Off-centre effect on dose reduction to anterior surfaces with organ-based tube-current modulation

Organ-based tube-current modulation (OB-TCM) is a new technique of radiation dose reduction to reduce superficial doses to eyes, thyroid, and breast. Several studies have investigated the dose reduction with OB-TCM. However, the superficial doses in case when bodies are located at off-centre positions, have not been reported. The purpose of this study was to evaluate the effect of shifting the patient's body off-centre on the reduction of dose to anterior surfaces of eyes and breast with OB-TCM. A 16-cm CT dose index phantom and a semi-anthropomorphic acrylic thorax phantom were used to represent a normal-sized adult. The effect of in-plane eye and breast shields in addition to OB-TCM on the radiation output was also studied in off-centre phantom locations using a CT ionization chamber. The superficial doses of phantoms at off-centre position of 50, 25, -25, -50, and -75 mm were measured using thermoluminescent dosimeter chips, and compared with superficial doses of phantoms located at the isocentre. The dose reduction trend is similar in the head and thoracic scanning regions, i.e. bismuth shielding alone offers less reduction than OB-TCM alone, and OB-TCM combined with bismuth shielding gives the best reduction. OB-TCM reduced by 26-28% of the anterior surface dose to the eyes, and increased by 25% the posterior surface dose. The free-in-air dose decreased as the chamber was placed away from the isocentre. OB-TCM could reduce by 22% the dose to the eyes, but gave the same dose to breast as automatic tube-current modulation. In phantoms placed at off-centre positions, OB-TCM increased by some 18% the dose to the eyes and doubled superficial dose to the breast in phantoms shifted downwards.