Objective: To dosimetrically compare multicatheter interstitial brachytherapy (MIBT) and intensity modulated radiotherapy (IMRT) for accelerated partial breast irradiation (APBI) with special focus on dose to normal tissues and organs at risk (OAR-s).
Material and methods: Thirty-four patients with early stage breast cancer treated with MIBT were selected for the study. For each patient an additional IMRT treatment plan was created using the same CT data and contours as used in MIBT plans. OAR-s included ipsilateral non-target and contralateral breast, lung of both sides, skin, ribs and heart for left sided lesions. The CTV was created from the outlined lumpectomy cavity with a total margin (surgical+radiation) of 20mm in six main directions. The PTV in IMRT plans was generated from CTV with an addition of isotropic 5mm margin. The prescribed dose was 30.1Gy with 7×4.3Gy fractionation for both techniques. From dose-volume histograms quality parameters including volumes receiving a given dose (e.g. V100, V90, V50) and doses to specified volumes (e.g. D0.01cm3, D0.1cm3, D1cm3) were calculated and compared.
Results: Except for high dose, non-target breast received less dose with MIBT. V90 was 3.6% vs. 4.8% and V50 was 13.7% vs. 25.5% for MIBT and IMRT, respectively. Ipsilateral lung was spared better with MIBT. Mean lung dose was 5.1% vs. 7.1%, [Formula: see text] was 39.0% vs. 54.3% and V5 was 32.9% vs. 41.7% in favour of MIBT. For left sided lesions the heart was generally irradiated by larger doses with MIBT. Mean heart dose was 4.5% vs. 2.0% and [Formula: see text] was 18.3% vs. 19.7%, correspondingly. Volumetric maximal skin doses were similar, but regarding dose to 0.1cm3 and 1cm3 of most exposed volume MIBT provided significantly less doses (76.6% vs. 94.4% and 60.2% vs. 87.8%, respectively). Ribs received less dose with MIBT with values of 45.6% vs. 69.3% for [Formula: see text] and 1.4% vs. 4.2cm3 for V50. Dose to contralateral breast and lung was low with both techniques. No significant differences were observed in maximal doses, but dose to volumes of 0.1cm3 and 1cm3 were less with MIBT for both organs. [Formula: see text] was 3.2% vs. 6.7% for breast and 3.7% vs. 5.6% for lung with MIBT and IMRT, respectively.
Conclusions: The target volume can be appropriately irradiated by both techniques, but MIBT generally spares normal tissues and organs at risk better than IMRT. Except for the heart, other critical structures receive less doses with brachytherapy. To observe whether these dosimetric findings translate into clinical outcome more studies are needed with assessment of toxicity profiles.
Keywords: APBI; Dosimetry; IMRT; Multicatheter brachytherapy.
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