Estimation of patient-size dependent imaging dose for stereoscopic/monoscopic real-time kV image guidance in lung and prostate SBRT

Abstract

Purpose: The purpose of this work is to quantify the dependence of patient-specific imaging dose on patient-size from ExacTrac stereoscopic/monoscopic real-time tumor monitoring during lung and prostate stereotactic body radiotherapy (SBRT). 
Approach: Thirty lung and 30 prostate SBRT patients that were treated with volumetric modulated arc therapy (VMAT) were selected and divided into three patient size categories. Imaging doses from all SBRT fractions were calculated retrospectively assuming patients went through real-time tumor monitoring during their actual VMAT treatment times. Treatment times were divided into periods of stereoscopic and monoscopic real-time imaging depending on the imaging view with linac gantry blockage. The computed tomography (CT) images and contours of the planning target volume (PTV) and organs at risk (OARs) were exported from the treatment planning system. Based on the CT data, patient-specific 3D imaging dose distributions were calculated in a validated Monte Carlo (MC) model using DOSEXYZnrc. Vendor-recommended imaging protocols (lung: 120-140 kV, 16-25 mAs; prostate: 110-130 kV, 25 mAs) were used for each patient size category. Patient-specific imaging doses received by PTV and OARs were evaluated using dose volume histograms, dose delivered to 50% of organ volume (D50), and 2% of organ volume (D2). 
Results: Bone and skin received the highest imaging dose. For the lung patients, the highest D2 for bone and skin were 4.30% and 1.98% of the prescription dose respectively. For prostate patients, the highest D2 were 2.53% and 1.35% of the prescription for bone and skin. Additional imaging dose to PTV as a percentage of the prescribed dose was at most 2.42% for lung and 0.29% for prostate patients. T-test results showed statistically significant difference in D2 and D50 between at least two patient size categories for PTVs and all the OARs. Larger patients received more skin dose in both lung and prostate patients. For the internal OARs, larger patients received more dose in lung treatment while the trend was opposite in prostate treatment. 
Conclusion: Patient-specific imaging dose was quantified for monoscopic/stereoscopic real-time kV image guidance in lung and prostate patients with respect to patient size. Additional skin dose was 1.98% (in lung patients) and 1.35% (in prostate patients) of the prescription which is within 5% recommended value by the AAPM Task Group 180. For internal OARs, larger patients received more dose in lung patients while the trend was the opposite for prostate patients. Patient size was an important factor to determine additional imaging dose.