Consider referral to a specialist sarcoma team for biopsy planning

3. If ultrasound is inconclusive:

   • MRI would be the next appropriate imaging study for soft tissue masses of the trunk 1

Important Caveats

• While ultrasound is highly accurate for superficial masses, its diagnostic accuracy decreases for deeper lesions 1
• Ultrasound is user-dependent, so the quality of the examination depends on the operator's experience 1
• Even with benign ultrasound features, any mass that is increasing in size warrants further evaluation 1
• The patient's family history may warrant genetic counseling if specific hereditary tumor syndromes are suspected 1

Ultrasound represents the optimal balance of diagnostic utility, cost-effectiveness, and patient comfort for the initial evaluation of this flank mass, allowing for appropriate triage to more advanced imaging or biopsy if necessary.