Program Information
Dose and Energy Spectra From Neutron Induced Radioactivity in Medical Linear Accelerators Following High Energy Total Body Irradiation
S Keehan1*, M Taylor1, R Smith2, L Dunn3, T Kron4, R Franich1 , (1) RMIT University, Melbourne, VIC, (2) William Buckland Radiotherapy Centre, Melbourne, VIC, (3) Australian Clinical Dosimetry Service, Yallambie, VIC, (4) Peter MacCallum Cancer Instit., Melbourne
Presentations
TU-G-CAMPUS-T-1 (Tuesday, July 14, 2015) 4:30 PM - 5:00 PM Room: Exhibit Hall
Purpose: To assess the risk posed by neutron induced activation of components in medical linear accelerators (linacs) following the delivery of high monitor unit 18 MV photon beams such as used in TBI.
Methods: Gamma spectroscopy was used to identify radioisotopes produced in components of a Varian 21EX and an Elekta Synergy following delivery of photon beams. Dose and risk estimates for TBI were assessed using dose deliveries from an actual patient treatment. A 1 litre spherical ion chamber (PTW, Germany) has been used to measure the dose at the beam exit window and at the total body irradiation (TBI) treatment couch following large and small field beams with long beam-on times. Measurements were also made outside of the closed jaws to quantify the benefit of the attenuation provided by the jaws.
Results: The radioisotopes produced in the linac head have been identified as ¹⁸⁷W, ⁵⁶Mn, ²⁴Na and ²⁸Al, which have half-lives from between 2.3 min to 24 hours. The dose at the beam exit window following an 18 MV 2197 MU TBI beam delivery was 12.6 μSv in ten minutes. The dose rate at the TBI treatment couch 4.8 m away is a factor of ten lower. For a typical TBI delivered in six fractions each consisting of four beams and an annual patient load of 24, the annual dose estimate for a staff member at the treatment couch for ten minutes is 750 μSv. This can be further reduced by a factor of about twelve if the jaws are closed before entering the room, resulting in a dose estimate of 65 μSv.
Conclusion: The dose resulting from the activation products for a representative TBI workload at our clinic of 24 patients per year is 750 μSv, which can be further reduced to 65 μSv by closing the jaws.
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