Performance of a fluorescent screen and CCD camera as a two- dimensional dosimetry system for dynamic treatment techniques S. N. Boon and P. van Luijk Kernfysisch Versneller Instituut, 9747 AA Groningen, the Netherlands T. Bo ¨ hringer, A. Coray, A. Lomax, E. Pedroni, and B. Schaffner Paul Scherrer Institut, CH-5234 Villigen, Switzerland J. M. Schippers a) Kernfysisch Versneller Instituut, 9747 AA Groningen, the Netherlands Received 18 February 2000; accepted for publication 31 May 2000 A two-dimensionally position sensitive dosimetry system has been tested for different dosimetric applications in a radiation therapy facility with a scanning proton beam. The system consists of a scintillating fluorescent screen, mounted at the beam-exit side of a phantom and it is observed by a charge coupled device CCD camera. The observed light distribution at the screen is equivalent to the two-dimensional 2D-dose distribution at the screen position. It has been found that the dosimetric properties of the system, measured in a scanning proton beam, are equal to those measured in a proton beam broadened by a scattering system. Measurements of the transversal dose distribution of a single pencil beam are consistent with dose measurements as well as with dose calculations in clinically relevant fields made with multiple pencil beams. Measurements of inho- mogeneous dose distributions have shown to be of sufficient accuracy to be suitable for the veri- fication of dose calculation algorithms. The good sensitivity and sub-mm spatial resolution of the system allows for the detection of deviations of a few percent in dose from the expected intended or calculated dose distribution. Its dosimetric properties and the immediate availability of the data make this device a useful tool in the quality control of scanning proton beams. © 2000 American Association of Physicists in Medicine. S0094-24050001909-X Key words: proton dosimetry, scintillator, fluorescent screen, CCD, scanning beam, dynamic treatment, intensity modulation, quality control I. INTRODUCTION Intensity-modulated beams offer many new possibilities to optimize the treatment results in modern radiation therapy. An intensity-modulated beam possesses a prescribed inten- sity variation in fact a dose variation over the transversal cross section of the radiation field. Dynamic radiation therapy is a way to accomplish such variations of the dose distribution within a field. One or several parameters of the dose delivery system are varied during the administration of the irradiation. Examples of such parameters are collimator leaf position, block position, or the position of a pencil beam within a field. The time that the field or beam is in a certain configuration is prescribed by a weight factor. Ad- vantages of an intensity-modulated beam, and thus of dy- namic beam delivery, of course not only apply for photon beams but also and even especially for applications with electron beams, proton beams, and heavy ion beams. 1,2 Using a dynamic treatment technique, and especially with proton beams, the shape of the total dose distribution deliv- ered in one single field can be rather complex Fig. 1.A detailed knowledge of the dose distribution is essential for matching several of such fields to the desired total dose dis- tribution. Here, we will discuss the applicability of a method to verify the dose distribution in a phantom, created by a dynamic treatment modality for proton beams. The aim of such a verification is threefold. First, it can be used to check if the shape and magnitude of a complex- shaped dose distribution in a homogeneous phantom agrees with the result of the treatment planning. Due to various reasons the real dose distribution may differ from the calcu- lated one. The second aim can then be to use this system to understand the effects that may be responsible for such de- viations, so that the precision of the dose calculations can be improved. For instance, one could think of the effects of inhomogeneities in the phantom. If the reason for the dis- crepancy is understood, one may decide to change the weight factors of certain pencil beams. For such applications it will be obvious that several phantom measurements are needed. Third, the dose verification in a phantom can be used as a means of inclusive verification of the functioning of the beam delivery system: has the dose administration been per- formed as prescribed? Standard dosimetry systems are not very suitable for dy- namic beam delivery systems. Usually dosimetric measure- ments are done with an ionization chamber at various posi- tions in a water phantom. However, since the dose delivery varies in time, a measurement of the dose distribution in three dimensions is very time consuming: for each point of measurement the full beam delivery sequence has to be re- peated. Another disadvantage of such methods is that an er- ror in the beam delivery is not detected if it has caused a 2198 2198 Med. Phys. 27 „10…, October 2000 0094-2405Õ2000Õ27„10…Õ2198Õ11Õ$17.00 © 2000 Am. Assoc. Phys. Med.