Intensity-modulated radiation therapy (IMRT) uses solid compensators or multileaf collimators to modulate the intensity of radiation in each field, delivering highly conformal dose distributions. This technique allows treating volumes with concave shapes when the target is close to a critical structure. The movement of multileaf collimator under computer control can modulate the dose in 3 main ways: IMRT with static field with segments, IMRT with dynamic delivery, and IMRT rotational therapy. Volumetric modulated arc therapy (VMAT) is a novel radiation technique that creates conformal distributions with variable gantry speed, dynamic movements of multileaf collimator, and variations in dose rate. The purpose of this study was to review the basis of VMAT, highlighting the differences with other IMRT techniques.
The absorbed dose outside of the direct fields of radiotherapy treatment (or peripheral dose, PD) is responsible for radiation exposure of the fetus in pregnant women. Because the radiological protection of the unborn child is of particular concern in the early period of the pregnancy, the aim of this study is to estimate the PD in order to assess the absorbed dose in the uterus in a pregnant patient irradiated for breast carcinoma therapy. The treatment was simulated on an Alderson-Rando anthropomorphic phantom, and the radiation dose to the fetus was measured using an ionisation chamber and thermoluminescence dosemeters. Two similar treatments plans with and without wedges were delivered, using a 6 MV photon beam with two isocentric opposite tangential fields with a total dose of 50 Gy, in accordance with common established procedures. Average field parameters for more than 300 patients were studied. Measurements showed the fetal dose to be slightly lower than 50 mGy, a level at which the risk to the fetus is uncertain, although several authors consider this value as the dose threshold for deterministic effects. The planning system (PS) underestimated PD values and no significant influence was found with the use of wedge filters.
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