(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: To measure two dimensional (2D) dose deposition matrix for partial and full gantry rotation cone beam computed tomography (CBCT) protocols
Methods: To study 2D dose matrix of CBCT along direction of gantry rotation, we designed and fabricated circular phantom made of PMMA . Phantom has diameter of 16cm. Center removable slab is with TLD grooves of dimensions 5mm diameter and arranged in circular fashion. TLD grooves are at distance7cm from center of phantom are analogous to CTDI phantom measurement at periphery location. The consecutive TLD grooves are arranged 10 degree apart. Two TLD (LiF:Mg,TI) were kept in TLD groove of 2D Matrix phantom. 2D matrix phantom was set on machine couch such that center plane was at Isocenter level. To get a reasonable signal, dosimeters were irradiated five times for partial and full CBCT imaging protocols on Varian clinac IX machine. TLDs were evaluated using commercial TLD-reader system and average of two TLD readings was used to estimate the dose.
Results: We observe that for full gantry rotation CBCT protocol, which uses 360° rotation for image acquisition, produces uniform dose distribution. It is observed 2D dose spread is uniform along all the gantry angles and for partial CBCT which uses 200° gantry rotation for image acquisition produces non-uniform 2D dose distribution with peak dose occurring in central plane on side of phantom closest to focal spot. It is observed dose spread is more towards gantry angle 270 as compared to gantry 90, this is because during partial CBCT, images are acquired by rotating the gantry in anticlockwise direction from 22 to 178 degree which results in kV source moving from 292 to 88 degree
Conclusion: With advancement in CBCT technology user has choice of selecting the gantry trajectory; 2D matrix phantom will help to measure 2D dose distribution efficiently.
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