Abstract
Purpose: The output factor, Scp, of small MLC‐shaped beams suffer from two competing effects: The loss of lateral CPE which tends to reduce the dose, and the extra leakage which tends to increase it. Scp measurement is further complicated by the size of the chamber used. We show that for small MLC‐beams, CPE is regained. Subsequently, we find measurement‐based analytical formula to calculate the output factors of small MLC‐shaped beams that is independent of chamber size. Method and Materials: A series of Scp measurements, with three photon energies, were taken using various chambers and various MLC‐shaped and jaw‐shaped fields ranging from 1×1 to 10×10 cm2. Results: The measured Scp of small MLC‐shaped beams was found to closely follow the linear trend of the larger fields. The linear curve joining the Scp of large fields was called the CPE‐line and it was extrapolated towards the small fields. For each energy/chamber combination, the CPE linear fitting parameters were calculated and chamber‐size effect was found to be minimal. The parameters were averaged over all chambers to present formula that is dependent only on energy and field size. Conclusion: Establishing CPE conditions for small MLC‐shaped beams allows the use of measurement‐based formula to calculate Scp. The formula is a function of field size and energy but not of chamber size. The removal of chamber dependence reduces the potential of underestimating Scp and consequently overdosing the patient. We propose that our formalism can be used in treatment planning systems for modeling of Scp for small MLC‐shaped beams for IMRT or SRS treatment or used by physicists to estimate the expected Scp value for the MLC‐shaped beams and compare them to their measured data.