Dosimetric evaluation of magnetic resonance imaging based synthetic computed tomography for head and neck photon and proton therapy

Abstract Background Accurate target delineation is essential for precise delivery of proton therapy. A magnetic resonance imaging (MRI)‐only radiotherapy workflow may improve delineations, and thus improve proton therapy, but requires reliable synthetic computed tomography (sCT) for accurate dose calculations. Purpose The aim was to evaluate the dosimetric accuracy of commercial software‐based sCT for proton dose calculation for head and neck cancer (HNC), and benchmark it against the accuracy of photon dose calculation on the same sCT. Methods MRI and planning CT (pCT) were acquired for 20 HNC patients before receiving photon therapy. A commercial software created MRI‐based sCTs, which were co‐registered to pCTs. For each patient, photon and proton plans based on pCT, prescribing 68 Gy to the target, were created in RayStation and recalculated on sCT. Dose‐volume histogram (DVH) metrics and gamma index (2%/2 mm criteria) were analyzed, comparing pCT‐ and sCT‐calculated dose. Local and global gamma index were calculated for low and high‐dose thresholds. A gamma pass rate (GPR) was calculated for each plan and gamma index type. Results For photon and proton plans, the median difference in DVH metrics between pCT‐ and sCT‐calculated dose was 97.7%, while for proton plans it was > 93.3% except for body GPR with low‐dose threshold. Conclusion While sCT seems feasible for photon therapy, poorer agreement between pCT‐ and sCT‐calculated proton dose was found. Patient positioning differences between CT and MRI may partly explain the discrepancies between pCT‐ and sCT‐calculated proton dose.