SU‐F‐J‐126: Influence of Six Dimensional Motions in Frameless Stereotactic Dosimetry Incorporating Rotational Shifts as Equivalent Translational Shifts: A Feasibility Study for Elekta‐BrainLAB Stereotactic System

Abstract

Purpose: Six dimensional positional shifts (translational and rotational) determined by a volumetric imaging system were mathematically combined and incorporated as simple translational shifts and the resultant impact on dose characteristics was studied. Methods: Thirty patients who underwent either single fraction (12 Gy) or five fractions (5 Gy per fraction) stereotactic treatments were included in this study. They were immobilized using a double layered thermoplastic mask from BrainLAB. Isocenter matching was done using infrared marker of ExacTrac. An initial cone beam CT (CBCT) gave positional shifts in 6‐dimensions that were applied through 6‐D motion enabled couch. A verification CBCT was done following corrections before treatment. These 6‐D positional shifts determined at each imaging session from the first CBCT were mathematically combined to give three simple translational shifts. Doses were recalculated in the patient matrix with these positional errors present by moving the whole image dataset. Doses were also recalculated after second CBCT with only residual errors present. PTV dose statistics were compared. Results: For the approved plans V100%(PTV), V100%(GTV), D95%(PTV), D95%(GTV), D1%(PTV) and D1%(GTV) were 96.2±3.0%, 98.2±1.4%, 102%±1.7%, 103±1.2%, 107.9±8.9% and 109.3±7.5% of prescription dose respectively. With the positional errors present (after 1st CBCT) the corresponding values were 86.7±4.9%, 91.3±2.9%, 89.6±4.2%, 95.9±3.7%, 108.3±9.9% and 108.6±4.5%. Post‐correction (after 2nd CBCT) with only residual errors present, values were 94.5±5.7%, 97.3±2.9%, 99.3%±3.2%, 102%±2.1%, 107.6±8.5% and 109.0±7.6% respectively. Significant and nominal OAR dose variation was observed between pre‐ and post‐table corrections. Conclusion: Positional errors significantly affect PTV dose statistics. They need to be corrected before delivery of stereotactic treatments although the magnitude of dose changes can vary from patient‐to‐patient depending on the tumor location. As expected after the table corrections, residual errors result in insignificant dose deviations. For frameless stereotactic treatments having a six‐dimensional motion enabled couch is highly recommended to reduce quantum of dose deviations.

Methodology to reduce 6D patient positional shifts into a 3D linear shift and its verification in frameless stereotactic radiotherapy

“…The described methodology reduces the 6D positional shifts to composite 3D translational shifts, which can include rotational shifts as well. There is only one research group besides ours, which attempted the conversion of 6D shifts to 3D shifts using a matrix method (Onimaru et al 2002, Sarkar et al 2016. However, the approach adopted by Onimaru et al, is controversial, as the matrix method cannot be applied in this case, and a geometrical approach is required to tackle this problem in the 6D Euclidian space (Sarkar et al 2015).…”

Standardization of volumetric modulated arc therapy‐based frameless stereotactic technique using a multidimensional ensemble‐aided knowledge‐based planning

“…Subsequently CT images, along with the structure set and isocenter information, were pushed to the Monaco (Elekta CMS, Sunnyvale, CA) workstation for VMAT planning. Details of our stereotactic planning strategies can be found in previous articles …”