A new approach in dose measurement and error analysis for narrow photon beams (beamlets) shaped by different multileaf collimators using a small detector.

Abstract

Dose measurement for narrow stereotactic beams and intensity modulation radiotherapy beamlets is difficult and error-prone due to the lack of lateral electron equilibrium. A small detector position error and finite sensitive volume as well as the nonfocus collimation could result in considerable (> 10%) measurement errors. A new method is introduced here to measure the dose and error components so that the accuracy and precision of the dose measurement can be improved. Based on superposition principle, we can create exactly the small field of interest by subtraction of a reference open field (O-field) and two strip fields (S-fields) from the sum of four quadrant fields (Q-fields). The position effect on the dose measurement is determined by the standard deviation of the four Q-field readings. The collimator leaf-edge effect (LEE) is quantified by the difference between the readings of the two S-fields using a detector that has very small sensitive volume. The detector-volume effect can be analytically estimated from the integrals of the dose distributions of the two S-fields over the detector volume. Using a pinpoint ion chamber (PTW N31006) and a stereotactic silicon diode detector (Scanditronix, DEB050), we have measured scatter factors (SF) and tissue-maximum ratios for 6-MV x-ray fields with sizes of 3 x 3 and 6 x 6 mm2 shaped by a BrainLAB micromultileaf collimator (microMLC) (M3), 4.4 x 4.4 and 8.8 x 8.8 mm2 shaped by a 3DLine double-focus MLC, and 5 x 5 and 10 x 10 mm2 by a Varian Millennium MLC. Our experimental results demonstrate that the large errors are often caused by a small setup error or measuring point displacement from the central ray of the beam. The LEE is almost independent of the depth but closely related to the field size and the type of MLC. The volume effect becomes significant when the detector diameter is comparable to the half size of the small fields. Application of the new method using different detectors had achieved less than 8.3% total experiment error for all of the small fields of interest except for the SF of the 3 x 3 mm2 field from the pinpoint ion chamber that has 15% volume effect. Importantly, the new method using a solid water phantom is clinically convenient and highly reproducible.