Session Item

Accelerated partial breast irradiation (APBI) represents a valid alternative treatment for selected patients (pts) with early breast cancer (EBC). Aim of our study is to evaluate the reproducibility and tolerability of APBI with volumetric modulated Arc therapy (VMAT) and daily image-guided radiotherapy (IGRT) in a selected subgroup of pts.

From March 2021 to July 2022, we enrolled 63 EBC pts aged > 60 years underwent lumpectomy and sentinel lymph node (SLN) biopsy, with at least 4 titanium clips on the tumor bed. Prevalent histology was ductal and the molecular subtype was Luminal A. T stage was T1-T2, tumor grade was G1-G2 with negative margins and SLN. A computed tomography scan with two-millimeter slice thickness was obtained. The clinical target volume (CTV) was drawn with an isotropic 3-dimensional margin of 1.5 cm around the surgical clips to include radiological abnormalities and seromas, if present, and to exclude the pectoral muscle when possible. The CTV was limited to 3 mm from the skin surface. An isotropic margin of 0.5 cm was added to obtain the planning target volume (PTV). The ipsilateral and contralateral lung, heart, left coronary artery (LAD) for the left breast, and contralateral breast were contoured as organs at risk (OARs). A total dose of 26 Gy in 5 once-daily fractions was prescribed. These constraints were adopted for VMAT optimization: PTV coverage V95% = 95%; maximum dose (Dmax) on PTV < 107%, for ipsilateral lung V10 < 20%, contralateral lung V5 < 10%, heart V3 < 10% for left breast and < 8% for right breast, LAD mean dose (Dmean) ≤ 5 Gy, contralateral breast Dmax < 1 Gy and Dmean ≤ 3 Gy. Daily cone beam CT (CBCT) was performed for pts set-up. Varian's intrafraction motion review ability to automatically detect fiducial markers was utilized using KV images triggered every 60 degrees of gantry rotation during treatment. Marker displacement from expected position was used to evaluate the reliability of CBCT pts positioning. Toxicities were evaluated using Common Terminology Criteria of Adverse Events (CTCAE) version 5.0 scale at the end of RT and 1 month after RT. The first pts treated were also evaluated at 6 months after RT.

All dose constraints were widely respected and PTV coverage was V97% = 97%. The median displacement of the marker was greater for the cross- plane axis affected by respiratory motion but remained at approximately 3 mm. RT was well tolerated. None of the pts reported side effects during RT; during follow-up, grade 1 breast pain occurred in 7 patients, and grade 1 skin induration and hyperpigmentation occurred in 5 patients.

APBI with VMAT technique was well tolerated and had a very low toxicity profile. Intrafraction marker detection showed no significant bias with CBCT imaging and may prevent off-target irradiation when Auto Beam hold is enabled.