Effect of tertiary multileaf collimator (MLC) on foetal dose during three-dimensional conformal radiation therapy (3DCRT) of a brain tumour during pregnancy.

Background And Purpose: The aim of this work was to measure the dose to foetus both in vivo and in vitro during three-dimensional conformal radiation therapy (3DCRT) in a pregnant patient with a pituitary adenoma. The study was then extended to assess the components contributing to the foetal dose such as collimator scatter, internal scatter, head leakage, wedge scatter and multileaf collimator (MLC) effect.

Patients And Methods: A 30-year-old pregnant woman with a non-functioning pituitary macroadenoma was planned for 3DCRT with 6MV X-ray using four equally weighted MLC-shaped non-coplanar wedged portals. In vivo dosimetry was carried out using thermoluminescent (TL) phosphor powder, which was placed at different positions on the patient, corresponding to different locations in the uterus and also at external os. In vitro measurements were also performed on a simulated phantom using the same set-up parameters and beam arrangement to verify the in vivo measured dose. Experiments were carried out to measure the respective contributions of different components towards peripheral dose.

Results: In vitro measured dose to foetus was found to be slightly more than that of in vivo measurement with a maximum of 0.044% of the prescribed dose of 45Gy, which corresponded to 0.0199+/-0.0008Gy. Thermoluminescence dosimeter (TLD) kept at the external os of the patient showed a dose of 0.031% of the prescribed dose. Among the various components of the peripheral dose (foetal dose) measured, head leakage was found to be the leading cause contributing 52%, followed by wedge scatter (31%), collimator scatter (14%) and internal scatter (13%). The use of MLC reduced not only the volume of normal brain irradiation as compared to open fields but also the peripheral dose by 10%.

Conclusion: Radiotherapy of brain tumours during pregnancy poses a unique clinical situation and decisions to deliver radiotherapy should be taken after detailed in vitro and in vivo dosimetric measurements. Our findings suggest that the beam arrangement using 3-4-fields generally used for 3DCRT of brain tumour with MLC for optimal coverage can be employed for pregnant patients even in early trimester. A possible increase in foetal dose from wedges to a large extent can be compensated with the use of MLC.