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Out-Of-Field Fetal Dose Measurement From a Head-And-Neck Treatment with VMAT: An Anthropomorphic Phantom Study

D Roa

D Roa*, Y Lin, N Hanna, M Al-Ghazi, J Kuo, University Of California, Irvine, Orange, CA

SU-C-137-1 Sunday 1:00PM - 1:55PM Room: 137

Purpose: The irradiation of pregnant patients is an uncommon occurrence but if discovered requires accurate fetal dose estimates. For this purpose, we measured the out-of-field dose to the fetal region from the primary dose given to a head-and-neck tumor(s) irradiated with a volumetric-modulated arc therapy (VMAT) technique.

Methods:A VMAT-RapidArc plan was generated in ECLIPSE for a head-and-neck case extrapolated to an anthropomorphic phantom. The clinical plan, including target and OAR contours was co-registered to the phantom CT study. The phantom received a total dose of 60 Gy in 2 Gy/fx using a 6 MV photon beam from a Varian Trilogy irradiated using two 358o axial arcs.
108 TLDs and 6 Gafchromic films were mounted outside and inside the phantom to measure the out-of-field surface and internal scatter dose. Films were mounted axially in the phantom so that ~60% of the area provided internal scatter and ~40% leakage dose data.

Results:At 9 cm inferior to the primary treatment site, the leakage dose (~150 cGy) was about two times higher than internal scattering (~80 cGy). At 30 cm, the leakage (~40 cGy) was 2-3 times higher than the internal scatter dose (~15cGy). The highest dose measured by TLDs at 25 cm away from the target was 20 cGy (0.3 % of total). Although, the data showed higher dose contribution from leakage than internal scattering, the combined amount was small with respect to the primary dose. The estimated fetal dose from these measurements amounted to 0.1-0.3 cGy (at 45-60 cm from the target) which could be slightly higher compared to the established NCRP #91 recommendation (0.5 mSv).

Conclusion:Fetal dose measurements at different stages of pregnancy using a VMAT-RapidArc technique are underway. This data should provide updated and comprehensive information to complement TG-36.

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