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Introduction
Radiotherapy during pregnancy represents a clinical and radiological challenge, in which reliable fetal dose estimation is essential to balance maternal therapeutic benefit and fetal radiation risk. A fetal absorbed dose of 100 mGy is widely cited as a practical reference level in radiological protection guidance. Although analytical models and advanced simulations are used for out-of-field dose estimation, their routine clinical application is limited. In this context, due to the multifactorial nature of fetal out-of-field dose (patient scatter, collimator scatter, and head leakage) and the associated limitations of non-in vivo estimation methods, in vivo dosimetry using optically stimulated luminescence (OSL) detectors arises as a practical and non-invasive approach for conservative fetal dose estimation.
Materials and Methods
A modified whole-body anthropomorphic phantom is used to simulate a pregnant patient, incorporating an abdominal insert that allows for a fixed detector placement at a depth of 10 cm., representative of fetal position. Treatments of the central nervous system, head and neck, breast, and apical lung (SBRT) have been evaluated using 6 MV photon beams with, volumetric modulated arc therapy (VMAT) with flattening filter (WFF), and VMAT flattening-filter-free (FFF) techniques.
All treatment plans are first administered with and without abdominal shielding using a 4-cm-thick Cerrobend dome to quantify the effect of shielding on fetal dose. for the unshielded configurations, additional abdominal surface dose measurements were performed using OSL detectors to establish the relationship between surface dose and fetal dose. Fetal dose measurements were obtained using OSL detectors and Gafchromic EBT3.
Results
Measured fetal doses showed a clear dependence on treatment tissue and irradiation technique. Treatments distant from the uterus, such as central nervous system and apical lung, resulted in fetal doses below 100 mGy. Whereas breast irradiation systematically exceeded this level. VMAT-FFF consistently produced lower fetal doses than VMAT-WFF.
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