TABLE 2.

Summary of common image and dose based similarity metrics

	Metric	Formula
Image similarity	MAE/ME ↓	$\frac{1}{n}\sum _{i=1}^n|CBC{T}_i-C{T}_i|$/$\frac{1}{n}\sum _{i=1}^n(CBC{T}_i-C{T}_i)$
	MSE/RMSE ↓	$\frac{1}{n}\sum _{i=1}^n{(CBC{T}_i-C{T}_i)}^2$/$\sqrt{MSE}$
	PSNR ↑	$20\mathrm{lo}{\mathrm{g}}_{10}\left(\frac{MA{X}_{CT}}{RMSE}\right)$
	SSIM ↑	$\frac{\left(2{\mu }_{CBCT}{\mu }_{CT}+c_1\right)\left(2{\sigma }_{CBCT,CT}+c_2\right)}{\left({\mu }_{CBCT}^2+{\mu }_{CT}^2+c_1\right)\left({\sigma }_{CBCT}^2+{\sigma }_{CT}^2+c_2\right)}$ with ${\mu }_x=mean$; ${\sigma }_x^2=variance$; ${\sigma }_{x,y}=covariance$; $c_1={(k_{1}L)}^2$; $c_2={(k_{2}L)}^2$; $L=luminance$; $k_1=0.01$; $k_2=0.03$
	DICE ↑	$\frac{2|Are{a}_{CBCT}\cap Are{a}_{GroundTruth}|}{|Are{a}_{CBCT}|+|Are{a}_{GroundTruth}|}$ with $\cap =intersection$
Dosimetric similarity	DPR ↑	Fraction of voxels where DD ⩽ x% with $DD=\frac{D_{CBCT}-D_{CT}}{D_{CT}}\mathrm{\Delta }100$ where $D=dose$
	DVH ↑	Cumulative histogram of dose–volume frequency distribution for a given volume
	GPR ↑	Fraction of voxels where $\gamma \le 1$

Note: Arrows indicate better result.

Abbreviations: DPR, dose difference pass rate; DVH, dose–volume histogram; GPR, gamma pass rate; MAE, mean absolute error; ME, mean error; MSE, mean squared error; PSNR, peak signal‐to‐noise ratio; RMSE, root mean square error; SSIM, structural similarity.