Dose accumulation in IMRT for left breast cancer determined by applying deformation registration

To evaluate the dosimetric changes in the target volume and organs at risk (OARs) of patients with left breast cancer (LBC) who underwent intensity-modulated radiation therapy (IMRT) based on a deformation registration (DF) method.

Sixteen patients with LBC treated with 6 MV X-ray IMRT were retrospectively analyzed. All targets included the lymph node drainage area and chest wall. All patients underwent CT for simulation of the primary positioning and repositioning. Primary and secondary treatment plans were developed using primary positioning CT (CT₁) and repositioning CT (CT₂) images to obtain plan₁ and plan₂, respectively. Rigid and DF of the dose distribution of plan₂ to CT₁ were applied; the results were then added to the dose distribution of plan₁, yielding plan_rig and plan_def, respectively. The dosimetric differences between the target and OAR volumes of the four plans were compared.

The clinical target volume of CT₂ was 8.74% less than that of CT₁. The planned target volume of CT₂ was 11.20% less than that of CT₁. The Dice similarity coefficients (DSCs) of the heart, left lung and right lung were significantly lower after than before DF (0.94±0.01 vs. 0.89±0.05, 0.96±0.01 vs. 0.91±0.03, and 0.96±0.01 vs. 0.92±0.03, respectively; t=3.83, 7.28, and 6.70, P<0.05, respectively). There were no significant differences in the dose-volume indices of the heart or left lung between plan₁ and the other plans, while the dose-volume indices were higher in plan_rig than in plan_def.

Because of small changes in the target and OAR volumes during radiotherapy, we suggest the first IMRT plan could be used to evaluate the dose-volume indices of the lungs and heart for these patients.