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Background Significant calcification in a coronary chronic total occlusion (CTO) is associated with percutaneous coronary intervention (PCI) failure. However, CTO PCI failure also occurs in the absence of significant calcification for unclear reasons.Aims We aimed to characterise the non-calcified areas of a CTO (in Hounsfield Units [HU]) on computed tomography coronary angiography (CTCA) and correlate with CTO PCI outcomes.Methods Out of a total of 499 CTO PCI patients (2017-2024), 51/82 patients who underwent CTCA and CTO PCI had an analysable CTCA and were included.Results Patients with excess calcfication (full moon) were excluded. Mean proximal cap attenuation was similar and >120 HU (p=0.98). Mean CTO body HU was lower in failed compared to successful CTO PCI (55.6 ±59.2 HU vs 72.2 ±49.1HU, p=0.3). Distal cap HU was significantly higher in failed CTO PCI (mean 140.3 ±76.1 vs 97.0 ±51.5, p=0.029). A V-shaped attenuation of the mean HU (F=7.667, p=0.0011) and median HU (H = 15.90, p = 0.0004) at the proximal cap, body and distal cap was seen in failed CTO PCI. No significant difference was identified in successful CTO PCI (H = 3.60, p = 0.165) (figure 1).Conclusions In CTOs without significant calification, a V-shaped curve of attenuation seen on CTCA (low body HU and high distal cap HU) is associated with CTO PCI failure. It is unclear what this attenuation pattern represents –possibly higher weight wires are required to cross such CTOS. Larger prospective studies are required before definitive conclusions can be made. How to assess a CTO on CTCA1) Assess only calcified portion2) CT-RECTOR score only3) CTO length only4) Assess both calcified and non calcified plaqueAbstract 26 Figure 15) There is no needto assess a CTO on CTCA