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Achieving Sub-Millimeter Accuracy in Radiation Therapy with a Self-Leveling Surveyor Laser

S Hancock

S Hancock1*, M Nyflot2, E Ford3, (1) Southeast Missouri Hospital, Cape Girardeau, MO, (2) University of Washington Medical Center, Seattle, WA, (3) University of Washington, Seattle, WA

SU-E-T-132 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Sub-millimeter accuracy is increasingly important in radiation therapy as the field trends towards highly conformal and high dose-per-fraction therapy. We developed a method for achieving sub-millimeter accuracy in external beam radiation therapy using a surveyor self-leveling laser for precision alignment of radiotherapy vault lasers.

An off-the-shelf self-leveling laser (Stanley FatMax) that projects in three orthogonal directions was used to align the radiotherapy vault lasers. The position of the room lasers were adjusted to the principal planes containing the radiation isocenter, using the surveyor laser to achieve colinearity over large distances. The overall alignment of the lasers to the radiation isocenter is accomplished with a BB aligned to the radiation isocenter using an existing digital methodology for minimizing target positioning error. Once the lasers are aligned, daily QA is possible using a blank sheet of paper to check colinearity.

Excellent laser colinearity (< 1mm) is achieved and maintained across the length of the room. Daily laser QA can be performed in less than 15 seconds with a sensitivity of approximately 0.2 mm. Laser alignment can be adjusted in less than 5 minutes using a remote laser control to reestablish colinearity of multiple lasers. Monthly QA results for a single linac demonstrated consistent maintenance of sub-mm alignment of the kV imager to the radiation isocenter and the lasers to the kV imager. One year of monthly QA results using a BB phantom demonstrated a mean deviation of kV imager from radiation isocenter of 0.31 mm and a mean deviation of lasers to kV imager of 0.38 mm, with maximum deviations of 0.51 mm and 0.54 mm respectively.

Sub-millimeter coincidence of room lasers and kV imager with radiation isocenter can be efficiently achieved and maintained using a surveyor self-leveling laser with a BB phantom.

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