Beam rate influence on dose distribution and fluence map in IMRT dynamic technique

Abstract

Aim: To examine the impact of beam rate on dose distribution in IMRT plans and then to evaluate agreement of calculated and measured dose distributions for various beam rate values.

Background: Accelerators used in radiotherapy utilize some beam rate modes which can shorten irradiation time and thus reduce ability of patient movement during a treatment session. This aspect should be considered in high conformal dynamic techniques.

Materials and methods: Dose calculation was done for two different beam rates (100 MU/min and 600 MU/min) in an IMRT plan. For both, a comparison of Radiation Planning Index (RPI) and MU was conducted. Secondly, the comparison of optimal fluence maps and corresponding actual fluence maps was done. Next, actual fluence maps were measured and compared with the calculated ones. Gamma index was used for that assessment. Additionally, positions of each leaf of the MLC were controlled by home made software.

Results: Dose distribution obtained for lower beam rates was slightly better than for higher beam rates in terms of target coverage and risk structure protection. Lower numbers of MUs were achieved in 100 MU/min plans than in 600 MU/min plans. Actual fluence maps converted from optimal ones demonstrated more similarity in 100 MU/min plans. Better conformity of the measured maps to the calculated ones was obtained when a lower beam rate was applied. However, these differences were small. No correlation was found between quality of fluence map conversion and leaf motion accuracy.

Conclusion: Execution of dynamic techniques is dependent on beam rate. However, these differences are minor. Analysis shows a slight superiority of a lower beam rate. It does not significantly affect treatment accuracy.

Keywords: Dose rate; Fluence map; Gamma index; IMRT.

Fig. 1

Comparison of average RPI value calculated for plans with 100 MU/min and 600 MU/min beam rate. For most cases average RPI value is higher in 100 MU/min than in 600 MU/min plans, what means better dose distribution in plans with lower beam rate.

Fig. 2

Comparison of number of monitor units (MUs) calculated in 100 MU/min and 600 MU/min plans. For all cases lower MU number is achieved in 100 MU/min plans than in 600 MU/min plans.

Fig. 3

Optimal fluence maps vs. actual fluence maps. Comparison of gamma index calculated for plans with 100 MU/min and 600 MU/min beam rate. Lower gamma index value indicates better congruity of optimal and actual fluence maps. For all cases lower gamma index for 100 MU/min was achieved.

Fig. 4

Calculated vs. measured actual fluence maps. Comparison of gamma index (GI₊₁) calculated for 100 MU/min and 600 MU/min in area limited to treatment field plus 1 cm. Lower gamma index value indicates better congruity of calculated and measured actual fluence maps. For all cases lower value of GI₊₁ for 100 MU/min was obtained.

Fig. 5

Comparison of summary leaves position inaccuracy detected for 100 MU/min and 600 MU/min beam rates. Leaves position error is expressed by sum of integrals calculated for root mean square curve and named relative integral. For all cases bigger error occurs in 600 MU/min plans.

Fig. 6

Changes of average Radiation Planning Index (RPI) with the beam rate.

Fig. 7

Changes of total number of monitor units (MUs) per treatment plan with the beam rate.

Fig. 8

Dependence between inaccuracy of map conversion represented by gamma index calculated in OmniPro-I’mRT software and treatment execution represented by GI₊₁ calculated in Portal Dosimetry option for (A) 100 MU/min and (B) 600 MU/min beam rates. The dashed line indicates 95% confidence range (Statistica v.8).

Fig. 9

Dependence between inaccuracy of map conversion represented by gamma index calculated in OmniPro-I’mRT software and treatment execution represented by leaves motion precision, calculated in MLCtracker software for (A) 100 MU/min and (B) 600 MU/min beam rates. The dashed line indicates 95% confidence range (Statistica v.8).