Evaluation of a diode array for QA measurements on a helical tomotherapy unit K. M. Langen, a S. L. Meeks, D. O. Poole, T. H. Wagner, T. R. Willoughby, O. A. Zeidan, and P. A. Kupelian Department of Radiation Oncology, M. D. Anderson Cancer Center Orlando, Orlando, Florida K. J. Ruchala and G. H. Olivera TomoTherapy Inc., Madison, Wisconsin Received 23 May 2005; accepted for publication 1 September 2005; published 21 October 2005 A helical tomotherapy system is used in our clinic to deliver intensity-modulated radiation therapy IMRT treatments. Since this machine is designed to deliver IMRT treatments, the traditional field flatness requirements are no longer applicable. This allows the unit to operate without a field flatness filter and consequently the 400 mm wide fan beam is highly inhomogeneous in intensity. The shape of this beam profile is mapped during machine commissioning and for quality assurance purposes the shape of the beam profile needs to be monitored. The use of a commercial diode array for quality assurance measurements is investigated. Central axis beam profiles were acquired at different depths using solid water built-up material. These profiles were compared with ion chamber scans taken in a water tank to test the accuracy of the diode array measurements. The sensitivity of the diode array to variations in the beam profile was checked. Over a seven week period, beam profiles were repeatedly measured. The observed variations are compared with those observed with an on-board beam profile monitor. The diode measurements were in agreement with the ion cham- ber scans. In the high dose, low gradient region the average ratio between the diode and ion chamber readings was 1.000± 0.005 ±1 standard deviation. In the penumbra region the agreement was poorer but all diodes passed the distance to agreement DTA requirement of 2 mm. The trend in the beam profile variations that was measured with the diode array device was in agreement with the on-board monitor. While the calculated amount of variation differs between the devices, both were sensitive to subtle variations in the beam profile. The diode array is a valuable tool to quickly and accurately monitor the beam profile on a helical tomotherapy unit. © 2005 American Asso- ciation of Physicists in Medicine. DOI: 10.1118/1.2089547 Key words: tomotherapy, quality assurance, diode array I. INTRODUCTION A TomoTherapy™ Hi * Art II unit delivers radiation therapy plans using a helical tomotherapy technique. 1 A linear accel- erator is mounted on a ring gantry that continuously rotates while the patient is translated along the axis of gantry rota- tion during treatment delivery. The beam has a fan geometry and a 64 leaf binary collimator is used to subdivide this fan beam into beamlets. Intensity modulation is achieved by a temporal modulation of the collimator leaves. The unit is designed for intensity modulated treatment delivery and therefore the traditional requirement of a flat radiation field across the treatment field does not exist any- more. The TomoTherapy Hi * Art unit takes advantage of this and a field flattening filter is omitted. The advantage of this design is a relatively high machine output and a radiation field that varies less in energy across the field. 1 During machine commissioning, the nonuniform beam profile is mapped using an ion chamber scan in a water tank. To calculate collimator leaf opening times, the relative out- put for each leaf’s off-axis location is determined from the beam profile and taken into account. It is therefore necessary to periodically monitor the shape of the beam profile and to verify its consistency. Water tank measurements are tedious on a monthly basis and the verification of nonuniform fields with film dosimetry requires the acquisition of a film calibra- tion curve. The measurement of the beam profile with a com- mercially available two-dimensional 2-D diode array was therefore investigated. Advantages are the immediate avail- ability of the results and ease of use. The diode array investigated was designed for use on a TomoTherapy unit and it is manufactured by Sun Nuclear Corp. of Melbourne, FL. This device, henceforth referred to as TomoDOSE™, was used for all test measurements re- ported. First, the TomoDOSE readings are compared with ion chamber scans in a water tank to test the diode array’s accuracy. Second, the sensitivity of the diode array to subtle changes in the beam profile was tested. To monitor the beam profile, TomoTherapy’s on-board detector array can also be used. Over the course of seven weeks, the monitoring of the beam profile via this on-board detector array was compared with repeated TomoDOSE measurements. Incidentally, a continuous change in the beam profile was observed during this period. These changes were subtle yet persistent and were used to diagnose deterioration of the x-ray target. Prior to this observation the x-ray target had been used for 18 months. The x-ray target was replaced at the end of the seven week observation period. 3424 3424 Med. Phys. 32 „11…, November 2005 0094-2405/2005/32„11…/3424/7/$22.50 © 2005 Am. Assoc. Phys. Med.