Dose to craniofacial region through portal imaging of pediatric brain tumors

The purpose of this study was to determine dose to the planning target volume (PTV) and organs at risk (OARs) from portal imaging (PI) of the craniofacial region in pediatric brain tumor patients treated with intensity-modulated radiation therapy (IMRT). Twenty pediatric brain tumor patients were retrospectively studied. Each received portal imaging of treatment fields and orthogonal setup fields in the craniofacial region. The number of PI and monitor units used for PI were documented for each patient. Dose distributions and dose-volume histograms were generated to quantify the maximum, minimum, and mean dose to the PTV, and the mean dose to OARs through PI acquisition. The doses resulting from PI are reported as percentage of prescribed dose. The average maximum, minimum, and mean doses to PTV from PI were 2.9 ± 0.7%, 2.2 ± 1.0%, and 2.5 ± 0.7%, respectively. The mean dose to the OARs from PI were brainstem 2.8 ± 1.1%, optic nerves/chiasm 2.6 ± 0.9%, cochlea 2.6 ± 0.9%, hypothalamus/pituitary 2.4 ± 0.6%, temporal lobes 2.3 ± 0.6%, thyroid 1.6 ± 0.8%, and eyes 2.6 ± 0.9%. The mean number of portal images and the mean number of PI monitor units per patient were 58.8 and 173.3, respectively. The dose from PI while treating pediatric brain tumors using IMRT is significant (2%-3% of the prescribed dose). This may result in exceeding the tolerance limit of many critical structures and lead to unwanted late complications and secondary malignancies. Dose contributions from PI should be considered in the final documented dose. Attempts must be made in PI practices to lower the imaging dose when feasible.