Session Item

Modern radiotherapy techniques often use the multileaf collimator (MLC) to create highly conformal dose distributions. Therefore, the MLC leaf positioning accuracy during  treatment delivery is very important. Any missteps during preparation such as transfer error or machine miscalibration could lead to MLC leaf mispositioning, and in turn, dosimetric deviation. This study aims to examine the possibility to detect any dosimetric deviation caused by systematic or incidental MLC positioning errors using Octavius 4D system (PTW).

A lung IMRT treatment plan and a SRS dynamic arc brain metastases plan, both optimized using Pinnacle TPS, were included in this study. For both plans, incidental and systematic faults were introduced by misplacing one MLC leaf or all MLC leaves of the entire leaf bank by 1-3mm, respectively. All plans were delivered on an Elekta Synergy linac with Agility collimator. Patient QA of treatment plans with large fields (long IMRT) were carried out using Octavius 4D 1500 system while SRS plans were measured using Octavius 4D 1000 SRS system. The detector size of both systems are 0.06 and 0.003cc, respectively. The 3D-measured doses were analyzed using Verisoft software with gamma criterion of dose difference ranging from 2-5% and distance to agreement (DTA) ranging from 1-3mm, with cut-off value of 50%.

Table 1 shows the results of gamma evaluation for the lung IMRT plans. For single MLC positioning fault of up to 3mm, the panel fails to detect any dose deviation as the resulting gamma values evaluated using different gamma criterions were comparable to the original plan without any MLC positioning error. However when the entire leaf bank is shifted, the results is apparent even with an induced positioning error of only 1mm. The results of the SRS dynamic arc measurements are shown in Figure 1. When gamma criterion of 3%/3mm is applied with a passing threshold of 95%, all deviations went undetected except the shift of the entire leaf bank by 3mm . The Octavius SRS 1000 system could possibly detect an incidental MLC positioning errors of up to 1mm when the analysis is carried out using gamma criterion of 3%/1mm. When DTA of >1mm is applied, all incidental errors of up to 3mm would go unnoticed. The use of more stringent gamma criterion is crucial in detecting such small errors, although it might also increase the incident of false negative findings.

Incidental MLC positioning errors might go undetected when patient QA is carried out using the Octavius 4D 1500 system. However, for treatments with large fields that require larger margin, the clinical dosimetric impact of such incidental errors might be insignificant. In contrast, this impact could be profound in SRS treatment which usually apply margins of only 1-2mm.  The Octavius SRS 1000 is capable of detecting such small positioning errors when a DTA of 1mm is use for analysis. The use of a larger DTA could potentially mask the present of small MLC positioning errors in SRS plan.