Intensity modulated radiation therapy (IMRT) surface dose measurements using a PTW advanced Markus chamber

A new PTW advanced Markus ionization chamber has been implemented in IMRT fields, to measure surface dose at ICRU and ICRP reference depth of 0.07 mm [ICRU Report 39, Determination of dose equivalents resulting from external radiation sources, 1985; ICRP Publication 60, 1990 recommendations of the International Commission on Radiological Protection, 1991]. This chamber has a small radius with a revised guard ring design, therefore less prone to surface over-response effects. The over response correction for advanced Markus chamber is 3.3%, which is significantly smaller than 10.1% which was the original Markus chamber over response. After over response correction, the surface dose can be accurately measured by either data extrapolation or by adding one layer of plastic sheet protector to the top of Markus chamber. The surface dose measurements for small fields, e.g 3 x 3 cm, the polarity effect of advanced Markus chamber is 12%, which is significantly higher than the 5% polarity effect of the original Markus. For a 12 x 12 cm field size, surface dose (at 0.07 mm) measured by advanced Markus chamber is 19.8% for open field and 19.2% for an unmodulated step-and-shoot IMRT field. The variation in surface dose due to intensity modulated IMRT fields has also been investigated. For an intensity modulated, step-wedge IMRT field, surface dose varies from 15.7 + or - 1% for the highest intensity segment to 26.9 + or - 1% for the lowest intensity segment. The results of chamber measurements have been compared against EBT type GAFCHROMIC film results.