## 1 Introduction  
Enchondromas are common, benign cartilage tumors that mostly develop in the metaphysis of long bones in close proximity to growth plate cartilage [1; 2; 3; 4]. Ollier disease, also known as enchondromatosis, is a nonhereditary disorder defined by the presence of multiple enchondromas, while Maffucci syndrome is that condition in which multiple enchondromatosis is associated with soft tissue hemangiomas [6]. The prevalence of Ollier disease is estimated to be 1/100,000, while there are about 200 reports of Maffucci syndrome. Both conditions usually present during childhood as a possible consequence of a differentiation disorder during endochondral ossification of growth plate tissue [7]. Clinical problems caused by enchondromas depend on the extent of skeletal involvement and include skeletal deformities associated or not with pathological fracture, limb-length discrepancy, and the potential risk for malignant transformation to chondrosarcoma [3; 4; 5]. The diagnosis of Ollier disease is mainly based on clinical and radiological evaluations, which typically show an irregular distribution of multiple, radiolucent, homogenous lesions with an oval or elongated shape and a well-defined, slightly thickened bony margin. Since children affected by Ollier disease will undergo multiple radiographs during childhood for diagnosis and follow-up, the need for accurate assessment of enchondromas has to balance with the risks associated with repeated radiation exposure. Repeated exposure of children to radiation is a major concern, having been reported to be 2 to 5 times more radiosensitive than adults [8] and to have the highest risk of cancer development, as radiologic stochastic damage has a latency of one or even two decades. Thus, in the last few decades, much effort has gone into developing radiologic techniques able to keep radiation doses \"as low as reasonably achievable\" (ALARA), while not compromising image quality, especially in pediatric imaging [9]. EOS (EOS Imaging, Paris, France) is a relatively novel low-dose imaging system that is capable of producing radiographic images with very low radiation exposure [10]. It exploits a biplanar slot-scanning technology that employs highly sensitive gaseous photon detectors [11; 12; 13]. It consists of a cabin in which the patient is positioned standing erect, and the two linear X-ray sources and two gaseous detector arrays allow for the patient to be scanned in two orthogonal planes simultaneously. The X-ray sources move in synchronous fashion so that the beam is always horizontal to the patient, thus avoiding the magnification error (the so-called \"parallax effect\") of teleradiography. Once acquisition parameters have been set on the basis of three predefined morphotypes, the scan can start. The scan has a variable duration of up to twenty seconds, making this modality sensitive to motion artifacts. However, the acquisition can be shortened, further minimizing the dose with minimal reduction of image quality [14]. In general, the EOS system is excellent in the evaluation of biomechanical alignment of the spine and of the lower limbs, while it is not routinely used in the evaluation of focal bone lesions. Indeed, the diagnostic accuracy of the EOS system to identify and measure bone tumors has not been investigated previously. Moreover, the reduction of dose obtained by the EOS system has been established. Nevertheless, this exam can be acquired with different speeds, which are associated with different radiation doses and image quality, but a comparison of radiation doses of different sets of EOS images and conventional radiography to image the whole skeletal system has not been previously reported. The rationale for conducting this study was the lack of data concerning the potential use of this low-dose imaging examination to follow patients with multifocal bone lesions, namely enchondromatosis, in order to decrease the radiation burden on young patients affected by these conditions but providing images with enough quality to be used for oncologic purposes.