Abstract
The purpose of the study was to analyze the effects of the number of intensity levels on treatment planning outcome of static IMRT method with dynamic IMRT method and also to investigate the integral dose to non-target tissues in both the methods. The IMRT planning was carried out using Eclipse treatment planning system with millennium 120 multileaf collimator (Varian Clinac- 2100 DHX). Five cases each of head and neck, cervix and esophagus cancer were selected for this study. For each case, planning was carried out using both delivery methods. Further for the static IMRT, different numbers of intensity levels ranging from 5 to 20 were studied. The optimization values were kept common for both the techniques and only the leaf motion calculation was varied. The parameters associated with the Dose volume histograms were examined for a more quantitative comparison. The integral doses (0.5 Gy to 30 Gy) of Non-target tissues were also calculated for both techniques. Analyses were performed using a t test to determine difference in any of the parameters examined. For three sites studied, there were no significant changes observed between static IMRT (above 10 intensity levels) and dynamic IMRT method. However there were significant differences observed with 5 intensity level static IMRT plans compared to dynamic IMRT plans. There were no significant changes observed in normal tissue dose values between static IMRT plans and dynamic IMRT plans. The total number of monitor unit was more for dynamic IMRT plans compared to static IMRT plans for all three sites. The integral doses from 0.5 Gy to 30 Gy were analyzed and no significant changes were observed between static IMRT and dynamic IMRT plans.
Highlights
Intensity modulated radiation therapy refers to a radiation therapy technique in which nonuniform fluence is delivered to the patient from any given position of the treatment beam to optimize the composite dose distribution
We evaluated dynamic IMRT and static IMRT treatments with 5, 10 and 20 intensity levels in terms of target coverage and homogeneity, dose to OAR and integral dose to NTT
From the results obtained in all three treatments sites: head and neck, cervix and esophagus, it is clear that dynamic IMRT is superior in target coverage and target homogeneity compared to static IMRT plans
Summary
Intensity modulated radiation therapy refers to a radiation therapy technique in which nonuniform fluence is delivered to the patient from any given position of the treatment beam to optimize the composite dose distribution. Depending on the relationship between MLC leaf movements and radiation dose delivery, the delivery can generally be divided into step-and-shoot delivery (static IMRT) and sliding window delivery (dynamic IMRT). The former is the simplest computer controlled delivery scheme of the fixed-gantry IMRT, in which MLC leaf movements and dose deliveries are done at different instances. The simultaneous delivery of dose and leaf movement makes the dynamic mode become more advantageous than static mode, since the delivered intensity profile almost matches the fluence created by treatment planning system (TPS). In the static mode delivery, alternatively, the fluence created by TPS is converted into discrete intensity levels. The static IMRT with increased intensity levels is nearly comparable with dynamic IMRT provided the increased intensity levels makes large number of beam segments with relatively short beam on time [2,3]
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