Measured peripheral dose in pediatric radiation therapy: a comparison of intensity-modulated and conformal techniques

The interest in IMRT for the treatment of pediatric malignancies has raised concern about possible increased total body dose. This study examines the pediatric peripheral dose resulting from IMRT compared to 3D conformal therapy.

Five brain or base of skull pediatric cases were planned with both IMRT and 3D conformal techniques. A pediatric-sized anthropomorphic phantom was created and ion chambers were placed at interest points approximating the position of the thyroid, breast, ovary and testes. Measured peripheral doses at the interest points were compared for both IMRT and 3D conformal techniques for the 5 cases.

While tumor coverage was similar for both techniques, the IMRT delivery resulted in lower peripheral doses at points near the target (thyroid) presumably due to reduced internal scatter from a smaller effective field size for sliding window dynamic multi-leaf collimation. The IMRT delivery resulted in higher doses to the more distant points, presumably due to the higher monitor units and resulting increased head leakage. Since the magnitude of dose at the distant points was much smaller than that of the thyroid point, the overall absolute peripheral dose was similar for both techniques.

Peripheral dose is difficult to predict by monitor units alone. In this study, interest points closer to the beam received less dose with IMRT. This difference may result from the competing factors of reduced internal scatter from dynamic multileaf collimation IMRT and reduced head leakage for 3D conformal therapy.