Heard you can diagnose ganglion cyst by transillumination with flashlight.?

Theoretically: A ganglion is a fluid filled cyst that should transilluminate - if it is large enough and the skin over it is thin enough. In practice the overlying skin and subcutaneous tissue may be too dark or dense to allow this to happen.

If large: Yes if this is large and you have an intense light source. Turn off the lights and place your light source on the opposite side of the cyst. It may glow as the light shines through it.

One Factor: In diagnosis of ganglions, we often do transilluminate the lesion with a small penlight. This alone, however, is not diagnostic. We evaluated the nature of the lesion, associated symptoms, its location and its features on clinical examination coupled with necessary imaging and sometimes pathology before making the final call.