PII S0360-3016(98)00048-0 ● Physics Contribution NEW METHOD TO OBTAIN THE MIDPLANE DOSE USING PORTAL IN VIVO DOSIMETRY RONALD BOELLAARD, M.SC.,* MARION ESSERS,PH.D.,* † MARCEL VAN HERK,PH.D.* AND BEN J. MIJNHEER,PH.D.* *Radiotherapy Department, The Netherlands Cancer Institute/Antoni van Leeuwenhoek Huis, Amsterdam, The Netherlands; and † Department of Clinical Physics, Dr. Daniel den Hoed Cancer Center, Rotterdam, The Netherlands. Purpose: The aim of this study was to develop a method to derive the midplane dose [i.e., the two-dimensional (2D) dose distribution in the middle of a patient irradiated with high-energy photon beams] from transmission dose data measured with an electronic portal imaging device (EPID). A prerequisite for this method was that it could be used without additional patient information (i.e., independent of a treatment-planning system). Second, we compared the new method with several existing (conventional) methods that derive the midline dose from entrance and exit dose measurements. Methods and Materials: The proposed method first calculates the 2D contribution of the primary and scattered dose component at the exit side of the patient or phantom from the measured transmission dose. Then, a correction is applied for the difference in contribution for both dose components between exit side and midplane, yielding the midplane dose. To test the method, we performed EPID transmission dose measurements and entrance, midplane, and exit dose measurements using an ionization chamber in homogeneous and symmetrical inhomogeneous phantoms. The various methods to derive the midplane dose were also tested for asymmetrical inhomogeneous phantoms applying two opposing fields. A number of combinations of inhomogeneities (air, cork, and aluminum), phantom thicknesses, field sizes, and a few irregularly shaped fields were investigated, while each experiment was performed in 4-, 8-, and 18-MV open and wedged beams. Results: Our new method can be used to assess the midplane dose for most clinical situations within 2% relative to ionization chamber measurements. Similar results were found with other methods. In the presence of large asymmetrical inhomogeneities (e.g., lungs), discrepancies of about 8% have been found (for small field sizes) using our transmission dose method, owing to the absence of lateral electron equilibrium. Applying the other methods, differences between predicted and measured midplane doses were even larger, up to 10%. For large field sizes, the agreement between measured and predicted midplane dose was within 3% using our transmission dose method. Conclusions: Using our new method, midplane doses were estimated with a similar or higher accuracy compared with existing conventional methods for in vivo dosimetry. The advantage of our new method is that the midplane dose can be determined in the entire (2D) field. With our method, portal in vivo dosimetry is an accurate alternative for conventional in vivo dosimetry. © 1998 Elsevier Science Inc. In vivo dosimetry, Portal imaging, Midline dose, Midplane dose. INTRODUCTION During radiotherapy, high accuracy in dose delivery is re- quired because the relationships between absorbed dose and local tumor control and particularly normal tissue damage are very steep. Uncertainty in dose delivery to the specifi- cation point of about 3– 4% [1 standard deviation (SD)] is considered a criterion for good clinical practice (1–3). Therefore, in many institutions, in vivo dosimetry using diodes or TLDs is performed to check the actual dose delivery (4 –12). Recently, the use of electronic portal im- aging devices (EPIDs) for in vivo dosimetry has also been explored (13–22). In some institutions, entrance in vivo dose measurements are performed routinely (7, 12). In general, these measure- ments serve to check the source-skin distance (SSD) the output and performance of the treatment device, treatment unit setup parameters, calculation of the number of monitor units, and data transfer. However, not all errors in dose delivery can be traced using entrance dose measurements. Therefore, exit dose measurements are sometimes per- formed as well (5, 8). These measurements are particularly useful if the relative dose calculations are not accurate enough, e.g., if a simple algorithm for inhomogeneity cor- rections is used, or if no computed tomographic (CT) data Reprint requests to: R. Boellaard, M.Sc., Radiotherapy De- partment, The Netherlands Cancer Institute/Antoni van Leeu- wenhoek Huis, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands. Acknowledgments—This work was financially supported by the Dutch Foundation of Technical Sciences (STW), Grant BGN 33.3240. Accepted for publication 3 December 1997. Int. J. Radiation Oncology Biol. Phys., Vol. 41, No. 2, pp. 465– 474, 1998 Copyright © 1998 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/98 $19.00 + .00 465