Letter: Neurosurgery Fellowships and the Residencies That Enfold Them: A Nationwide Correlational Analysis

To the Editor: We read with great interest the article by Bashir et al, “Neurosurgery Fellowships and the Residencies That Enfold Them: A Nationwide Correlational Analysis.”1 We commend the authors on a timely and valuable contribution to neurosurgical literature, highlighting how standardizing fellowship structures could enhance training across programs. Pursuing neurosurgery in the United States entails a rigorous, 7-year commitment, traditionally divided into junior and senior years. Residents gradually build clinical and operative skills, often with 6 to 24 months of dedicated research time during postgraduate year (PGY)–4 or PGY-5. Although originally intended for laboratory-based projects, this time is now flexibly used for graduate degrees, neuroanatomical studies, global health, or additional clinical training.1-3 A widely adopted strategy for residents to explore subspecialties is through enfolded fellowships. These experiences span a variety of subspecialties, including spine, neuro-oncology, functional, endovascular, epilepsy, and skull base surgery. These are among the most frequently Committee on Advanced Subspecialty Training–accredited fellowships (Table) and reflect both resident demand and subspecialty growth. Enfolded fellowships allow residents to develop clinical expertise in their niche with strong mentorship, although they differ in structure, timing, and depth. Consideration of an enfolded fellowship has a wide range of strengths and limitations, each of which is further modulated by whether the fellowship is being considered a complement to a planned postgraduate fellowship or as an alternative.1 Postgraduate fellowships, by contrast, occur after residency and offer additional focused training. The decision to pursue one model over the other—or both—depends on individual career goals, program flexibility, and institutional resources. Our goal here is to provide an overview of the evolving fellowship landscape and highlight key considerations for trainees making decisions about their future specialization.4,5 TABLE. - Public CAST-Accredited Fellowship Data Based on Neurosurgical Subspecialty Neurosurgical subspecialty fellowship No. of CAST-accredited fellowship programs Skull base 20 Spine 59 Neurocritical care 23 CNS endovascular 82 Cerebrovascular 12 Neurosurgical oncology 25 Peripheral nerve 10 Stereotactic and functional 44 Neurotrauma 6 CAST, Committee on Advanced Subspecialty Training. NEUROSURGICAL RESIDENCY PROGRAMS: DOES ONE SIZE FIT ALL? Though all neurosurgery residency programs aim for surgical excellence and high-quality patient care, their secondary emphases differ. Some programs focus heavily on academic productivity, producing research-oriented neurosurgeons; others emphasize technical mastery or leadership in clinical practice. These differences affect the feasibility of offering enfolded fellowships. Factors such as call burden, resident cohort size, scheduling flexibility, support from advanced practice providers, case volume, and faculty mentorship influence the ability to pursue these opportunities. Some programs even facilitate extramural enfolded fellowships, though these arrangements are difficult due to Accreditation Council for Graduate Medical Education restrictions and limitations on case counting toward graduation requirements. Given how frequently programs evolve—whether due to leadership changes, shifting faculty interests, or changing class sizes—candidates should be cautious about using the promise of an enfolded fellowship as a decisive factor in program ranking. NOVEL CURRICULAR STRATEGIES AND PEDAGOGICAL TRENDS Recent innovations reflect growing interest in enfolded fellowships. As of the 2024 to 2025 Electronic Residency Application Service cycle, the University of Arizona College of Medicine Phoenix and Ascension St. Vincent Hospital have introduced models where PGY-6 is the chief year and PGY-7 is dedicated to an enfolded fellowship.6 These PGY-7 years are formally designated as “transition to practice” years, allowing residents to function with greater independence while focusing on subspecialty training.7,8 Similar structures exist at institutions such as Mayo Clinic and Rush University, where residents may pursue subspecialty or career development during their final year.9,10 Although formal data are limited, a review of program websites and curricula indicates that a growing number of residencies now offer some kind of enfolded fellowship opportunity, marking a shift in neurosurgical training culture. ENFOLDED FELLOWSHIPS: ON TIMING AND OPPORTUNITY A key debate centers on the timing of enfolded fellowships. Many educators argue that pursuing these experiences after the chief year is ideal. This aligns with American Board of Neurological Surgery recommendations, which express caution around certifying graduates whose subspecialty training occurred before they held leadership responsibilities.11 There are exceptions; for instance, early enfolded experiences may be appropriate for procedural areas such as endovascular or functional neurosurgery. However, for most residents, a PGY-7 fellowship is more valuable. This structure allows residents to first hone their leadership and operative independence during the chief year and then apply that maturity in an advanced clinical fellowship. The fellow role acts as a bridge to junior attending status, reinforcing both confidence and competence. Nonetheless, given the constraints of residency design, programs should remain open to flexible models to ensure broader access to subspecialty exposure. POSTGRADUATE FELLOWSHIPS: MORE IS MORE Enfolded fellowships are sometimes seen as a way to shorten training. However, postgraduate fellowships offer unique advantages. Chief among these is the opportunity to train outside one's home institution. Exposure to a different clinical environment, surgical philosophy, and mentorship team can foster professional and personal growth. It also helps trainees build a broader collaborative network and gain familiarity with varying institutional cultures. Therefore, we recommend viewing enfolded and postgraduate fellowships as complementary rather than competing pathways. When carefully coordinated, they allow for more robust and diverse subspecialty training. For example, one could pursue an enfolded skull base fellowship followed by a postgraduate open cerebrovascular fellowship—or an enfolded endovascular year followed by comprehensive dual training in open and endovascular cerebrovascular neurosurgery.12