Verification of positioning accuracy of MLC leaves of 8 medical accelerators in IMRT in Henan province

Abstract

Objective To measure the positioning accuracy of multi-leaf collimators (MLC) leaves by using radiochromic films, with aim to providing data in support of IAEA′s methodology validation. Methods The present study focused on 8 accelerators (Varian, Elekta and Siemens machines) at 6 hospitals in Henan province with highly skilled physicists. The 25 cm × 25 cm radiochromic films were put on 30 cm × 30 cm homogeneous solid phantoms and covered with a 2.0-cm-thick homogeneous solid phantom slabs. The CT-scanned images were transmitted to TPS for plan formulation. With 6 MV X-ray, MLC created 5 strip 3 cm×0.6 cm picket fence field, each with 3.0 cm strip separation. The SSD was 100 cm at the maximum dose point, with a 250 MU to each strip. The irradiated radiochromic films were returned to IAEA for analysis within one week and compared with the values given by IAEA. Results The difference of film-measured and TPS-planned positions of MLC leaves for each strip picket fence should be within ± 0.5 mm as required by IAEA. For 7 of 8 accelerators selected, the differences of accurately measured MLC leaf positions were all within ±0.5 mm, which were in line with the IAEA requirements, with only other one being beyond -0.5 mm, not consistent with the IAEA requirements. The differences of film-measured actual widths between each pair and all pairs of leaves were within ±0.75 mm as required by IAEA for 7 accelerators, whereas the other one was outside ±0.75 mm, not consistent with the IAEA requirements. The standard deviation of film-measured actual width of MCL leaf between each pair and all pairs for 7 accelerators were ≤0.3 mm as required by IAEA, whereas the other one was 0.5 mm, not consistent with IAEA requirements. Conclusions The MLC positioning accuracy of a few of medical accelerators in Henan is not qualified. It is of great significance to carry out regular quality examination as well as third-party verification to ensure precise delivery of radiotherapy. Key words: Intensity modulated radiation therapy; MLC leaves; Radiochromic film; Quality audit