Kilo-Voltage Imaging Doses to Organs-at-Risk in CBCT-guided Breast Radiation Therapy

Abstract

Recently kilo-voltage cone beam computed tomography (kVCBCT) scans have been widely used in the external beam radiation therapy of breast cancer patients to improve daily setup accuracy so that high dose of tumoricidal radiation can be delivered accurately to the partial volume, as well as to the whole breast. Although early studies investigated the kV imaging doses from kVCBCT scans to critical structures in the pelvis and head regions, the knowledge of dose depositions to the various organs-at-risk (OARs) in thoracic kVCBCT scans of the patient chest is largely missing. Therefore, the goal of this study is to systematically investigate the kV imaging doses to the OARs during CBCT-guided radiation therapy of breast cancer patients. Twenty breast cancer patients in supine orientation were retrospectively assorted into two groups by the lesion site (left vs. right breast). Critical structures such as the heart, the lungs, the normal breast and the thyroid were delineated on the Eclipse treatment planning system based on the planning CT images, which together were subsequently converted into patient CT phantoms for Monte Carlo simulations with an in-house DICOM utility tool. A benchmarked EGS4 Monte Carlo code was used to calculate the 3D dose distributions in the patients scanned by the kVCBCT with a half-fan pelvis protocol pre-defined by the manufacturer. The correlation between the imaging doses received by the OARs and the patient size as well as the lesion site was studied. Furthermore, a similar study was carried out on an adult female anthropomorphic phantom as defined in ICRP report 110 with cortical bone and red bone marrow delineated in the ribs, scapulae, clavicles, thoracic and cervical spines. The mean doses deposited to the heart, the lungs, the normal breast and the thyroid were found to be 3.1, 3.2, 3.3, and 1.5 cGy per CBCT scan, respectively. There were no significant differences observed between the dose depositions to the left and the right lungs, due primarily to the 364° arc rotation defined for the half-fan pelvis scan. A strong correlation was observed between the patient size and the imaging doses, with lower organ doses in the patients of larger sizes. The surface dose for the skin of the breast could reach 7-8 cGy per scan due to proximity to the kV source. The ICRP phantom study indicated that the kVCBCT-contributed mean doses to the cortical bones and the red bone marrows in the thoracic region were 6.1 and 4.0 cGy, respectively for each scan. Compared to the negligible doses (< 0.2% of therapeutic dose except for ipsilateral lung) from the mega-voltage photon beams in breast radiation therapy, each kVCBCT scan contributed 4-10 times more doses to the various OARs on average. Caution should be exercised to avoid multiple kVCBCT acquisitions for each fraction in CBCT-guided breast radiation therapy.