Session Item

Stereotactic irradiation requires high precision in both imaging and dosimetry. For quality assurance of these requirements Sun Nuclear has developed the chamber array SRS MapCHECK together with the phantom StereoPHAN. The StereoPHAN is a multipurpose Phantom made of PMMA, which includes various inserts for determining geometric and dosimetric accuracy. The agreement between the planned dose using a special stereotactic beam model for the TPS and the measured dose with the SRS MapCHECK inside the StereoPHAN was investigated.

The delivered dose was measured using the SRS MapCHECK, which is a 2D array of 1013 semiconductor detectors. Absolute dose and profile measurements of quadratic fields (same as for measuring output factors) as well as for different MLC field configuration at fixed jaws were performed. In addition, conformal-arc plans and VMAT plans for phantom based PTV as well as for real patients were verified.

The measurements of the output factors for quadratic fields (1x1cm2 to 8x8cm2) showed a high agreement compared to the calculated values (see Figure 1). The measurements at fixed field sizes (2x2cm2, 3x3cm2, 4x4cm2 and 8x8cm2) and different MLC configurations also showed a high agreement with an average deviation of 1.2% (see Figure 2). The verification of conformal arc (gamma criteria 1%/1mm) showed a passing rate of 98.8%, for phantom based VMAT plans the passing rate is of the order of 98.4% and finally VMAT patient plans have a passing rate of 99%.

Measurements obtained with the SRS MapCHECK - for phantom based as well as for real patient plans - are highly consistent with the calculated dose even using a gamma criteria of 1%/1mm. However the measurements with fixed field size but different MLC configuration systematically show higher measured values than those calculated by the planning system. This deviation is about 1.2%. Overall the study shows that the StereoPHAN with the SRS MapCHECK is a suitable measuring device for dosimetry in stereotactic radiotherapy.