Verification of ZAP-X treatment plans with the Octavius 1600 SRS detector array
PO-1759
Abstract
Verification of ZAP-X treatment plans with the Octavius 1600 SRS detector array
Authors: Katrin Büsing1, Joerg Harmsen2, Peter D. Klassen3, Hui Khee Looe1, Bjoern Poppe4, Daniela Poppinga5
1Carl-von-Ossietzky University, University Clinic for Medical Radiation Physics, Oldenburg, Germany; 2Practice for Radiation Therapy Nordhorn-Meppen, Radiation Therapy Meppen, Meppen, Germany; 3St. Bonifatius Hospital , ZAP-X Center, Lingen, Germany; 4Carl-von-Ossietzky University, University Clinic for Medical Radiation Physics, Oldenburg , Germany; 5PTW Freiburg, PTW, Freiburg, Germany
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Purpose or Objective
The ZAP-X is a novel self-shielding system designed for stereotactic radiotherapy of brain lesions. The linear accelerator with 3 MV nominal photon beam is mounted gyroscopically on two axes allowing 4 pi radiation angle. The irradiation is performed with collimators between 4 mm and 25 mm in diameter. As with other radiotherapy modalities, the treatment plans require independent verification to ensure patient safety. However, this task is complicated by the use of multiple isocenters and the large number of non-coplanar beam angles. The aim of this work is to establish a clinical workflow for patient plan verification based on the liquid-filled ionization chamber array OCTAVIUS 1600 SRS (PTW Freiburg, Germany).
Material and Methods
A planning CT was performed with the OCTAVIUS 1600 SRS array positioned between 3 cm RW3 solid water beneath and above is. The treatment plan to be verified was recalculated on this planning CT, where the target was placed in the middle of the array’s sensitive area, without changing the relative position of the isocenters to each other, the gantry positions, cone s and MU per beam.
During the measurement, the array with the RW3 was placed on the patient couch using the same setup as in the planning CT. All measurements were performed in clinical mode, so that the workflow started with the patient positioning based on 2D kV images. Thereby, the setup with the array was automatically aligned to the first isocenter. For each isocenter, the treatment delivery was measured and verified separately. The patient positioning workflow was repeated between the isocenters to check the array alignment. The measured and calculated dose distributions were compared using global gamma-index criteria of 1 mm / 3%.
Results
By using the measurement setup in this study, a realistic clinical workflow including the automatic positioning using the 2D kV images is possible. For more than 70% of the measured isocenters, the comparison between the measured and calculated dose distribution resulted in a gamma-index passing rate of over 90%. Close inspections on the results of isocenters with lower gamma index passing rates indicated a directional dependency of the 1600 SRS array.
Conclusion
This study demonstrated the first clinical feasible workflow of patient plan verification using the high-resolution OCTAVIUS 1600 SRS array at the ZAP-X stereotactic platform. The initial results indicated the importance to account for the angle-dependent detector’s response. In a further step, correction strategy taking this aspect into account will be studied.