Investigation of Dose Delivery from On-Board CBCT for Head and Pelvis Regions
Abstract
Introduction: The Varian 23EX Clinac at Siriraj Hospital is a linear accelerator with integrated On-Board Imager (OBI) system capable of producing cone beam computed tomography (CBCT) images of the patient in the treatment position. The increased use of CBCT for patient setup may signifi cantly increase the dose to the patient’s normal tissues. Objective: To investigate the skin and organ dose from the CBCT system from the clinical protocol setup, with fi xed technical setting for head and neck and pelvic scans. Materials and Methods: CBCT scans were acquired in full-fan and half-fan mode with full and half bowtie fi lters for head and pelvic area, respectively. The technical setting of 100 kV, 20 mA and 20 ms for head region and 125 kV, 80 mA and 13 ms for pelvic were used. TLDs-100H were used for Rando phantom skin dose measurement at the point of interest for head and neck and pelvic area. Organ doses were also measured inside Rando phantom. Results: For head and neck CBCT, The lateral surface dose was higher than anterior ( include critical organ such as eyes, lip, chin and neck). The peripheral organ dose at Lt. lat area dose (0.28 cGy) was less than central dose ( 0.4 cGy), but Right lateral area such as temporal and parotid was about the same as the central dose (brainstem). For pelvic CBCT, The lateral surface dose was not symmetrical, with Left lateral side being ~ 20% higher than Rt. Lat. The anterior (AP) surface dose was higher than lateral especially at the pubic area (5.2 cGy). The dose at bladder, rectum and bowl were about the same as the central dose (~ 3 cGy). The highest dose was in the left hip joint region up to 7 cGy. Conclusions: The use of CBCT for treatment verifi cation provided a high dose to the patient. It is important to consider about the optimal protocol and amount of times to verify the patient position to keep additional doses as low as reasonably practicable.
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