## 1 Introduction  
Medical imaging often employs tomography as an effective tool to help detecting and characterizing both pathological and physiological conditions in tissues. Tomography provides 2D/3D images of the tissue under investigation and is performed exploiting approaches that typically involve ionizing radiations or complex magnetic field analysis (e.g., X-Ray CT, MRI). A different non-invasive experimental approach involves injecting low-power laser light in the Near-Infrared (NIR) spectrum and studying photon paths through the tissue to derive its optical properties, i.e., absorption and scattering. Thus, performing optical measurements with a set of properly arranged sources and detectors enables a full 3D reconstruction of the optical properties that correlate with the nature of imaged tissue. Such an approach is known as Diffuse Optical Tomography (DOT), because light propagation follows a quasi-diffusion like model when working in the NIR on large (cm\({}^{3}\)) tissue volumes (e.g., for clinical applications). Other approaches involve the detection of the diffused fluorescence signal (Fluorescence DOT) emitted by a specific fluorescent marker after being injected into the tissue [1]. Specifically, Time Domain DOT (TD-DOT) is performed by injecting pulsed laser light and detecting the effects of propagation through the tissue (i.e., amplitude attenuation, delay and temporal broadening of the pulse) to retrieve the optical properties of the tissue [2]. TD-DOT is potentially powerful for applications needing in-depth body exploration (such as brain activation [3], thyroid and breast imaging), and can be implemented in compact hand-held probes for easier non-invasive clinical examinations [4]. The main advantage of the TD approach is that it enables one to uncouple the effects of scattering from the effects of absorption in a single measurement, with no need for multiple distance measurements. Furthermore, when reflectance geometry is used, the photon Time-of-Flight (TOF) provides information on the depth probed by the measurement. More generally, the TD approach, exploiting time as an additional dimension, reduces --at least in principle--the required number of sources and detectors with respect to continuous wave or frequency domain operation, and that enables simpler implementations for medical use [5]. In the last years an important technology push has made the TD approach more attractive: Detectors like Silicon Photomultipliers (SiPMs), adapted from different applications, proved to be cheaper, more compact and sensitive than conventional detectors (e.g., Photomultiplier Tubes) and started to be widely applied as single-photon detectors in optical measurements [4; 6]. At the same time, SiPMs can be developed for hardware gating of the photon distribution, thus opening to higher penetration depths, but still keeping small dimension, which is not trivial for gated detectors [5]. Despite recent technology innovations, there is plenty of room for improvement in TD-DOT in reflectance geometry on the software side (e.g., reconstruction algorithms), as well as on the hardware side (e.g., more compact multiwavelength or tunable sources). Thus, anticipating a consistent framework for this modality, a clear and widely shared approach to performance quantification is essential for comparing results from different implementations, driving improvements in hardware and data analysis, and anticipating problems that may arise in the clinical environment. Breast cancer applications are crucial, as it is the most common cancer amongst women, with about 1 in 8 females being diagnosed with the disease during their lifetime [7; 8]. Survival rate significantly improves when an early diagnosis is reached. Nowadays screening mammograms play a key role in the prevention programs, but X-ray mammography still suffers from a low sensitivity that generally leads to unnecessary, social-care expensive, painful and invasive examinations [9; 10].