Validation of the Tena pregnancy phantom and fetal dose assessment in proton scanning beam therapy

Intensity modulated proton therapy (IMPT) is the preferred option during pregnancy, as it reduces out-of-field doses compared to photon techniques. A physical pregnancy phantom was validated for in-field proton dosimetry and used to assess fetal dose across four IMPT plans.

The 18-week pregnancy Tena phantom was composed of bone, soft tissue, and lung substitutes. Proton relative stopping power (RSP) for Tena tissues was measured and compared with treatment planning system (TPS) and Monte Carlo (MC) calculations. Experimental TPS dose verification was performed using gamma index (GI). Fetal dose was measured for IMPT of glioma, Hodgkin lymphoma without (HL) and with a range shifter (HL-RS), and submandibular gland (neck) cancer using a Timepix and bubble detectors.

Differences between TPS-assigned and MC-simulated relative to the measured RSP values were up to -7.4 %. GI(3 %/3 mm) values were above 93.38 %. The neutron dose equivalent in the fetus position ranged between 2.5 and 49.4 µSv/Gy(RBE) for glioma and HL-RS plans, respectively. The HL plan reduced neutron dose equivalent to 15.8 µSv/Gy(RBE), while for the neck 20 µSv/Gy(RBE) was measured. Neutrons were dominant with ∼ 80 % contribution to the total dose equivalent. A summed fetal dose was calculated considering the prescribed dose per treatment and ranged between 0.17 mSv and 1.89 mSv for glioma and HL-RS, respectively.

The Tena phantom is suitable for proton dosimetry and enables accurate TPS calculations. The use of a range shifter increased the fetal dose by more than threefold. Fetal doses for all IMPT plans remained below 2 mSv.