## 2 Materials and Methods  
All the eligible patients had a diagnosis of unresectable LA-NSCLC and were candidates for standard CRT followed or not by durvalumab as decided by a multidisciplinary meeting. The tumor was staged with total body CT scan and 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT). In addition, all patients underwent a lung perfusion assessment with Tc-99 MAA SPECT-CT scan within 10 days before starting the treatment. A standard RT treatment plan was obtained using a four-dimensional simulation CT. All treatment plans were delivered using volumetric modulated arc therapy (VMAT) technique and daily image-guided radiation therapy (IGRT).  
All treatment volumes were identified and contoured following the ESTRO ACROP guidelines [12]. The ICRU 83 [13] normalization and optimization guidelines were used as a reference for PTV coverage, with V95% > 95% and D105% < 5% so that a minimum dose of 95%, a maximum of 105% and an average of 95% of the prescribed dose were accepted. According to the QUANTEC [14], dose constraints for organs at risk were defined as follow: mean lung dose 20-23 Gy, mean dose to the esophagus < 34 Gy, volume of the lung receiving 20 Gy (V20) < 35% and volume of the lung receiving 5 Gy (V5) < 60%.  
In addition, over the standard plan obtained for treatment, an in silico one was calculated for each patient introducing lung perfusion provided by SPECT CT scan. For this second treatment plan obtained after the SPECT optimization, deformable co-registration between CT simulation images and the SPECT/CT ones was performed using the VelocityTM software 4.0 (Varian Medical Systems, Crawley, UK). First performed a fusion between the simulation CT and the PET CT. Then, we automatically fused them with the corresponding PET perfusion images (Figure 1). The planning system used was Pinnacle. From the literature, the best perfused lung areas could be considered those with a perfusion range ranging from 20% of maximum perfusion to 80% of maximum perfusion [15].  
Since the exploratory nature of this study, the regions of interest representing lung volumes with a perfusion of at least 20% 40%, 60% and 80% of the maximum perfusion value were created on the treatment plan (xx%IsoROI) (Figure 2).  
Finally, a second plan optimization was developed giving priority to 80% isoROI with the purpose to preserve the most perfused lung areas and to maintain the same OARs doses obtained from the standard plan.  
The dosimetric parameters that most frequently correlate with pulmonary toxicity, such as mean lung dose (Dm), lung V5, lung V20 and the PTV volume (cc), were analyzed. PTV coverage dose, mean heart and esophageal dose were also considered.  
Volumes, ROI x%--PTV mean dose, V20 and V5 were extracted for each isoROI. The mean values of these parameters were used to compare the standard and the second RT treatment plan. The \(t\)-test was applied, and a \(p\) value \(<\) 0.05 was considered statistically significant.  
Figure 1: Deformable co-registration between CT simulation images and the SPECT/CT.  
Figure 2: Red: PTV; green: isoROI 20%; magenta: isoROI 40%; yellow: isoROI 60%; light-blue: isoROI 80%.