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. 2025 Dec;26(12):e70335.
doi: 10.1002/acm2.70335.

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

Marte Kåstad Høiskar  1 , René M Winter  1 , Natalie Hornik  1   2 , Mirjam Delange Alsaker  3 , Kajsa Fridstrøm  1   3 , Sigrun Saur Almberg  3 , Kathrine Røe Redalen  1
Affiliations

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

Marte Kåstad Høiskar et al. J Appl Clin Med Phys. 2025 Dec.

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 < 0.4 Gy with an interquartile range < ± 0.7 Gy for all structures and metrics, except for mean dose to oesophagus (median = -0.3 Gy, range: -2.0 Gy to 0.7 Gy), oral cavity (median = 1.8 Gy, range: 0.6 Gy to 3.0 Gy), and larynx (median = -0.1 Gy, range: -1.5 Gy to 0.6 Gy) for proton plans. The median GPR for photon plans 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.

Keywords: Head and neck cancer; MRI‐based synthetic CT; Proton therapy.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
Example of planning CT (pCT), synthetic CT (sCT) and their difference. (a) pCT, (b) sCT, and (c) the difference between them for one patient with well registered pCT and sCT. Here, a slice in the lower head and neck region is shown. HU, Hounsfield unit.
FIGURE 2
FIGURE 2
Evaluation of the Hounsfield unit (HU) accuracy of synthetic CT (sCT) and its spatial‐structural agreement with planning CT (pCT). (a) The mean absolute error (MAE) of HU, (b) Dice score, (c) average symmetrical surface distance (ASSD), and (d) Hausdorff distance comparing the sCT to the pCT for the whole body, soft tissue, bone and air. The box extends from the first quartile to the third quartile, while the whiskers extend 1.5 times the interquartile range beyond the box. The green horizontal line is the median, and outliers are shown as circles.
FIGURE 3
FIGURE 3
The absolute difference in dose‐volume histogram metrics comparing planning CT‐calculated treatment plans with synthetic CT‐calculated plans for both photons and protons. The high‐risk clinical target volume that is prescribed 68 Gy is called CTVp68. D 2 and D 99.5 are the dose given to 2% and 99.5% of the structure's volume, while D mean is the mean dose given to the structure. *Statistically significant differences between photon and proton dose differences.
FIGURE 4
FIGURE 4
Example of dose difference maps and corresponding gamma index maps for one patient. The dose difference between planning CT (pCT)‐ and synthetic CT (sCT)‐calculated plans (left column) and the local gamma index, comparing pCT‐ and sCT‐calculated plans, calculated with a dose cutoff of 10% (middle column) and 90% (right column) of prescribed dose is shown for both photon (first and third row) and proton (second and fourth row) plans. Here, two different slices of the same patient are given as examples. Black arrows indicate the treatment beam direction in proton plans; for photons, volumetric modulated arc therapy was used.
FIGURE 5
FIGURE 5
Gamma pass rates for photon and proton plans. Global (left) and local (right) gamma index calculated with gamma criterion of 2%/2 mm and with either low (top row) or high (bottom row) dose threshold. Here, boxplots of the patients’ gamma pass rates are shown for photon and proton plans, for the whole body or the union of clinical target volumes (CTVs).
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