Dosimetric analysis of breast cancer tumor bed boost: An interstitial brachytherapy vs. external beam radiation therapy comparison for deeply seated tumors

PurposeTo dosimetrically compare interstitial brachytherapy (MIBT) vs. EBRT (3DCRT and high-energy electron beams) for deep-seated tumor bed boosts (depth ≥4 cm) in early-stage breast cancer.Methods and MaterialsPlanning CTs of fifteen left-side breast cancer patients previously treated with MIBT boost chosen for this study. MIBT, 3DCRT (three-field technique), and enface high-energy electron (15–18 MeV) plans retrospectively generated on these images. To minimize intrapatient target contour inconsistency, due to a technical limitation for transferring identical contours from brachytherapy to EBRT planning system, spherical volumes delineated as hypothetical CTVs (CTV-H) (depth ≥4 cm with considering the geometry of the brachytherapy implant) instead of original lumpectomy cavities (which had irregular contours). In EBRT, PTV-H=CTV-H+5 mm. To account for beam penumbra, additional PTV-H to beam-edge margins added (3DCRT = 5 mm; electron = 10 mm). Included organs at risk (OARs) were ipsilateral breast, skin, ribs, lung, and heart. Prescribed dose-fractionations were 12 Gy/3fractions (MIBT) and 16 Gy/8fractions (EBRT) (BED = 24 Gy, breast cancer Alpha/Beta = 4 Gy). Biologically equivalent DVH parameters for all techniques compared.ResultsMean CTV-H depth was 6 cm. Normal breast V_25%–V_100%; skin V_10%–V_90%; rib V_25%–V_75%; lung V_5%–V_25%; heart V_10%; mean lung dose; ribs/lung D_max were lower in MIBT vs. 3CDRT. MIBT reduced breast V_25%–V_125%; skin V_25%–V_125%; rib V_25%–V_75% and V_100%; lung V_25%–V_90%; heart V_10%–V_50%; skin/ribs/lung D_max compared to electrons. In contrast, breast V_125%–V_250% and V_175%–V_250% were increased in MIBT vs. 3DCRT and electron plans, respectively. Electron plans had the minimum mean heart dose.ConclusionsFrom a dosimetric point of view, in deeply-seated lumpectomy beds, MIBT boost better protects OARs from exposure to medium and high doses of radiation compared to 3DCRT and high energy electron beams (except more ipsilateral breast hot spots).