X-ray is the Next Investigation
Plain radiographs in two orthogonal planes should always be the first investigation for any elderly patient with a well-demarcated, red, painful thigh lesion, as this rapidly excludes bone involvement, periosteal reaction, or underlying pathology that could masquerade as a superficial process. 1
Why X-ray Takes Priority
The clinical presentation—well-demarcated, red, and painful—could represent several entities ranging from benign (cellulitis, abscess) to serious (underlying osteomyelitis, bone tumor with soft tissue extension). The critical principle is that you must never assume a superficial presentation means superficial disease, as this could represent overlying inflammation from underlying bone pathology. 1
The Diagnostic Algorithm
Step 1: Plain X-ray in two planes is mandatory as the initial imaging study and should never be skipped, regardless of how superficial the lesion appears clinically. 2, 3, 1
Step 2: If X-ray is normal but symptoms persist (particularly pain, which is highly concerning), proceed to MRI to evaluate soft tissue and bone marrow pathology. 3, 1
Step 3: If X-ray shows suspicious findings (bone destruction, periosteal reaction, calcification), obtain MRI of the entire bone with adjacent joints for local staging, then refer to a specialized sarcoma center before any biopsy. 2, 3, 1
Why Not Punch Biopsy First?
Punch biopsy should never be performed before adequate imaging, as this fundamental error can contaminate tissue planes and compromise future surgical management if malignancy is present. 1 While punch biopsy is useful for primary cutaneous lesions, 4, 5 performing it before imaging in a painful thigh lesion violates the cardinal rule: image before you cut. 3, 1
The presence of pain—especially in an elderly patient—warrants radiological assessment to exclude bone pathology before any tissue sampling. 2, 1 Bone sarcomas and aggressive lesions are frequently difficult to recognize clinically even by experienced physicians, and premature biopsy at a non-specialized center can lead to inadequate sampling or inappropriate surgical contamination. 3
Why Not MRI or Ultrasound First?
While MRI is superior for soft tissue characterization and local staging, it is indicated only when malignancy cannot be excluded with certainty on plain radiographs. 2, 3, 1 Starting with MRI bypasses the rapid, inexpensive screening that X-ray provides for bone involvement. 1
Ultrasound has no role in the initial evaluation of a painful thigh lesion where bone pathology must be excluded. 1
Critical Pitfall to Avoid
A recent injury or trauma does not rule out malignancy and must not prevent appropriate diagnostic procedures. 2, 3 The pain in this elderly patient could represent pathologic fracture, bone tumor, or infection—all of which require X-ray as the first step.
Answer: C. X-ray