Can the daily position of bolus material influence radiotherapy treatment?

Daily bolus positioning implies a high degree of variability, which can affect the dose distribution within the planning target volume (PTV) and the organs at risk (OAR). We carried out a retrospective study to evaluate bolus positioning in patients with breast cancer.

We evaluated 7 cases with left and 5 cases with right chest-wall with comprehensive nodal region irradiation in which bolus material was used to obtain better skin surface coverage. The bolus positioning on the daily cone-beam computed tomography (CBCT) images was compared to the reference image from the treatment planning system. Deviations from the reference position of the bolus were categorized as positive shifts (PosS) or negative shifts (NegS), depending on the material's overlapping with its planned position. Subsequently, a second plan was calculated using the information from the CBCT images for comparison with the original treatment plan. We performed a statistical and dosimetric analysis on the results.

For both the 95% dose coverage for the PTV for the chest wall and for the lymph node regions, about 2% variation between initial and recalculated plans was seen, with a shift of the hotspots' position in some cases. The average mean heart dose was 4.1 ± 0.3 Gy, whereas the values for PosS and NegS mean heart doses were 3.8 ± 0.4 Gy and 4.0 ± 0.6 Gy, respectively. In contrast to the original values for the ipsilateral lung V5 (57.1 ± 12.9%), V20 (30.2 ± 2.7%), and Dmean (15.0 ± 1.7 Gy), the values for PosS were 56.1 ± 4.2% for V5:, 30.1 ± 3.3% for V20, and 14.9 ± 1.2 Gy for Dmean while for NegS we obtained 56.9 ± 8.9% for V5, 30.0 ± 2.3% for V20, and 15.2 ± 1.8 Gy for Dmean.

We observed dosimetric differences between the initial and given treatment plans depending on the position of the bolus for all cases, indifferent of the shift direction. Although the differences were not statistically significant, we identified a few specific instances where the variations might cause uncertainties regarding doses to the organs at risk (OAR). We suggest therefore that strategies for correct daily reproducibility of the bolus need to be implemented on a departmental level.