Impact of attenuation correction of radiotherapy hardware for positron emission tomography‐magnetic resonance in ano‐rectal radiotherapy patients

Background Positron Emission Tomography‐Magnetic Resonance (PET‐MR) scanners could improve ano‐rectal radiotherapy planning through improved Gross Tumour Volume (GTV) delineation and enabling dose painting strategies using metabolic measurements. This requires accurate quantitative PET images acquired in the radiotherapy treatment position. Purpose This study aimed to evaluate the impact on GTV delineation and metabolic parameter measurement of using novel Attenuation Correction (AC) maps that included the radiotherapy flat couch, coil bridge and anterior coil to see if they were necessary. Methods Seventeen ano‐rectal radiotherapy patients received a 18F$\mathrm{^{18}F}$‐FluoroDeoxyGlucose PET‐MR scan in the radiotherapy position. PET images were reconstructed without (CTACstd$\mathrm{CTAC_{std}}$) and with (CTACcba$\mathrm{CTAC_{cba}}$) the radiotherapy hardware included. Both AC maps used the same Computed Tomography image for patient AC. Semi‐manual and threshold GTVs were delineated on both PET images, the volumes compared and the Dice coefficient calculated. Metabolic parameters: Standardized Uptake Values SUVmax$\mathrm{SUV_{max}}$, SUVmean$\mathrm{SUV_{mean}}$ and Total Lesion Glycolysis (TLG) were compared using paired t‐tests with a Bonferroni corrected significance level of p=0.05/8=0.006$p = 0.05/8 = 0.006$. Results Differences in semi‐manual GTV volumes between CTACcba$\mathrm{CTAC_{cba}}$ and CTACstd$\mathrm{CTAC_{std}}$ were approaching statistical significance (difference −15.9%±1.6%$-15.9\%\pm 1.6\%$, p=0.007$p = 0.007$), with larger differences in low FDG‐avid tumours (SUVmean