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Clinical dosimetry with MOSFET dosimeters to determine the dose along the field junction in a split beam technique
Esther Bloemen-van Gurp1, Wim du Bois, Peter Visser
1Maastro Clinic/Radiotherapeutic Institute Limburg, Maastricht and Heerlen, The Netherlands.
Background And Purpose:
We report on the accuracy of metal oxide-silicon semiconductor field effect transistor (MOSFET) detectors to measure dose distributions in the region of a field junction in a split beam technique, compared to ionisation chamber and photographic film. We present a study on five patients receiving loco-regional treatment for breast cancer.
Materials And Methods:
The dose variation at the junction was measured with the patient dose verification system model TN-RD-50 (MOSFET system). Phantom measurements were performed to investigate the MOSFET accuracy in a half-field matching method and the influence of factors related to the accelerator. The aim of the patient measurements was to determine the effect of patient related factors on the dose at the junction over a period of 10 irradiation fractions.
Results:
The MOSFET detector overestimates the dose in the junction area, therefore a correction factor was determined to correct the MOSFET results. Phantom measurements showed overdosages as well as underdosages, depending on the matching field direction (X or Y). Patient measurements showed dose values that deviated up to 133%+/-3% (2 S.D.). The average values of 10 irradiation sessions showed a continuous, almost linear, variation of dose value across the junction.
Conclusions:
(1) Significant dose deviations in the junction area average out over repeated treatments; (2) one individual measurement in the junction area should not lead to any action; (3) a linac QA procedure to check the dose at the junction line should simulate the clinical situation sufficiently; and (4) MOSFET dosimeters overestimate dose values in the penumbra region.
