Dosimetric Comparison of HDR Brachytherapy and VMAT for Accelerated Partial Breast Irradiation (APBI)

Abstract

Accelerated partial breast irradiation is an option for some early stage breast cancer patients. The rationale of APBI consists of irradiating only the lumpectomy cavity and a margin that defines a volume where about 95% of the recurrence occurs after surgery. A smaller treated volume means a higher achievable dose per fraction and fewer fractions for a treatment with similar tumor control and toxicity rates. Its utilisation is increasing and a variety of technics can be used. The objective of this study is to compare the dosimetry of APBI plans delivered by high dose rate (HDR) brachytherapy and volumetric modulated arc therapy (VMAT). Twelve patients treated with brachytherapy were retrieved from our database. Eight patients were treated by APBI and received 33 Gy in 9 fractions BID (prescribed at V95≥5 %). Four patients received a boost as part of their treatment and their plans were adjusted to match the APBI prescription. All patients had a pre-implantation CT scan which was used to create VMAT plans following the OPAR protocol. The 30 Gy in 5 fractions prescription dose (prescribed at D100≥95%) was used to yield a biological equivalent dose similar to the HDR prescription. The t-test was used to compare HDR and VMAT dose distributions in terms of Dmax, Dmin, Dmean, and Vdose to targets and organs at risk. Firstly, the cavity’s volumes were similar (VmeanVMAT=17.38±23.95 cc; VmeanHDR=18.40±20.13 cc) but not the CTV (VmeanVMAT=74.45±71.04 cc; VmeanHDR=18.40±20.13 cc). The HDR offers a higher cavity Dmean (DmeanVMAT=30.2±0.4 Gy; DmeanHDR=44.6±3.5 Gy). Small differences (p<0.001) were observed to the conformity index (CIVMAT=1.19±0.07 ; CIHDR=1.12±0.12), the ipsilateral lung V30 (V30VMAT=5.3±3.5 %; V30HDR=1.2±1.5 %) the contralateral breast Dmax (DmaxVMAT=0.622±0.236 Gy; DmaxHDR=0.246±0.163 Gy), and the heart Dmean (DmeanVMAT=0.433±0.193 Gy; DmeanHDR=0.624±0.355 Gy). No statistical difference were found in the dose to the contralateral lung, the contralateral breast Dmean (DmeanVMAT=0.141±0.063 Gy; DmeanHDR=0.047±0.029 Gy), the ipsilateral lung V10 (V10VMAT=12.8±5.6 %; V10HDR=12.7±9.3 %) and the heart Dmax (DmaxVMAT=0.472±0.633 Gy; DmaxHDR=0.324±0.280 Gy). Comparing VMAT and HDR APBI plans is complex due to their intrinsic differences, such as the prescription of the dose. That and the small number of patients included limit the conclusions that can be draw from our data. With that in mind, in this study, HDR brachytherapy allowed a higher dose at the cavity while effectively sparing OAR. Tough, both techniques offer a safe treatment, an adequate dose to the cavity and doses to the OAR well below the constraints, with minor dosimetric differences. Those nuances can guide the choice of a method to use for a specific patient considering some particularities such as the tumor location, the patient’s comorbidities, previous irradiation and the preference of the patient.