Analysis of interfractional set-up errors and intrafractional organ motions during IMRT for head and neck tumors to define an appropriate planning target volume (PTV)- and planning organs at risk volume (PRV)-margins.

BACKGROUND AND PURPOSE: To analyze the interfractional set-up errors and intrafractional organ motions and to define appropriate planning target volume (PTV)- and planning organs at risk volume (PRV)-margins in intensity-modulated radiotherapy (IMRT) for head and neck tumors. PATIENTS AND METHODS: Twenty-two patients with head and neck or brain tumors who were treated with IMRT were enrolled. The set-up errors were defined as the displacements of the coordinates of bony landmarks on the beam films from those on the simulation films. The organ motions were determined as the displacements of the coordinates of the landmarks on the images recorded every 3 min for 15 min on the X-ray simulator from those on the initial image. RESULTS: The standard deviations (SDs) of the systematic set-up errors (Sigma-INTER) and organ motions (Sigma-intra) distributed with a range of 0.7-1.3 and 0.2-0.8 mm, respectively. The average of the SDs of the random set-up errors (sigma-INTER) and organ motions (sigma-intra) ranged from 0.7 to 1.6 mm and from 0.3 to 0.6 mm, respectively. Appropriate PTV-margins and PRV-margins for all the landmarks ranged from 2.0 to 3.6 mm and from 1.8 to 2.4 mm, respectively. CONCLUSIONS: We have adopted a PTV-margin of 5mm and a PRV-margin of 3mm for head and neck IMRT at our department.

[Accuracy of dynamic multileaf position and influence of beam profile according to change in dose monitor unit in QA plan of intensity modulated radiotherapy].

The number of dose monitor units in IMRT QA plans differs form the actual conditions of IMRT in dosimetry for verification of dynamic multileaf-IMRT (DMLC-IMRT). We measured the accuracy of the position of the dynamic multileaf collimator and the dose profile for various numbers of dose monitor units, and verified the accuracy of dosimetry in an IMRT QA plan. The accuracy of the position of the dynamic multileaf collimator was measured by using the software of the external irradiation device, and the dose profile was measured by using the semiconductor profiler. Deviation in the position of the dynamic multileaf collimator increased as the number of dose monitor units decreased. When deviation in the position of the dynamic multileaf collimator was large and the gap width of the multileaf collimator was narrow, the change in dose profile was large. Therefore, verification of IMRT QA plans requires a phantom and measurement device close to the actual conditions of IMRT.