Comparison of integral doses to normal tissue and organs at risk between interstitial high-dose-rate brachytherapy and modern external-beam radiotherapy techniques in breast and head and neck cancer patients.

Background: Although a cornerstone of modern cancer treatment, radiotherapy (RT) is associated with a risk of secondary cancer due to irradiation of non-target tissues. Techniques such as intensity-modulated RT (IMRT), volumetric modulated RT (VMAT), and stereotactic body RT (SBRT) provide highly conformal target dose distributions and reduce doses to nearby organs at risk (OARs), albeit at the cost of larger normal tissue volumes being irradiated with lower doses. In brachytherapy (BT), the low-value isodoses cannot be changed: they are a consequence of the inverse-square law. This study evaluates and compares the normal tissue integral doses (NTIDs) delivered by BT and modern external-beam RT (EBRT) techniques in breast and head and neck (H&N) cancer patients.

Methods: Included were the BT and IMRT plans of 34 women with early-stage breast cancer treated with interstitial high-dose-rate (HDR) BT and two groups of head and neck (H&N) patients: 38 patients with mobile tongue, floor of the mouth, and base of the tongue cancer treated definitively with interstitial HDR BT for whom VMAT treatment plans were also prepared and 20 patients with tongue and floor of the mouth tumors who received postoperative interstitial HDR BT for whom VMAT and stereotactic CyberKnife (CK, Accuray Inc., Sunnyvale, CA, USA) plans were also created. The NTIDs for three normal tissue volumes (NT_V10, NT_V5, NT_V2) and OARs were calculated and compared.

Results: Brachytherapy resulted in 39%, 32%, and 26% lower NTIDs compared to IMRT for NT_V10, NT_V5, and NT_V2, respectively, in patients with breast cancer. In H&N cancer, the NTIDs were always lower for BT compared to VMAT. The reductions in NTID achieved with BT were 45%, 36% and 27% with the same planning target volumes in BT and VMAT, and 56%, 48% and 37% with larger planning target volumes in VMAT. For CK, the NTID reductions were 54%, 49% and 41% compared to BT. In breast cases, BT resulted in a significant reduction in ipsilateral lung NTID, and in H&N cases, salivary glands NTIDs were always lower with BT than with VMAT.

Conclusion: For patients with breast and head and neck cancer, interstitial BT results in lower integral dose to normal tissue and OARs compared to modern EBRT techniques. The clinical implications require further detailed investigation.