Evaluation of Optimal Combination of Planning Parameters (Field Width, Pitch, and Modulation Factor) in Helical...

The aim of the study was to find the most balanced plan with an optimal combination of planning parameters in helical tomotherapy (HT) for bilateral breast irradiation by evaluating dosimetric indices and time factors. In particular, we investigated the best combination of field width (FW), pitch, and modulation factor (MF).

A total of 90 plans (18 plans for each patient) was created in this study, with different combination of planning parameters (FW: 2.5 cm [F1] and 5 cm [F2]; pitch: 0.215 [P1], 0.287 [P2], and 0.43 [P3]; and MF: 2.0 [M1], 2.5 [M2], and 3.0 [M3]). Plans were analyzed using several dosimetric indices: homogeneity index, conformity index, dose near minimum D_98%, dose near maximum D_2%, and the coverage by D_95% of the target. Organ at risk (OAR) doses were evaluated by mean dose, V_5Gy and V_25Gy for the heart and mean dose V_5Gy and V_20Gy for both the lungs. Treatment time was also reported for all plans.

Reducing FW from 5 cm to 2.5 cm increased the treatment time by 40%-50% and improved homogeneity of the target. Tightening the pitch value from 0.43 to 0.215 improved target as well as OAR doses without increasing the treatment time. Increasing MF from 2 to 3 improved all the dosimetric indices and also increased treatment time.

On the basis of our analysis, a plan with FW 5 cm, pitch 0.215, and MF 2.5 can be considered as an optimal combination of planning parameters for bilateral breast irradiation in HT technique.