External beam focal boost radiotherapy to intraprostatic lesions in prostate cancer: a scoping review

Background: Focal boost external beam radiation therapy (EBRT) delivers an escalated dose to intraprostatic lesions (IPLs) in prostate cancer. It is an emerging field characterized by highly heterogeneous practices. Materials and methods: This scoping review was conducted with adherence to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews guidelines. Eligible reports were reports detailing treatment methods and outcomes of focal boost EBRT in prostate cancer. Results: Forty-two studies were reviewed. Fractionation shifted over time from conventional to moderate and ultra hypofractionation. Patient populations were largely high-risk localized disease. Assuming α/β = 1.5, equivalent IPL boost dose in 2 Gy/fraction spanned 71.3-138 Gy in 32-45 fractions with conventional fractionation, 81.5-112.5 Gy in 15-25 fractions with moderate hypofractionation, and 96.4-196.4 Gy in 2-5 fractions with ultra hypofractionation. Multiparametric magnetic resonance imaging (MRI) was used for IPL delineation in 39 studies, and positron emission tomography/computed tomography (PET/CT) in five studies. Five-year biochemical disease-free survival and overall survival were: conventional fractionation 92-98.2% & 100%, moderate 75.2-96.7% & not reported, and ultra hypofractionation 93.2-100% & 86-100%. Median ≥ Grade 2 genitourinary/gastrointestinal toxicities were < 45% and ≥ Grade 3 < 10%. Conclusions: Focal boost EBRT appears feasible and may improve biochemical control without increasing toxicity, particularly in high-risk disease. However, optimal dose/fractionation, organ at risk constraints, and long-term survival benefits remain undefined. Large, preferably multicenter prospective studies involving several hundred patients, with standardized endpoints and comprehensive toxicity assessment are warranted.