Feasibility of surface-guided radiotherapy for detecting patient instability during prostate radiotherapy

Purpose: Maintaining patient immobility during radiotherapy is essential for accurate dose delivery; however, patients may experience discomfort during treatment, leading to involuntary movement.Continuous imaging to detect intrafractional changes is not routinely feasible in conventional fractionation due to radiation exposure.This study aimed to evaluate the feasibility of knee-based surface-guided radiotherapy (SGRT) as a real-time method for detecting subtle patient motion during prostate radiotherapy.Material and Methods: A total of 125 patients undergoing prostate intensity-modulated radiation therapy (2,527 fractions) were monitored using an SGRT system with the knees as the region of interest.Mean and maximum displacement per fraction were analyzed.Clinical factors, including bladder volume, age, International Prostate Symptom Score, laxative use, and overactive bladder medication, were evaluated.A displacement threshold of ≥4 mm was defined as an operational level to identify notable motion events.Results: Displacement ≥4 mm was observed in 357 of 2,527 fractions (14.1%).Larger displacements were more frequent in the latter half of sessions and were often suggestive of urinary urgency.In a single case, motion improved following verbal instruction.Bladder volume was significantly associated with both mean displacement (p = 0.025) and the proportion of fractions with displacement ≥4 mm (p = 0.004), whereas other factors were not consistent predictors.Knee motion monitoring allowed real-time detection of patient movement requiring clinical attention.Conclusion: Knee-based SGRT appears to be a feasible, non-invasive method for monitoring patient motion.While not a surrogate for prostate displacement, it may help identify patient instability and support timely clinical responses.