patient's skull, and can be transferred to a similar ring that contains a simulated target. In this system, the phantom target is moved on the simulator to 3D coordinates. After adjusting the probe holder on the aiming bow so that the probe touches the desired target on the phantom, the transferable aiming bow is moved from the phantom base ring to the base ring on the patient. The probe is then lowered to the determined depth in order to reach the target point deep in the patient's brain. Treatment Stereotactic radiosurgery Stereotactic radiosurgery utilizes externally generated ionizing radiation to inactivate or eradicate defined targets in the head or spine without the need to make an incision. This concept requires steep dose gradients to reduce injury to adjacent normal tissue while maintaining treatment efficacy in the target. As a consequence of this definition, the overall treatment accuracy should match the treatment planning margins of 1–2 mm or better. To use this paradigm optimally and treat patients with the highest possible accuracy and precision, all errors, from image acquisition over treatment planning to mechanical aspects of the delivery of treatment and intra-fraction motion concerns, must be systematically optimized. To assure quality of patient care the procedure involves a multidisciplinary team consisting of a radiation oncologist, medical physicist, and radiation therapist. Dedicated, commercially available stereotactic radiosurgery programs are provided by the irrespective Gamma Knife, CyberKnife, and Novalis Radiosurgery devices. Stereotactic radiosurgery provides an efficient, safe, and minimal invasive treatment alternative for patients diagnosed with malignant, benign and functional indications in the brain and spine, including but not limited to both primary and secondary tumors. Stereotactic radiosurgery is a well-described management option for most metastases, meningiomas, schwannomas, pituitary adenomas, arteriovenous malformations, and trigeminal neuralgia, among others. Irrespective of the similarities between the concepts of stereotactic radiosurgery and fractionated radiotherapy and although both treatment modalities are reported to have identical outcomes for certain indications, the intent of both approaches is fundamentally different. The aim of stereotactic radiosurgery is to destroy target tissue while preserving adjacent normal tissue, where fractionated radiotherapy relies on a different sensitivity of the target and the surrounding normal tissue to the total accumulated radiation dose. Historically, the field of fractionated radiotherapy evolved from the original concept of stereotactic radiosurgery following discovery of the principles of radiobiology: repair, reassortment, repopulation, and reoxygenation. Today, both treatment techniques are complementary as tumors that may