A biopsy device includes a housing having a vacuum port, a cutter hub and attached cutter being slidably mounted in the housing so that the cutter hub and cutter are movable between a proximal, armed position, and a distal, fired position. A sealed cutter spring well with a cutter firing spring seated therein is disposed within the housing, wherein a proximal end of the cutter hub comprises a movable distal wall of the spring well so that proximal movement of the cutter hub compresses the cutter firing spring. A vacuum lumen is in communication with each of the vacuum port and spring well. A cutter arming valve assembly having a valve member is disposed in the vacuum lumen and selectively configurable to place the spring well in communication with the vacuum port for arming the cutter, or with atmosphere for firing the cutter.
