Case Report: Proton Therapy of a Pregnant Patient with Small-Cell Lung Cancer in the Third Trimester Using Synthetic Computed Tomographies for Tumor Monitoring

We report a case of a 32-week pregnant patient (at the initial radiotherapy intake) with stage III small cell lung cancer (SCLC) treated with concurrent chemo-proton radiotherapy.A total dose of 45 Gy(RBE) was delivered in 30 fractions (1.5 Gy(RBE) twice daily). Fetal dose estimates were calculated for both photon and proton radiotherapy scenarios. Synthetic CTs (sCTs) generated from daily cone-beam CTs (CBCT) enabled daily dose evaluation and treatment dose accumulation while minimizing imaging dose.The estimated cumulative fetal dose was 45.5 mSv, versus an estimated 182 mSv (without shielding) if photon therapy had been used for the whole treatment. Using sCTs instead of repeated weekly CTs saved 6 mSv. One plan adaptation was necessary after 10 days of treatment due to tumor shrinkage. The accumulated V95% of the clinical target volume (CTV) computed on the sCTs remained above 96.5% during the whole treatment course. A healthy boy was delivered at 37.5 weeks.This case illustrates how proton therapy and sCTs can safely minimize fetal exposure in pregnant patients requiring thoracic radiotherapy, even in the third trimester.