SU-FF-T-536: Cone Beam CT Observation of Intra-Fraction Shift During Intracranial Treatments for Patients Held in a Mask

Abstract

Purpose: When patients are held in a mask for co-planar intracranial IMRT the question remains as to how much the patient shifts during the 10–20 minute treatment. This study was undertaken to obtain data using cone beam CT (CBCT) measurements and determine the range of this intra-fraction motion. It was also unknown what effect the couch movement, which included tilts up to 3 degrees, would have on the stability of the patient positioning. Method and Materials: Intracranial patients being treated on a new Synergy Linac with 4 mm MMLC, CBCT and a Hexapod couch (Elekta Inc) were immobilized at CT planning and during treatment using a Uniframe mask and “Bear Claw” shoulder restraint (WFR Aquaplast) . A CBCT scan was taken once the patient was set-up to marks on the mask (Initial Scan), after Hexapod adjustment (Pre-Treatment) and immediately after treatment (Post-Treatment Scan). CBCT to the planning CT fusion was accomplished with the system software package and semi-automatically determines the three translations and three rotations which are required. The Hexapod couch was used to adjust positioning only at the Initial Scan. Results: Based upon 64 set-ups analyzed: after Hexapod adjustment 53% of Pre-Treatment CBCTs indicated that the anatomic isocentre was within 0.5 mm of the plan; 38% were within 0.5 mm of the plan according to the Post-Treatment scan. The median 3D vector shift from Pre- to Posttreatment was 0.35 mm, its average 0.55 mm, and its standard deviation 0.56 mm. (Note, however, that the CBCT isocentre is not perfectly coincident (< 1 mm) with the therapy beam isocentre). Conclusion: The average intra-fraction motion of this patient cohort was observed to be sub-millimeter.