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Treatment Room Activation After 15 MV Single Fraction Radiation Treatments Delivered Using Varian's TrueBeam and Trilogy Linear Accelerators

M Yan

M Yan1*, L Fanchon1,2, M Williamson1, M Hunt1, A Kirov1, (1) Memorial Sloan-Kettering Cancer Center, New York, NY, (2) Universite Joseph Fourier,Grenoble, France

SU-E-T-262 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To measure radiation levels in treatment room due to activation after 15MV single fraction radiation treatment (SFRT) delivered to a solid water phantom.

Methods: We performed radiation surveys of two LINAC treatment rooms immediately after 15 MV SFRT. We delivered a sequence of two 15 MV single fraction IMRT treatments to a phantom at the end of a typical treatment day. The first treatment delivered was 6201MU (about 12 Gy) and the second one, 15 minutes later was 12711 MU (24 Gy). Both were delivered to the pelvic region of a solid water anthropomorphic phantom. In a second technique, a 15 MV VMAT SFRT (4326 MU) was delivered using the Varian TrueBeam LINAC. Radiation measurements were recorded repetitively at four locations using a thin windowed Geiger Muller detector, a sodium iodide photon spectrometer and a pressurized ionization chamber. The four locations surveyed were: the top of the collimator head, the collimator window surface, the isocenter, and the inferior end of the patient support assembly.

Results: Radiation levels at the isocenter at the end of the treatment day and before the two IMRT SFRTs varied from 0.06 to 0.1 mR/h. Within 2-3 minutes after finishing the second IMRT SFRT the radiation levels were approximately 10 and 1.4 mR/h at isocenter for the TrueBeam and Trilogy rooms respectively and around 0.6 mR/h for the TrueBeam VMAT SFRT. Closing the MLC and the jaws significantly reduces the radiation level at isocenter. The average half life of the mixture of radionuclides produced is about 10 minutes.

Conclusions: High dose single fraction IMRT treatments with 15 MV photons produce elevated treatment room activation as compared to conventional IMRT. In addition, activation levels varied between the TrueBeam and Trilogy for similar SFRT schemes.

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