Photoneutron activation of an IMRT QA device and the radiation safety implications

A. Eagle,¹ M. Mann¹, J. Washington,² L. Sweeney,² D. Kaurin,² S. Qui,³ W. Simon,⁴ F. Newman⁵

¹Lutheran Medical Center, Wheat Ridge, CO; ²Northwest Medical Physics Center, Lynnwood, WA; ³University Of Colorado, Denver, Aurora, CO; ⁴Sun Nuclear Corporation, Melbourne, FL, ⁵University of Colorado Health Science, Aurora, CO

AAPM/COMP Annual Scientific Meeting, Vancouver, BC (2011).

Purpose: To characterize the activation of the Sun Nuclear MapCheck2 diode array when exposed to high energy photons.

Methods: A Sun Nuclear MapCheck2 diode array was exposed to 6, 15 and 18 MV photons. After activation, radiation levels were measured and analyzed with a calibrated survey meter and with the device moved well away from the linac. Additionally, gamma camera images and energy spectra were obtained.

Results: As expected, activation increases with increasing exposure and photonenergy. Irradiating the device at 100 cm SSD for 1000 and 3000 MUs yielded survey meter readings at the surface of the device of 30 and 60 mR/hr respectively from 18 MV photons, and 2 and 5 mR/hr from 15 MV photons. 6 MV photons produced no detectable activation. The observed half‐life was approximately 10 minutes. Gamma cameraimages showed the activated material to be approximately uniformly distributed. The energy spectra showed a strong peak centered on 511 keV, consistent with annihilation photons from positron emission. These data are consistent with the Cu63 (y, n) Cu62 reaction. The decreased activation from 15 MV photons is also consistent with Cu63 activation which has a strongly peaked cross section centered around 16 to 18 MeV. Also observed were strong energy peaks centered near 76 keV and 177 keV, which require further analysis.

Conclusions: Although normal operation of this device does not usually require such high doses, the Medical Physicist should be aware of the possibly higher than expected activation of any such QA device and the resultant exposure. Without proper precautions this could have ALARA consequences. For example, Cu63 accounts for nearly 70% of naturally occurring copper and is used extensively in virtually all similar measuring devices. Though the Cu63 (y, n) Cu62 dominates, the other peaks will be discussed as well.