Yes, POSTCAVAS is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, S 1, T 1, C 3, A 1, V 4, A 1, S 1.

Is it a valid word?

POSTCAVAS is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

receives blood from lower limbs and abdominal organs and empties into the posterior part of the right atrium of the heart; formed from the union of the two iliac veins

Also: inferior vena cava.