Total scatter factor for clinically relevant fields in stereotactic radiosurgery dosimetry

In SRS, the actual planned clinical m3 mMLC fields are irregular and shaped by the mMLC leaves; they are more complex than those set for the commissioning data measurements and hence, the scatter contributions may be different to those pertaining to the regular fields.<br><br>In this study, RDFs were measured using a range of m3 mMLC (BrainLAB, Heimstetten, Germany) fields of different shapes and same equivalent square areas in order to validate this notion of equivalency between irregular and square fields. The range of the m3 mMLC fields includes the square mMLC fields. Given that RDFs are measured and defined for a set of jaws-mMLC fields, their variation with respect to jaws was also investigated, including its impact on the MU accuracy: for the same m3 mMLC setting, different jaws openings were tested to notice the effects of jaws only on RDF. Thus, measurements of RDF were performed and examined for ten irregular m3 mMLC fields of 12.3 x 12.3 to 51.0 x 51.0 mm2 combined with fields set by the jaws of 2.5 x 2.5 to 9.8 x 9.8 cm2. Using the BrainSCAN TPS, iPlan, each m3 mMLC field in the lowest range was generated under three different shapes of the same area. The jaws fields were then set from the Linac console. The detectors used for dose measurements were IC3, IC13, stereotactic diode (SFD) and the PRESAGE dosimeter. Calculations of RDF using iPlan were also performed and compared to each other for different shapes and same equivalent square of m3 mMLC fields.<br><br>Examined in this research were the following questions: 1) the validity of the equivalent square concept, 2) the validity of irregular m3 mMLC fields in the commissioning of BrainSCAN TPS, and, 3) the safety of the jaws in the static position throughout a treatment plan and delivery.<br><br>It was found that RDF were responsive to mMLC shapes even if the areas are the same. The response was different from one detector to another due to the size and type of detector. The response of RDF for different mMLC shapes was also more pronounced for the smallest range of the fields. Thus, the equivalent square concept is more valid for the larger mMLC fields.<br><br>Also, the results have shown that RDFs are responsive to jaws opening only in the smallest mMLC fields: it is more practical to keep the jaws at the same position during a given treatment plan and delivery for that range of mMLC fields, as this allows reducing the mechanical uncertainty related to the movements of the jaws particularly when dealing with many treatment fields.<br>