Cone-beam CT dosimetry for the positional variation in isocenter: a Monte Carlo study

This study investigated variations in kilovoltage cone-beam computed tomography (kV-CBCT) dosimetry for a change in the isocenter positions in the head-and-neck, lung-breast, and pelvis Rando phantoms using Monte Carlo simulations. Phase space kV-CBCT beams with energy of 120 kVp for the XVI system equipped to the Elekta Synergy medical linear accelerator were used to calculate dose distributions for the head-and-neck, lung-breast, and pelvis phantoms, with isocenters in the original positions near to the center of each phantom using the EGSnrc-based Monte Carlo code. Dose calculations were repeated with kV-CBCT isocenters shifted from the original positions along the x, y, and z axes. The relative dose ratio profiles, corresponding to different isocenter positions normalized to the original ones, were compared to evaluate the variations in the dose distribution due to isocentric shifts. Moreover, the relative dose at the isocenter, maximum dose, and mean dose for each isocenteric shift in the three phantoms were calculated. In this study, the original isocenter/center of the phantom was used as the reference for dose normalization. When the isocenter was chosen off from the center of the head-and-neck phantom, the maximum and mean imaging doses were found, varying by 14% and 15% in the anterior-posterior directions and 9% and 7% in the left-right directions. These dosimetric variations were slightly larger (17% and 17% in the anterior-posterior directions and 19% and 16% in the left-right directions) for the lung-breast phantom. For the pelvis phantom, the maximum and mean imaging doses varied by 10% and 22% in the anterior-posterior directions and 9% and 14% in the left-right directions with changes in the isocenter positions. Since the typical imaging dose is in the range of 2-8 cGy for the soft tissue per kV-CBCT acquisition, the variation in the imaging dose due to the patient setup error (approximately 2 cm) is insignificant compared to the original designed imaging dose. Moreover, the information on imaging dose varying as a function of the choices of isocenter or imaging center is useful for the physician and planner when they design the treatment plan for image-guided radiotherapy.