Diaphragm Based Position Verification to Improve Daily Target Dose Coverage in Proton and Photon Radiotherapy Treatment of Distal Esophageal Cancer.

Abstract Purpose In modern conformal radiotherapy of distal esophageal cancer, target coverage can be affected by variations in the diaphragm position. We investigated if daily position verification (PV) extended by a diaphragm position correction would optimize target dose coverage for esophageal cancer treatment. Materials and Methods For fifteen esophageal cancer patients, intensity modulated proton therapy (IMPT) and volumetric modulated arc therapy (VMAT) plans were computed. Displacements of the target volume were correlated with diaphragm displacements using repeated 4D computed tomography (CT) images to determine the correction needed to account for diaphragm variations. Afterwards, target coverage was evaluated for three PV approaches based on: (1) bony anatomy (PV_B), (2) bony anatomy corrected for the diaphragm position (PV_BD), and (3) target volume (PV_T). Results The cranial-caudal mean target displacement was congruent with almost half of the diaphragm displacement (y=0.459x), which was used for the diaphragm correction in PV_BD. Target dose coverage using PV_B was adequate for most patients with diaphragm displacements up till 10 mm (≥94% of the dose in 98% of the volume [D98%]). For larger displacements, the target coverage was better maintained by PV_T and PV_BD. Overall, PV_BD accounted best for target displacements, especially in combination with tissue density variations (D98%: IMPT 94% ± 5%, VMAT 96% ± 5%). Diaphragm displacements of more than 10 mm were observed in 22% (4DCTs) and 20% ([4D]CBCTs) of the cases. Conclusion PV_B was sufficient to achieve adequate target dose coverage in case of small deviations in diaphragm position. However, large deviations of the diaphragm were best mitigated by PV_BD. To detect the cases where target dose coverage could be compromised due to diaphragm position variations, we recommend monitoring of the diaphragm position prior to treatment through online imaging.