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Toward A Filmless Gamma Knife (GK) Annual QA

T Qu, B Rudek, K Bernstein*, NYU Langone Health, Brookline, MA


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Gafchromic film is used in GK annual QA for three purposes: 1) measure the relative output factors (ROF) for comparison with those in the treatment planning system (RTP) GammaPlan; 2) confirm that the center of the patient positioning system (PPS) is coincident with the radiation isocenter; 3) compare the measured dose profile to the RTP profiles.
There is a big disadvantage of using film as the dosimeter and the ruler - it is complex and time-consuming and difficult to automate. In this study, we investige using a small volume PTW micro diamond detector to replace the film in GK annual QA.

Methods: A 3D printed holder positions the detector at the center of the PPS. SunNuclear pcElectrometer is used with a single continuous acquition of all the differential and integrated data . For the in air scan in x direction, 11 shots are planned from 95mm to 105 mm with 1mm increment, with 6 sec in each shot. Repeat for y and z directioin.

Results: 1) The ROF measurements are 0.812 and 0.895 for 4mm and 8mm cones, respectively. The results are -0.2% and -0.6% different from the ROF in GP.
2) The Source – Target coincidence measurements are within 0.5 mm for the directions of x, y, and z, respectively. The results are more easily reproduced than film and measurements can be automated.
3) The FWHM are 6.3, 6.3, and 4.9 mm for the x, y, and z direction. Compare with that from GP, the errors are -2%, -2%, and 3%, respectively. Dose from phantom scattering should be considered to compare in-air profile with the in phantom profile.

Conclusion: results are very encouraging that expensive hard to reproduce film QA can be replaced with in-air measurements with a small field detector.

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TH- External Beam- Photons: gammaknife

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