EVALUATION OF DOSIMETRIC EFFECT OF RESPIRATORY GATING ON LUNG IMRT DELIVERY

Abstract

Purpose: For treatment of 3 dimensional conformal or intensity modulated radiation therapy (IMRT) of lung cancer, it is essential that respiratory gating is used to reduce the margins of clinical target volume (CTV). In this study, we evaluate the accuracy of dose in gated-IMRT when dynamic multileaf collimator (DMLC) mode was selected.

Materials and Methods: The Real-time Positioning Management (RPM) respiratory gating was installed on General Electric computed tomography (CT) simulator to view the movement of tumor and the other one gating on Varian Clinac 23 EX linear accelerator to deliver the dose at selected phases of breathing. The beam intensities of IMRT are varied by using DMLC which this mode of MLC may be introduced the dose errors from leaf lag and this error may be exacerbated when the gating is used. The 1 cm leaves gap and wedge shape patterns were created by using MLC shaper software to verify the accuracy of dose in gating method. These patterns were compared between gated and nongated delivery at 300 monitor unit/min dose rate and 1.25 cm/s leaves speed. For gated delivery, the mechanical motion device which Varian supplied was placed nearby the solid water phantom to simulate the breathing motion. Kodak X-Omat Verification (XV) film was employed to measure dose distributions of these patterns, while enhance dose range (Kodak EDR2) film and 0.13 cm3 ionization chamber with DOSE1 dosemeter were used for a lung IMRT pre-treatment verification. OmniProTM I'mRT software was the tool to analyze the film, 3% dose difference at low dose gradient region and 3 mm distance of isodose difference at high dose gradient region ( 3/3) were set in the clinical criteria for quantitative evaluation of dose distributions.

Result: The Kodak X-Omat V films ofa 1 cm wide leaf gap sliding across a 10 cm wide field of undergated and nongated showed the uniform dose distribution across the field. The comparison of central axis profile was almost congruent and there are a few area that 3/3 is higher than unity. For 14x14 cm?’ wedge field, no gamma value higher than unity appeared, while the isodose comparison also well result. In case of lung IMRT QA, the ratio of point dose from chamber between gated and nongated was 1.0033 which is so minute discrepancy. EDR2 film confirmed the impression result because it showed small area that 3/3 larger than unity and isodose lines of both gated and nongated were nearly congruent too.

Conclusions: For moderate dose rate of 300 MU/min, the dosimetric difference between with and without gated DMLC deliveries was so small. So, the repeat beam-on and beam-off from Real-time positioning management (RPM) gating have insignificant impact on the dosimetry of DMLC-IMRT.