A variable length interstitial probe apparatus includes: a probe for effecting thermal therapy and/or cryotherapy to a tissue; a flexible umbilical sheath permanently affixed to the probe, including at least one interface for supplying energy, cooling fluid, cooling gas, heating fluid, and/or heating gas to the probe; and an adjustable depth stop configured to slide along a length of a shaft region of the probe, and lock to the shaft region at a selected position. The adjustable depth stop is configured to engage a probe driver and/or a skull mount apparatus to stabilize positioning of the probe and to control a depth of entry of the probe into a patient. The probe may be configured to effect temperature modulation therapy, where processing circuitry activates a modulation pattern of thermal therapy emission and cryogenic therapy emission for applying a thermal dose to the tissue.