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Photoneutron Equivalent Dose Measurements in GRID Therapy Using TLD Types 600/700 and Polycarbonate Film

S Gholami1*, S Tajiki2, A Hakimi3, A Meigooni4, F Kalantari Mahmoudabadi5, (1) Cancer institute, Tehran University of Medical Sciences, Tehran, ,IR, (2) Cancer Institute, Imam Khomeini Hospital, TUMS, (3) Health Physics And Dosimetry Research Laboratory, Amirkabir University, (4) Comprehensive Cancer Center of Nevada, Las Vegas, NV, (5) University of Arkansas for Medical Sciences, Little Rock, AR

Presentations

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

Room: AAPM ePoster Library

Purpose:
GRID therapy is a radiation therapy technique for treatment of bulky malignant tumors. The aim of this study is to measure photoneutron dose in a GRID field with high energy photon beam.

Methods:
A GRID block was mounted to Varian Clinac 2100c linear accelerator to perform photon and photoneutron equivalent dose measurements for 18 MV photon beam.
Dosimetric parameters of photon beam such as: valley to peak ratio, output factor, profiles and PDDs were measured using a pinpoint ionization chamber (PTW, Freiburg, Germany) in a water phantom. Combination of TLD types 600 and 700 were utilized to measure thermal neutron equivalent dose. In addition, polycarbonate film was considered for fast photoneutron dosimetry.
Dose measurements were performed on the surface and at depth of maximum dose (dmax) in a Solid Water™ slab phantom for each dosimeter type. Thermal and fast photoneutron doses were reported at different distances of the radiation field edge.

Results:
The valley to peak ratio and the output factor for 18 MV photon beam were obtained about 35% and 0.72, respectively. Fast and thermal photoneutron equivalent dose were reduced in the GRID field compared to an open field (without GRID), on the surface and dmax depth of Solid Water™ down to about 33% and 48%, respectively.

Conclusion:
Both thermal and fast neutrons were decreased dramatically with the GRID, so their risks are clinically insignificant. However, the possible high dose to the radiation worker from the radiation activation on the surface of the GRID should be considered.

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