Dosimetric study of optimum treatment mode applied in intensity-modulated radiotherapy for postoperative rectal cancer

Objective  To explore dosimetric characteristics of multivariate combination optimal treatment model such as treatment positions (supine/prone position), collimator intensification modemultiple static segments (MSS) and sliding window (SW), dose calculation algorithmAnisotropic Analytical Algorithm (AAA)and Pencil Beam Convolution (PBC), high energy X-ray (6MV and 15MV), the number of radiation fields (7 fields versus 9 fields) and the size of calculation grids (0.25 cm versus 0.50 cm) during postoperative intensity modulated radiotherapy for rectal cancer.

Methods  Controlled single variable was applied to compare the dosimetric differences of these six conditions (a total of 12 variables) on target volume and most critical organs at risk. Then, based on the above results, the dosimetric effects on planning target volume and most critical organ at risk were compared between group A which was composed up of relatively superior six variables and group B which was composed up of relatively inferior six variables. The dosimetric parameters included the dose distribution of planning target volume and most critical organs at risk, conformal index (CI), homogeneity index (HI), monitor units (MU) and beam-on time.

Results  Based on the statistical results under the influence of multiple variables, group A with prone position, AAA dose algorithm, dynamic SW multileaf collimators (MLC) motion mode, 15 MV X-ray, mesh size calculated by 0.25 cm and 9 field equipartition of 6 variables in series had 1.2% of decrease in mean dosage (D_mean), 10.0% of increase in CI, and 30.3% of decrease in planning target volume (PTV), while 3.0% of decrease in D_max of small intestine, 31.2% of decrease in dosage for bladder volume of 4 000 cm³ (V₄₀), and 3.6% of decrease in femoral head in maximum dosage (D_max) when compared with group B with supine position, PBC algorithm, MSS motion mode, 6MV X-ray, 0.50 cm calculation grids and 7 fields equipartition.

Conclusion  The treatment mode of prone position, AAA algorithm, MLC motion mode of dynamic SW, 15MV X-ray, 0.25 cm calculation grids and 9 fields is supposed to be chosen as a priority for post-operative rectal cancer of intensity-modulated radiotherapy technique.