The relation of the dose distributions with the dose rates in static intensity modulated radiotherapy using Varian's linear accelarator

Abstract

Objective To evaluate the dose distributions with different dose rates, and approach a reference to the dose rate for radiation. Methods Three classic static intensity modulated radiotherapy (IMRT) plans of prostate cancer, lymphoma and nasopharyngeal carcinoma were chosen for the study. For each plan, the dose verification of three different dose rates (100, 300 and 600 MU/min) was performed with the Varian 600CD linear accelarator by using the 2-DICA of I'mRT Matrixx. With the Pinncale planning system, each segment was used as a beam to form another IMRT plan. The OmniPro-I'mRT V1.6 was applied to compare the segments in the two IMRT plans, and then the actual weights were obtained. The simulated plans at different dose rate were designed when setting the weights back into the planning system. Results With the increase of dose rate, the passing ratio of the verification decreased and the D_(max), D_(min), D_(mean) and D_(95) of the planning tumor volume increased. The high dose area expanded significantly in target regions, and the 95% isodose line extended. At the dose rate of 600 MU/min, The D_(95) of GTV_(nd) in nasopharyngeal carcinoma increased by 5.33% than the original plan with the V_(110) up to 19. 38%. The irradiation dose of the organs at risk (OARs) increased. For the case of lymphoma, the V_(20) of the lungs in the original plan and the three simulated plans were 31.77%, 32. 11%, 32.60% and 33.26%, respectively. For the case of nasopharyngeal carcinoma, the V_(30) of the right parotid were 48.75%, 49. 56%, 51.65% and 53.91%, respectively. Conclusions With the increase of dose rate in static IMRT , the actual dose distribution deviates the original plan , and the high dose area and the OARs dose increases. The higher dose rate is suboptimal when the dose of the OARs is proximate to the tolerance limit. Key words: Intensity-modulated radiation therapy; Dose rate; Two-dimensional ion chamber array; Dose distribution