Improving geometric sparing and therapeutic effectiveness of lung SBRT for central and ultra-central tumors

Hatim Fakir; Gurpreet K Randhawa; Barbara Millman; Joanna Laba

doi:10.1016/j.meddos.2021.05.004

Improving geometric sparing and therapeutic effectiveness of lung SBRT for central and ultra-central tumors

Med Dosim. 2021;46(4):398-403. doi: 10.1016/j.meddos.2021.05.004. Epub 2021 Jun 23.

Authors

Hatim Fakir¹, Gurpreet K Randhawa², Barbara Millman³, Joanna Laba⁴

Affiliations

¹ London Health Sciences Centre, London, ON, Canada; Department of Medical Biophysics, Western University, London, ON, Canada; Department of Oncology, Western University, London, ON, Canada. Electronic address: hatim.fakir@lhsc.on.ca.
² London Health Sciences Centre, London, ON, Canada; Gurpreet Randhawa current address: University of Waterloo, ON, Canada.
³ London Health Sciences Centre, London, ON, Canada.
⁴ London Health Sciences Centre, London, ON, Canada; Department of Oncology, Western University, London, ON, Canada.

PMID: 34172369
DOI: 10.1016/j.meddos.2021.05.004

Abstract

The use of stereotactic body radiotherapy (SBRT) for central- and ultra-central lung tumors is a major therapeutic challenge since there are trade-offs between delivering adequate dose to the tumor and minimizing toxicity to critical mediastinal organs. This work investigates improving the therapeutic effectiveness of such SBRT treatments by enhancing the geometric sparing of normal tissue and systematically applying a planning target volume (PTV) margin smaller than the conventional values. Using plans from 10 previously SBRT-treated patients, we retrospectively created highly conformal plans with a reduced PTV margin of 2 mm and compared them to the clinical plans with a standard 5 mm PTV margin. We compared various dosimetric and biological parameters. We calculated the geometrical sparing factor (GSF) (ratio of biological dose between normal tissue and targets) for the mediastinal organs and the uncomplicated tumor control probability (UTCP) for the esophagus. We tracked tumor fraction doses using cone-beam computed tomography (CBCT) images. With geometric sparing, the median dose for critical mediastinal organs (proximal bronchial tree, great vessels, esophagus, and heart) dropped by 10 Gy (p ≤ 0.006). Dose sparing for the spinal cord and chest wall was 5 Gy and 8 Gy, respectively (p = 0.002). The geometrical sparing factor (GSF) dropped by 50% for the esophagus and the proximal bronchial tree (PBT) and 40% for the great vessels (p < 0.05). The CBCT fractional tumor dose varied by 2.7% (0.2 Gy) for the initially intended treatment volume and 4% (0.3 Gy) when accounting for daily volume changes. The expected delivered dose was above the prescribed value. Systematically reducing the PTV margin to 2 mm in lung SBRT of central and ultra-central tumors is feasible and ensures consistency in contouring and dose prescribing. It allows safe delivery of highly conformal treatments with significantly higher therapeutic effectiveness, potentially reducing treatment-related complications. Consequently, it may enable safer dose escalation, more effective fractionations, and safer management of retreatments and treatments of multiple synchronous lung tumors.

Keywords: Biologically effective dose; Central tumor; Lung cancer; Stereotactic Body Radiation Therapy (SBRT); Stereotactic radiotherapy; Ultra-central tumor.

MeSH terms

Humans
Lung
Lung Neoplasms* / radiotherapy
Lung Neoplasms* / surgery
Radiosurgery*
Radiotherapy Dosage
Radiotherapy Planning, Computer-Assisted
Radiotherapy, Intensity-Modulated*
Retrospective Studies