IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 52, NO. 10, OCTOBER 2005 1741 Implementation of a Water Compensator for Total Body Irradiation Paolo Gallina, Giulio Rosati*, and Aldo Rossi Abstract—This paper presents the design, implementation, and testing of an integrated system for improving dose homogeneity in total body irradiation (TBI). TBI is a radiation therapy tech- nique that consists in delivering a uniform X-ray dose to the en- tire body of the patient. Because of variations in patient’s tissues thickness and density, achieving a uniform dose over the entire body is one of the major challenges in TBI. The system proposed in this paper, whose main goal is to compensate for tissues hetero- geneities, is made up of a translating bed, a linear accelerator, a vision system for body thickness assessment, a dynamically con- trolled water filter, and a main control unit. The water filter, placed between the X-ray source and the patient, is made up of an array of 70 small water containers (cells). The water level in each cell is con- trolled in real time, so as to modify the dose distribution both in the transverse direction and in the longitudinal direction. A prototype of the water filter system was implemented and tested, achieving good results in terms of dose uniformity. Index Terms—Level control, total body irradiation, water, X-ray applications. I. INTRODUCTION T OTAL body irradiation (TBI) is a radiation therapy tech- nique employed in the treatment of patients suffering from various hematological diseases. In particular, TBI is a necessary component in patient conditioning before bone marrow trans- plantation in the treatment of acute leukemia and other hemato- logical diseases. Far from being a consolidated technique, TBI is delivered in a variety of protocols, employing different facilities and tools. Basically, every TBI protocol involves one or more external X-ray sources to deliver a given radiation dose to the entire body of the patient [1]. Unlike traditional radiotherapy, which targets a specific area of the body, TBI must encompass the entire body within the radiation field. A common way to ob- tain this configuration consists in projecting an horizontal X-ray beam over a long distance, the patient being placed within the field in a standing, semi-upright, or side-lying position (an inter- esting review about the different techniques employed to deliver TBI can be found in [2], whereas translating-bed techniques are referred to in the next paragraph). Since the whole dose is usu- ally delivered among several fractions (from 6 to 12) that can last up to 1 hour each plus up to 1 hour of setup time (this is Manuscript received January 16, 2004; revised February 6, 2005. This work was supported by University of Padua, Italy. Asterisk indicates corresponding author. P. Gallina is with the Department of Energetics, University of Trieste, 34127 Trieste, Italy (e-mail: pgallina@units.it). *G. Rosati is with the Department of Innovation in Mechanics and Man- agement, University of Padua, via Venezia 1, 35131 Padua, Italy (e-mail: giulio.rosati@unipd.it; web: www.mechatronics.it). A. Rossi is with the Department of Innovation in Mechanics and Manage- ment, University of Padua, 35131 Padua, Italy (e-mail: aldo.rossi@unipd.it). Digital Object Identifier 10.1109/TBME.2005.855715 the case, for example, of the side-lying patient technique em- ployed at the S. Bortolo Hospital of Vicenza, Italy), it is clear that patients are forced to hold an uncomfortable posture for a long time. Another major problem in performing an efficient TBI therapy is that a uniform dose must be delivered to the en- tire body. Local radiation absorption depends on tissue thick- ness and density (i.e., on tissue equivalent thickness). For this reason, care must be taken to deliver an adequate radiation dose to thicker parts of the patient (such as pelvis) without overdosing thinner parts (such as neck). In order to reach this goal, most TBI protocols include the use of metallic or plastic shielding blocks, placed between the patient and the source [2]–[6]. How- ever, shielding block design, construction, and positioning is an expensive and time-consuming procedure. Moreover, compen- sation accuracy is not always satisfactory, being affected by pa- tient’s movements during irradiation. As a result, the therapy turns out to be not repeatable and not always effective. In order to improve TBI, some new techniques have been de- veloped in the last decades. One of the most revolutionary con- sists in employing a translating unit to move the patient under the radiation source. In this way, only a small part (a sort of transverse slice) of patient’s body lies within the X-ray field at a time: the unit is moved slowly below the gantry, that gen- erates a vertical beam, while the patient comfortably rests in supine or prone position on the translation bed [7], [8]. Correct dose delivery is obtained by modifying in real-time the transla- tion velocity, whose value is calculated by taking into account some physical parameters such as patient’s local thicknesses and density, beam geometry, and dose rate. It has been proven that the translating unit technique, if compared to fixed-beam techniques, provides better dose uniformity and makes shielding blocks placement more accurate [9]. Moreover, translating-bed procedures are comfortable for patient and allow placing the patient closer to the X-ray source [10]. The main drawback of this approach is that dose uniformity can be achieved only in the translation direction. It is clear that, with the aim of im- proving this technique, a tissue density and thickness compen- sator must be introduced to achieve homogeneous dose distri- bution in the transverse direction. Moreover, the compensator should be suited to fit different patients in order to reduce setup time and costs. In this paper, we propose the use of a compensator based on water as compensating medium. The compensator is completed with a control system that dynamically changes the shape of the water volume interposed between the X-ray source and the patient. In this context, water-based compensation in TBI is a promising technique that has been recently investigated. Shigeo et al. carried out basic studies on a water compensator to be placed between the patient and a 10-MV linear accelerator [11]. By means of phantom tests, they calculated the shape of the 0018-9294/$20.00 © 2005 IEEE