Up to 40% of Obsessive-Compulsive Disorder (OCD) patients do not respond to conventional treatments (medications or behavior therapy). For some of them, a neurosurgical treatment can be indicated. A previous study, employing bilateral double-shot ventral capsular/ventral striatal (VC/VS) Gamma capsulotomy (GVC) for OCD has shown that this radiosurgical technique is potentially efficacious and relatively safe. However, a few patients may develop complications associated to radionecrosis (eg., brain cysts) in a long-term follow-up, which are probably associated to lesion volumes. Another study, with the preliminary results of smaller VC/VS Gamma capsulotomy lesions has suggested that this procedure is safe and remains efficacious. Our aim is to investigate the efficacy and safety profiles of smaller, single-shot VC/VS Gamma capsulotomy for OCD. This study will support the development of a future double-blind, randomized clinical trial of single-shot VC/VS Gamma capsulotomy.

Inclusion Criteria:

• A DSM-V diagnosis of OCD as the main diagnostic entity. If comorbid axis I or II disorders are present, OCD symptoms are the most troublesome among disorders.
• Age range between 18 and 65 years old.
• At least 5 years of OCD symptoms.
• "Yale-Brown Obsessive-Compulsive Scale" (YBOCS) scores greater than 26 (or greater than 13, for isolated obsessions or compulsions).
• Refractoriness criteria fulfilled.
• To be accepted by the "best estimate" method as a treatment refractory patient. It consists in the careful examination of each patient's history by two OCD specialists, so as to confirm refractoriness status.

Refractoriness criteria:

• At least 3 serotonin reuptake inhibitors (selective or not) have been tried before. One of these trials must have included clomipramine. All drugs were used for a minimum of 12 weeks, at the maximum doses or the maximally tolerated doses.
• Previous participation in a cognitive behavior therapy program (exposure and response prevention), for a minimum of 20 hours' time; or participation for some time, without subsequent adherence, due to severe OCD symptoms, and acceptance of the independent review board.
• YBOCS scores reductions after adequate drug therapy and psychotherapy not better than 25 %, or confirmation of no clinical improvements by the mental health professionals who were responsible for the treatments of the patient.
• By the end of adequately conducted pharmacological trials, "Clinical Global Impression" (CGI) scores not better than minimal improvement.
• Previous use of at least two antidepressant augmentation strategies (such as the association of a typical or atypical antipsychotic, another serotonin reuptake inhibitor, a benzodiazepine, an anticonvulsant or an antiglutamatergic medication), in adequate doses for a sufficient period of time, without satisfactory responses.

Exclusion Criteria:

• Less than 18 years or more than 65 years of age.
• Past history of head injury, with posttraumatic amnesia.
• Past or general medical condition, or neurological illness with brain compromise (severe and in active phase)
• History of physiological effects of a substance, as determinant of psychopathological symptoms, or cumulative signs of alcohol or drug abuse in the Central Nervous System (such cortical atrophy), confirmed by a neuroimaging scan.
• Pregnancy or lactation.
• Refusal to participate in radiosurgical procedures.
• Refusal to sign the Patient Information and Consent Form, or refusal to take part in this study.
• History of mental retardation and/or being unable to understand the Patient Information and Consent Form, confirmed by poor performance on neuropsychological tests.