Purpose: The addition of tumor treating fields (TTF) designed to target the infratentorial brain to radiotherapy for diffuse intrinsic pontine glioma (DIPG) will be explored in a phase II clinical trial. This study explored the dosimetric effect of concurrent TTF on target and structures-at-risk in pediatric patients with newly diagnosed DIPG undergoing IMRT.
Methods: Scalp dose with and without TTF electrodes was measured using a parallel-plate ion chamber in solid water for a 10x10 cm2 open field. Buildup thicknesses varying from surface to 2 mm simulated different depths in the skin. Beam incident angles varied from 0-90 degrees. Maximum dose increase at 1 mm depth was measured for three six-field IMRT patient plans.A treatment planning study was conducted by transferring clinical targets from ten patients ages 2-14 years onto age-appropriate ATOM phantom reference CTs with no TTF electrodes present. IMRT plans were generated with Eclipse AAA. TTF electrodes were contoured on CTs of the phantom with electrodes reconstructed with a metal artifact reduction algorithm. These contours were transferred to the reference CT and changes in target coverage and scalp dose (5 mm from skin surface) due to TTF electrodes were evaluated.
Results: Open field measurements demonstrated an increased sensitivity to electrode presence for shallower depths. The dose increase ranged from 150% at 2 mm to almost five-fold at the surface. Increases were greatest for enface beams. Maximum IMRT dose increase at 1 mm depth due to TTF electrode presence was approximately 150%. The treatment planning study showed minimal PTV coverage differences (ΔV95% < 1%) and slight volumetric scalp dose increase (D1cc +2%) with TTF electrodes present. Surface point doses calculated agreed with measurement results.
Conclusion: Presence of TTF electrodes during pediatric infratentorial IMRT delivery modestly increased the local scalp dose but had a minimal impact on volumetric dose metrics.