Pulmonary X-ray imaging together with pulse oximetry are harmful and invasive techniques used to monitor and diagnose the clinical course of lung dysfunction in preterm born infants which most of the cases suffer Respiratory Distress Syndrome (RDS) . Biophotonics@Tyndall is exploring Gas in Scattering Media Absorption Spectroscopy (GASMAS)  as a novel non-invasive technique to measure continuously absolute lung oxygen volume and concentration. This could assist and improve the assessment of lung function in neonates . <p> </p>In this paper, we present results of bench-top measurements carried out in the preclinical phase of GASMAS studies. We start with a detailed explanation of the manufacturing process of multi-structure thorax phantoms with realistic geometry based on organ segmentation from anonymized DICOM images of neonates. After segmentation, the organs are 3D printed and used to create negative rubber molds. The tissue optical properties of heart, bone and muscle are assigned by mixing the silicone matrix with different concentrations of absorbers and scatters, the lung is kept as a gas content cavity and the thorax phantom is build up by placing all organs inside out immersed in the muscle structure. <p> </p>The phantoms are used for quality control and validation of the system performance . Oxygen gas absorption imprints are measured for different light source-detector remittance configurations and the results are used to define the potential and limitations of the GASMAS technology in the development of a bed-side clinical device.