On beam quality and flatness of radiotherapy megavoltage photon beams.

Ratio of percentage depth dose (PDD) at two depths, PDD at a depth of 10 cm (PDD10), and beam flatness are monitored regularly for radiotherapy beams for quality assurance. The purpose of this study is to understand the effects of changes in one of these parameters on the other. Is it possible to monitor only the beam flatness and not PDD? The investigation has two components. Naturally occurring i.e., unintended changes in PDD ratio and in-plane flatness for 6 and 10 MV photon beams for one particular Siemens Artiste Linac are monitored for a period of about 4 years. Secondly, deliberate changes in the beam parameters are induced by changing the bending magnet current (BMI). Relationships between various beam parameters for unintended changes as well as deliberate changes are characterized. Long term unintentional changes of PDD ratio are found to have no systematic trend. The flatness in the inplane direction for 6 and 10 MV beams show slow increase of 0.43 and 0.75% respectively in about 4 years while the changes in the PDD ratio show no such trend. Over 10% changes in BMI are required to induce changes in the beam quality indices at 2% level. PDD ratio for the 10 MV beam is found to be less sensitive, while the depth of maximum dose, d(max), is more sensitive to the changes in BMI compared to the 6 MV beam. Tolerances are more stringent for PDD10 than PDD ratio for the 10 MV beam. PDD ratio, PDD10, and flatness must be monitored independently. Furthermore, off axis ratio alone cannot be used to monitor flatness. The effect of beam quality change in the absolute dose is clinically insignificant.