Dosimetric evaluation of the INTRABEAM system for breast intraoperative radiotherapy: A single-institution experience

Breast intraoperative radiotherapy (IORT) with the INTRABEAM system uses a 50 kV x-ray source to deliver a single fraction of radiation therapy to the lumpectomy cavity during breast-conserving surgery. We seek to perform a dosimetric analysis of the lumpectomy cavity for rigid spherical applicators. Water phantom measurements were acquired to validate the vendor-provided x-ray calibration. The planning target volume (PTV) was defined as a 10 mm expansion beyond the spherical applicator, a dose-volume histogram (DVH) was generated and dose-volume parameters [D_min, D_1mm, V₉₀, V₈₀, V₅₀, HI] were reported. Additionally, the therapeutic treatment depth using the 90 and 80% isodose level was computed [R₉₀, R₈₀]. When the percent depth dose (PDD) is normalized to the surface of the applicator, smaller applicators have a steeper PDD. For a prescription dose of 20 Gy to the surface of the applicator, the range of dose-volume parameters for the PTV was: 3.15 to 6.84 Gy for D_min, 16.2 to 17.6 Gy for D_1mm, 2.6 to 6.9% for V₉₀, 5.5 to 15.1% for V₈₀, and 21.1 to 55.6% for V₅₀. For applicators 15 to 50 mm in diameter, the reported values were: 6.35 to 2.9 for HI, 0.53 to 0.85 mm for R₉₀, and 1.18 to 1.85 mm for R₈₀. Smaller applicators have reduced PTV coverage but elevated HI because the attenuation of the beam proximal to the source is more pronounced. Additionally, the presence of the aluminum filter for small applicators (≤30 mm) increases PTV coverage but reduces the dose rate on the applicator surface. The delivery of IORT is performed in the OR without the use of image-based planning. To overcome this limitation, we have generated sample DVH's and report dosimetric parameters to offer clinicians a unique dosimetric perspective.