The relationship between image guided radiation therapy (IGRT) frequency and errors remains unclear. We present this data obtained from various IGRT frequencies in head and neck cancer patients. All consecutive head and neck cancer patients receiving radiation with curative intent and daily IGRT using kV cone beam CT (CBCT) scans, between June and December 2018, were included in the study. Translational displacements in medial-lateral (ML), superior-inferior (SI) and anterior-posterior (AP) direction were documented daily. Four IGRT scenarios were simulated retrospectively with reducing frequency, namely, 50% (Displacement data of every alternate fraction), 30% (Displacement data of 1st 3 fractions followed by once weekly), 20% (Displacement data of once weekly fraction) and 10% (Displacement data of 1st 3 fractions). Displacement data for once weekly was documented from shifts acquired on treatment day 1, 6,11, 16 and so on. Mean Displacements (MD), Systematic Errors (Σ), Random Errors (σ) and Planning Target Volume (PTV) margins were calculated for daily and simulated scenarios (Table 1). Thirty-one patients were analyzed with a median age of 54.5 years (Inter-Quartile Range - 42.75 - 67), predominantly males (77.4%), commonly involving oral cavity (54.8%) and oropharynx (25.8%) and treated post-operatively (64.5%). A total of 831 CBCT scans were analyzed, demonstrating MD in ML, SI and AP were normally distributed across all the image guidance scenarios, except in SI direction for daily format (Kolmogorov-Smirnov test of normality; Skewness - 3.32, p = .05). MD in the three directions were <0.1cm, reflected in Σ (< 0.2cm) for daily and simulated IGRT scenarios; σ for different formats revealed that in SI direction, for daily format was 0.7cm, whereas it was around 0.2cm for simulated scenarios and these differences were demonstrated in PTV margin calculation, 1cm in SI direction based upon daily IGRT data and around 0.5cm based upon simulated IGRT scenarios; margin estimation was similar in ML and AP direction for daily and simulated IGRT scenarios (Table 1). MD >0.5cm were around 2-3% in ML and SI direction, and 5% in AP direction; whereas MD >0.3cm were ranging between 10-20%. Systematic and random errors were similar across all simulated IGRT scenarios, inconclusive for superiority of one schedule over another, however higher random errors were observed with daily format in SI direction. This further validates the need for more data in determining the optimum IGRT frequency.Tabled 1Abstract 2565; TableIGRT FrequencyMLSIAPErrors (Σ = standard deviation (SD) from values of mean displacement for all patients; σ = root mean square of SD of all patients)ΣσΣσΣσDaily0.110.200.210.700.120.2250%0.110.210.120.210.140.2230%0.130.220.140.200.120.2520%0.150.240.150.200.130.2710%0.150.250.140.200.140.28PTV Margin (Van Herk’s Formula) (2.5Σ + 0.7σ) (cm)MLSIAPDaily0.421.000.4650%0.430.440.5030%0.490.480.4820%0.540.510.5010%0.560.490.56 Open table in a new tab