Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_5_3160 - Patient specific quality assurance for multiple tumor SRS treatment with a single isocenter

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Patient specific quality assurance for multiple tumor SRS treatment with a single isocenter
H. Jiang1, R. K. Badkul1, N. Demez2, K. Kauweloa2, and H. Saleh3; 1Department of Radiation Oncology, University of Kansas School of Medicine, Kansas City, KS, 2University of Kansas Cancer Center, Kansas City, KS, 3Department of Radiation Oncology, University of Kansas Medical Center, Kansas City, KS

Purpose/Objective(s): The standard method for treating brain metastases with SRS is to treat each tumor separately, with an isocenter placed at the tumor center. The treatment time is long for patients with many brain tumors. To reduce the treatment time, radiation delivery technique allowing multiple brain tumors being treated together with a single isocenter has been developed. However, this new technique poses tough challenges for linear accelerators and quality assurance checks. In this study we investigated the feasibility of using a secondary MU check software and an IMRT QA device for verifying single-isocenter, multi-tumor treatment plans.

Materials/Methods: Several multiple tumor SRS cases were planned with the treatment planning system. The distance from an isocener to any tumor was limited to no more than 5 cm, and hence some cases took 2 to 3 plans to treat all targets. Each plan included 4 to 8 dynamic conformal arcs. The treatment planning program automatically created a dose point for each tumor, and the dose to this dose point from each arc was calculated. The completed treatment plans with all dose points were exported to an independent MU check program for secondary check. The MU check program used patient CT data to more accurately calculate effective depth at each arc control point. Dose points were always positioned on the 100% isodose curve, and they might to be too close to the field edge for accurate calculations. Therefore, a searching radius of up to 2 mm was allowed in the 2nd MU check program. The check was considered a pass if the MU difference was within 5%. The same treatment plans were also exported to the linear accelerator for delivery. Each arc in a treatment plan had to be delivered multiple times on the QA device, each time with one tumor positioned at the diode plane of the QA device. All arcs were delivered at the planned couch angel to detect any error due to couch angle uncertainty. The measured and calculated were compared on an arc-to-arc base with a 3%/1mm criteria.

Results: The results were promising. Among the 120 arcs re-calculated with the 2nd check program, about 80% passed without neighborhood search. Another 15% passed the check with 1 mm of searching radius. The remaining 5% passed the check except one with 2 mm of searching radius. The reason for one arc that failed was unclear. Regarding to the measurement with the QA device, all 50 arcs passed the check, i.e. at least 95% of the points passed with the 3%/1mm criteria.

Conclusion: Treating multiple brain tumors with a single isocenter poses a stricter demand (e.g. 0.5mm and 0.5º) for linear accelerators. Stringent tests can provide proof whether a specific linear accelerator can be used for the new technique or not. The results at our institutions demonstrated our linear accelerator is capable of delivering such technique with acceptable accuracy.

Author Disclosure: H. Jiang: None. R.K. Badkul: None.

Hongyu Jiang, PhD

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