The Effect of Lumpectomy Cavity Changes on Planning Dose in Breast Radiotherapy Boost

The addition of a postoperative tumour bed (cavity) boost after whole-breast radiotherapy (RT) is known to decrease the risk of local in-breast recurrence. The purpose of this retrospective study is to assess breast excision cavity changes and planned doses at the time of initial boost RT treatments after the completion of whole-breast RT.

Over a 4-month period, women with highly visible cavity on computed tomography (CT) and treated with whole-breast RT followed by a conformal photon boost to the tumour bed were identified. The patient's day 1 boost online cone beam CT (CBCT) and corresponding setup corrections were overlaid with the planning CT image. The cavity and dose evaluation volumes (DEVs = 1 cm expansion of the cavity, excluding 5 mm of lung and skin) were assessed. Dosimetric evaluations were performed for maximum and minimum cavity doses, cavity volume receiving 90% (V90) and 95% (V95) of prescription dose, and DEV receiving 85% (V85), 95%, and 105% (V105) of the prescription dose. The two-tailed student t-test and Pearson's correlation test were performed for statistical analysis.

Eighteen patients were included in the analysis. The CT cavity (CAV_CT) volume (mean: 28.8 cc, SD 27.9) was larger than the CBCT cavity (CAV_CBCT) volume (mean: 21.1 cc, SD: 22.4) (P value= .003). The CT DEV similarly decreased at the time of boost; DEV_CT (mean: 113.9 cc, SD: 63.2) versus DEV_CBCT (mean: 92.3 cc, SD: 50.8), (P = .002). A strong correlation was observed between the initial CT cavity volume and cavity shrinkage observed at time of boost (day 1 CBCT) (Pearson's r = 0.697; r²= 0.486, P = .001). Both time periods from surgery to CT (median: 92 days, range: 31-227) and from surgery to day 1 CBCT (median: 141 days, range: 56-265) were not associated with cavity volume change. However, an inverse correlation was observed between time from initial CT to day 1 boost CBCT (median: 33 days, range: 24-54) and % cavity volume change (Pearson's r = -0.642; r²= 0.4121, P = .004). Dosimetric evaluations of the cavity and DEV showed no significant differences in maximum, minimum, and target dose coverage between the CT and boost volumes.

In patients with breast cancer treated with a tumour bed boost after whole-breast RT, the postoperative cavity can shrink between the time of CT and day 1 boost. The extent of cavity reduction was greatest in patients with large postoperative cavities on initial CT. Despite the cavity size change, conformal boost fields provide adequate dose coverage to the cavity.