Commissioning of a micro multi-leaf collimator and planning system for stereotactic radiosurgery

Abstract

A computer controlled micro multi-leaf collimator, m3 mMLC, has been commissioned for conformal, fixed-field radiosurgery applications. Measurements were made to characterise the basic dosimetric properties of the m3, such as leaf transmission, leakage and beam penumbra. In addition, the geometric and dosimetric accuracy of the m3 was verified when used in conjunction with a BrainSCAN v3.5 stereotactic planning system. The m3 was detachably mounted to a Varian Clinac 2100C accelerator delivering 6 MV X-rays. Leaf transmission, leakage, penumbra and multiple, conformal fixed field dose distributions were measured using calibrated film in solid water. Beam data were collected using a diamond detector in a scanning water tank and planned dose distributions were verified using LiF TLDs and film. A small, shaped phantom was also constructed to confirm field shaping accuracy using portal images. Mean transmission through the closed multi-leaves was 1.9 +/- 0.1% and leakage between leaves was 2.8 +/- 0.15%. Between opposing leaves abutting along the central beam-axis transmission was approximately 15 +/- 3%, but was reduced to a mean of 4.5 +/- 0.6% by moving the abutmen position 4.5 cm off-axis. Beam penumbrae were effectively constant as a function of increasing square field size and asymmetric fields and was seen to vary non-linearly when shaped to diagonal, straight edges. TMR, OAR and relative output beam data measurements of circular m3 fields were comparable to conventional, circular stereotactic collimators. Multiple, conformal field dose distributions were calculated with good spatial and dosimetric accuracy, with the planned 90% isodose curves agreeing with measurements to within 1-2 mm and to +/- 3% at isocentre. Portal films agreed with planned beams eye-view field shaping to within 1 mm. The m3 micro multi-leaf collimator is a stable, high precision field-shaping device suitable for small-field, radiosurgery applications. Dose distributions can be accurately calculated by a planning system using only a few beam data parameters.