Optical surface tracking for non-coplanar SRS treatments verified by film in a RandoAlderson phantom
PO-1340
Abstract
Optical surface tracking for non-coplanar SRS treatments verified by film in a RandoAlderson phantom
Authors: Oellers|, Michel(1)*[michel.oellers@maastro.nl];Swinnen|, Ans(1);Verhaegen|, Frank(1);
(1)MAASTRO Clinic, Medical Physics, Maastricht, The Netherlands;
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Purpose or Objective
To perform a dosimetric verification with EBT-XD film of a non-coplanar plan for multiple brain metastases monitored by a commercial optical surface tracking system (OST).
Material and Methods
A 3-camera OST system was used (Catalyst HDTM, C-RAD, Sweden) on a Varian Truebeam STx linac with a 6DoF couch. The set-up accuracy and agreement between the OST system and cone beam CT (CBCT) and kV-MV imaging at respectively couch angles 0o and 270o were examined. Then dosimetric film measurements at 3 depths in the Rando-Alderson phantom were performed based on a single isocenter non-coplanar VMAT plan containing 4 brain lesions. Initial set-up of the phantom was performed with CBCT at couch 0° and subsequently monitored by Catalyst HDTM at all other couch angles. The absolute measured film doses were compared to the calculated TPS doses.
Results
For repeated tests with the Rando-Alderson phantom, the deviations between rotational and translational isocenter corrections for CBCT and OST system are always within 0.2° (pitch, roll, yaw), and 0.1mm and 0.5mm (longitudinal, lateral, vertical) for couch positions 0° and 270°, respectively. Dose deviations between the absolute measured and calculated doses in the center of the 4 PTVs irradiated with a non-coplanar plan monitored by the OST system are -1.2%, -0.1%, -0.0% and -1.9% for PTV-1, PTV-2, PTV-3 and PTV-4, respectively. The dosimetric agreement between film and TPS presented by a local gamma evaluation criterion of 2%/2mm and 3%/1mm (both with cut-off dose value of 20%) yielded pass rates of 99.2%, 99.2%, 98.6%, 89.9% and 98.8%, 97.5%, 81.7%, 78.1% for PTV-1, PTV-2, PTV-3, and PTV-4 respectively.
Conclusion
Patient positioning for a non-coplanar single isocenter VMAT treatment of multiple brain metastases can be monitored with an OST system with submillimeter and subdegree accuracy.
Figure: A five arc single isocenter non-coplanar VMAT plan with 4 PTVs, verified using film dosimetry with EBT-XD films in the first 3 slices of the Rando-Alderson phantom head.
Table:The dosimetric agreement between film and TPS doses is presented by the deviations between measured and calculated doses in the center of the 4 PTVs and by a local gamma evaluation criterion of 2%/2mm as well as 3%/1mm for the 4 PTVs irradiated with single isocenter non-coplanar VMAT and monitored using Catalyst HDTM . To put these numbers into perspective, also the results for PTV-1 irradiated in a co-planar set-up (couch 0˚) are given (highlighted in bold).
PTV | D_film (Gy) | D_TPS (gy) | Δ(D_film-D_TPS) % | Agreement score (2%21mm) | Agreement score (3%/1mm) |
1 | 9.71 | 9.82 | -1.2% | 99.2% | 98.8% |
1 | 9.15 | 9.30 | -1.6% | 99.8% | 99.9% |
2 | 9.86 | 9.87 | -0.1% | 99.2% | 97.5% |
3 | 9.82 | 9.82 | -0.0% | 98.6% | 81.7% |
4 | 9.11 | 9.29 | -1.9% | 89.9% | 78.1% |