Abstract P066

Analysis of fetal dose exposure by modern radiation therapy in pregnant patients with supradiaphragmatic Hodgkin lymphoma - A phantom-based simulation

Background: Modern involved-site radiotherapy (ISRT) for Hodgkin lymphoma uses reduced field sizes and radiation doses with a subsequent low-toxicity profile. However, in the case of pregnant patients, even small doses may harm the mother and the unborn child. In lack of evidence-based data for this complicated treatment situation, we conducted a phantom-based simulation to analyze the dosimetric impact of modern cervical and mediastinal ISRT on the uterus.

Methods: Target volumes for cervical and mediastinal ISRT were contoured and used for calculation of three comparison plans (3D-CRT, IMRT and VMAT), respectively. Afterwards, dosimetric measurements were conducted using the humanoid Alderson-phantom. Thermoluminescent dosimeters (TLD) were placed at representative positions within the phantom to account for early and late stages of pregnancy, respectively. Overall, six measurements (two for every radiotherapy plan) with 38 TLD were conducted.

Results: With a RT dose of 19.8 Gy, the median total exposure to the uterus in early pregnancy was 8.8 mGy, 15.4 mGy and 9.9 mGy for 3D-CRT, IMRT and VMAT respectively. In late pregnancy, 12.6 mGy (3D-CRT), 19.7 mGy (IMRT) and 13.8 mGy (VMAT) were measured for a RT dose of 19.8 Gy and 19.5 mGy (3D-CRT), 30.4 mGy (IMRT) and 21.4 mGy (VMAT) for 30.6 Gy. By applying a tissue weighting factor of 0.05, IMRT and VMAT with 30.6 Gy exceeded an effective dose equivalent >1 mSv. In contrast, mediastinal ISRT resulted in higher uterine doses with 44 mGy, 63.8 mGy and 60.5 mGy for 3D-CRT, IMRT and VMAT respectively. In late pregnancy, 138.6 mGy (3D-CRT), 161.7 mGy (IMRT) and 161.7 mGy (VMAT) were estimated for a RT dose of 19.8 Gy, whereas 214.2 mGy (3D-conformal), 249.9 mGy (IMRT) and 249.9 mGy (VMAT) were calculated for 30.6 Gy. As a consequence, all three comparison plans resulted in an effective dose equivalent >1 mSv, both with a treatment dose of 19.8 Gy as well as 30.6 Gy.

Conclusion: The calculated RT doses at the uterus for cervical ISRT are overall low and only exceeded the legal limit of 1 mSv in the case of IMRT and VMAT (30.6 Gy). For the mediastinal ISRT, all three treatment technique exceeded the threshold of 1 mSv. Overall, the possible indication of radiotherapy in pregnant women always requires a careful risk-benefit consideration and individualized planning.

Authors

Michael Oertel, Gina Smeets, Isabel Vogt, Heidi Wolters, Christopher Kittel, Dominik A. Hering, Burkhard Greve, Uwe Haverkamp, Hans T. Eich