Does the protocol-required uniform margin around the CTV adequately account for setup inaccuracies in whole breast irradiation?

PURPOSE: To use cone-beam computed tomography (CBCT) imaging to determine the impacts of patient characteristics on the magnitude of geometric setup errors and obtain patient-specific planning target volume (PTV) margins from the correlated patient characteristics in whole breast irradiation (WBI).

METHODS

Between January 2019 and December 2019, a total of 97 patients who underwent breast-conserving surgery, followed by intensity-modulated radiation therapy in WBI, were scanned with pre-treatment CBCT for the first three treatment fractions and weekly for the subsequent fractions. Setup errors in the left-right (LR), superior-inferior (SI) and anterior-posterior (AP) directions were recorded and analyzed with patient characteristics-including age, tumor location, body mass index (BMI), chest circumference (CC) and breast volume (BV)-to examine the predictors for setup errors and obtain specific PTV margins.

RESULTS

A total of 679 CBCT images from 97 patients were acquired for analysis. The mean setup errors for the whole group were 2.32 ± 1.21 mm, 3.71 ± 2.21 mm and 2.75 ± 1.56 mm in the LR, SI and AP directions, respectively. Patients' BMI, CC and BV were moderately associated with setup errors, especially in the SI directions (R = 0.40, 0.43 and 0.22, respectively). Setup errors in the SI directions for patients with BMI > 23.8 kg/m2, CC > 89 cm and BV > 657 cm3 were 4.56 ± 2.59 mm, 4.77 ± 2.42 mm and 4.30 ± 2.43 mm, respectively, which were significantly greater than those of patients with BMI ≤ 23.8 kg/m2, CC ≤ 89 cm and BV ≤ 657 cm3 (P < 0.05). Correspondingly, the calculated PTV margins in patients with BMI > 23.8 kg/m2, CC > 89 cm and BV > 657 cm3 were 4.25/7.95/4.93 mm, 4.37/7.66/5.24 mm and 4.22/7.54/5.29 mm in the LR/SI/AP directions, respectively, compared with 3.64/4.64/5.09 mm, 3.31/4.50/4.82 mm and 3.29/5.74/4.73 mm for BMI ≤ 23.8 kg/m2, CC ≤ 89 cm and BV ≤ 657 cm3, respectively.

CONCLUSIONS

The magnitude of geometric setup errors was moderately correlated with BMI, CC and BV. It was recommended to set patient-specific PTV margins according to patient characteristics in the absence of daily image-guided treatment setup.