Clinical Example: Flashlight transillumination for tumor diagnosis

Imaging has come a long way in recent years, but there remains a middle ground between physical examination and MRI - light transillumination. I regularly use a penlight in the office as a low tech but often helpful diagnostic tool. Tumors fall into four groups based on opacity relative to surrounding tissues:
Finding
Likely pathology in the hand or finger
Bright: more transparent than surrounding tissues. The area lights up more than the surrounding tissues. Ganglion cyst filled with clear liquid
Equal: the area lights up to the same degree as surrounding tissues. Lipoma
Neurolemmoma
Indeterminate: the area seems darker than surrounding tissues to a variable degree. Giant cell tumor
Inclusion cyst
Thick skin callus
Dark: the area is clearly opaque, creating a shadow effect. Aneurism
Hemorrhage into a ganglion cyst
Calcification
Gouty tophi
Osteophyte

 
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Case 1: Mass arising a year after nail bed excision and skin graft for nail bed squamous cell carcinoma.
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Xrays show discrete calcification in the area, consistent with heterotopic ossification.
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Transillumination appearance, dark or opaque. 
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Case 2: Firm mass arising just distal to the PIP joint, fixed to deep structures. 
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Xray shows a contour change of the middle phalanx deep to the tumor.
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Transillumination is indeterminate, slightly darker than surrounding tissues.
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Intraoperative finding: classic giant cell tumor, arising from the PIP collateral ligament.
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Case 3: Painless dorsal middle phalanx mass.
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A standard plastic disposable flashlight is fine, but better if the tip is wrapped in opaque electrical tape to limit the light flare through the sides of the flashlight.
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Bright transillumination confirms the diagnosis of ganglion cyst. Treatment: observation.
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Case 4: similar to case 3.
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Flashlight
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A picture of the illumination is more obvious with the finger placed on a lit transparency light box.
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Case 5: illustrating the use of transillumination even in the darg skinned patient. Chronic dorsal distal phalanx tumor and concave nail deformity:
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Flashlight:
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Transillumination: clearly a mucous cyst.
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Intraoperative excision and joint debridement, demonstration of the deep pull out sutures used to close the deep skin layer of an eponychial splitting incision:
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Healed.
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