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Pediatric Total Body Irradiation Calculations and In-Vivo Dosimetry Using Diodes and OSLD's

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S Chungbin

S Chungbin1*, M Fatyga1 , (1) Mayo Clinic Arizona, Phoenix, AZ


SU-E-T-260 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To verify that a photon total body irradiation (TBI) calculation method scales properly from adult to pediatric dimensions and to determine TBI in-vivo dosimetry correction factors for diodes and optically stimulated luminescent dosimeters (OSLD's).

Methods: TBI technique used is 400 SAD 18 MV opposed laterals with beam spoiler. Water bags are used to supplement narrower lateral dimensions for patient treatments. To verify that dose calculations scale properly with decreasing dimensions, CAX doses were measured and compared to calculations for different rectangular phantom geometries: (L=length(cm), H=height(cm), d=depth(cm)): L(30)xH(30) (d=3-25), L(30)xH(12)(d=2-20), L(13)xH(13) (d=5-13), L(30)x(H=10-40) d=15, L(30-150) x H(10) (d=15). In infant geometry, measured off axis "leg" dose (L(30)xH(2.5-10.6), d=7)) was compared to CAX ("body" L(30)xH(10)(d=7) adjacent to "leg"). Entrance and exit doses were measured with surface diodes, diodes with buildup, OSLD's, as well as ion chambers for comparison. Correction factors ((ion chamber CAX dose)/(in vivo dose)) were calculated for surface diodes, diodes with buildup, OSLD's, and ion chamber.

Results: All rectangular phantom measurements agree with calculated within 2.5%. For L(30)xH(30), L(30)xH(12), L(13)xH(13), L(30)x(H=10-40) and L(30-80)xH(10) agreement was within 1%. For the infant geometry, the ratio of leg dose to CAX varies from 0.956 (h=2.5) to 0.995 (h=10.6). The range of in-vivo dosimetry entrance+exit to CAX dose correction factors varied by dosimeter (diode: 0.883-1.015, surface diode: 1.008-1.214, ion chamber: 0.924-1.084, OSLD: 0.920-1.106).

Conclusion: TBI calculations scaled properly to pediatric dimensions. In-vivo dosimetry with various detectors demonstrated similar trends with different magnitudes. OSLD measurements agreed well with ion chamber measurements.

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