Verification of positioning accuracy of MLC leaves of 8 medical accelerators in IMRT in Jiangsu province / 中华放射医学与防护杂志

ABSTRACT

Objective To measure the accuracy of multi-leaves collimator (MLC) leaves positions in intensity modulated radiation therapy (IMRT) for verification purpose.Methods Solid water homogeneous phantom with size of 30 cm× 30 cm was scanned by CT scanner.The scanned images were delivered to radiation therapy plan system (TPS) to formulate the therapy plan.The MLC leaves created 5 strips of exposure field,each 3 cm long and 0.6 cm wide.The strip-to-strip distance was 3 cm.With 6 MV X-rays,the SSD was 100 cm at the maximum dose point.The MU per strip was 250 MU.EBT2 radiochromic films each of 25 cm×25 cm were placed on the 30 cm×30 cm homogeneous solid phantom and covered with 1 cm thick solid phantom slabs for delivering of IMRT.Results For 7 of 8 accelerators,the differences of film-measured and TPS-planned MLC leaf position for every fence field were within ± 0.5 mm as required by IAEA,with only other one being-0.6 mm,not consistent with the IAEA requirements.The film-measured position difference between each pair and all pairs of leaves for 8 accelerators were all in line with IAEA's requirements.The film-measured actual width difference between each pair and all pairs of leaves was within ±0.75 mm as required by IAEA for 4 accelerators and outside ±0.75 mm for the other three,not consistent with the IAEA requirements.The standard deviation of film-measured actual width between each pair and all pairs of leaves for 6 accelerators were ≤ 0.3 mm,as required by IAEA,whereas ＞0.3 mm for the other two,not consistent with IAEA requirements.Conclusions The film dosimetric verification of IMRT is an important part of its quality assurance,featuring simple,reliable and highly accurate positioning and can meet measurement requirement.