An ultrasonic device for source to skin surface distance measurement in patient setup.

PURPOSE To develop an ultrasound-based source to skin surface distance (SSD) measurement technique and device for patient setup and test its feasibility and accuracy. METHODS AND MATERIALS The ultrasonic SSD measurement device (USD) prototype consists of two main parts: a probe plate with an ultrasonic transducer in the center and a control unit that displays the SSD in millimeters. The probe plate can be slid into the block tray accessory slot of any treatment machine at the time of the SSD measurement. The probe plate contains an ultrasonic transducer as both the source and the detector for measuring the distance between the transducer and the target surfaces on the basis of an echo-detecting technique. The device was calibrated by a mechanical ruler with an accuracy of 0.01 mm and corrected by an offset of 601.7 mm, which is the distance from the radiation source to the ultrasonic transducer surface for the Siemens Primus linear accelerator (Linac). The ultrasound device provided digital readout with an accuracy of +/-0.1 mm for a flat surface after calibration. The SSD measurement experiments were done with the USD, an optical distance indicator (ODI), and an AKTINA 53-104 Mechanical Front Pointer (FP) on a Siemens Primus Linac with a full-sized female phantom. Ten measurements were carried out at each gantry angle of 0 degrees , 52 degrees , 85 degrees , 90 degrees , and 227 degrees for anatomic locations of head, thorax, breast, and pelvis, to obtain the average values and standard deviations. RESULTS The comparison study with the ODI and FP showed that the USD had an accuracy of less than +/-1.0 mm and that USD measurements had the minimum standard deviations among the three methods; therefore, USD gave more consistent and accurate readouts for SSD measurement. When considering the FP as a reference, the USD yields smaller deviations than the ODI for all measured locations (less than +/-2 mm). The variation of USD digital readout with a room temperature change of +/-2 degrees C is +/-0.1 mm, which is sufficiently accurate for SSD measurement. CONCLUSIONS The USD method has the following advantages. First, it decreases patient setup time by avoiding problems related to the blocking of the device by the patient or by the immobilization device. Second, it is more accurate than the other two methods currently used, as the test data show. Last, the digital readout eliminates the possibility of human reading error associated with the visual scales.