VMAT CRANIOSPINAL PLANS QUALITY ASSURANCE AND 3D DOSIMETRY USING COMPASS/DOLPHIN

Introduction: one of the main challenges in VMAT craniospinal quality assurance (QA) patients is to be able to evaluate the full extention of the irradiated target volume. Regardless of the technique used in the planning, conventional approaches for dosimetric analysis are limited to the field junction regions and, in general, measurements are performed with a small volume ionization chamber or with two-dimensional systems only. 3D dosimetry allows a global and much more comprehensive assessment with statistical data comparing the calculated dose and the dose received by the patient, slice by slice, on any plane, not only for the treatment target but also for each involved organ at risk. Objective: the aim of this study was to quantitatively evaluate the patient-specific 3D dosimetry tool Compass with the 2D array Dolphin detector (IBA Dosimetry) for craniospinal cases planned with VMAT. Materials and Methods: a craniospinal patient was planned in supine position using 6MV Xray and 3 isocenters: one full-arc in the skull region, one in the thorax and the third in the lumbar spine, with partial arcs avoiding the metals of the table. Auxiliary volumes were employed in the field junction regions to force dose homogeneity during optimization and the treatment prescription was 24Gy in 12 fractions. Results: the irradiation time for the studied case was approximately 12min. For the target volume (PTV), the difference between mean doses (TPS vs. reconstructed dose calculation) was 5% for a 3%/3mm 3D gamma criterion and doses in most of organs at risk evaluated were similar, with differences between mean doses less than 2%. Analyzing the fields individually with the quick check tool, the mean difference found in 2D analysis was less than 2%. Field junction regions showed gamma values between 0 and 1, which represents a high proximity between TPS doses and reconstructed doses, one of the most critical points to be evaluated in craniospinal quality assurance cases. Conclusion: the results of this work showed that the Compass/Dolphin system can be used as a new pretreatment QA tool, more comprehensive, functional and efficient in error detection for VMAT craniospinal cases.