Brandon Disher, Presenter: Nothing to Disclose

George Hajdok, Abstract Co-Author: Nothing to Disclose

Stewart Gaede, Abstract Co-Author: Nothing to Disclose

Matthew Mulligan, Abstract Co-Author: Nothing to Disclose

Jerry J. Battista, Abstract Co-Author: Nothing to Disclose

Stereotactic Body Radiation Therapy (SBRT) is a treatment option for inoperable early stage lung cancer patients. SBRT uses tightly conformed megavoltage (MV) x-ray beams to ablate the tumour in only a few treatment sessions. Small MV x-ray fields may cause lateral electron disequilibrium (LED) to occur within lung tissue, which can reduce the dose to the tumour to a variable extent. These dose effects may be challenging to predict using commercially-available dose calculation algorithms. Thus, to avoid LED, previous authors suggested using low energy photons and larger fields for lung cancer patients. We propose a new form of SBRT, named LED-optimized SBRT (LED-SBRT), which utilizes RT parameters designed to exploit LED to advantage. It will be shown that LED-SBRT can be used to reduce the dose within healthy lung tissue while enabling escalation of tumour dose levels. 

The DOSXYZnrc Monte Carlo software was used to calculate dose within a typical SBRT patient. To mimic a 360o SBRT arc, 36 equally weighted fields were focused onto the small tumour (~1 cm). 6 or 18 MV x-ray energies were used to simulate different plans of various field sizes. The LED-optimized plan, 18MV(3x1cm2), was compared to a clinical standard arc using 6MV(3x3cm2) beam parameters. A planning target volume (PTV) was generated by considering the extent of tumour motion over the patient’s breathing cycle. All dose results were normalized such that at least 95% of the PTV received at least 54 Gy (i.e. D95). 

The LED-optimized plan produced a ‘hot spot’ at the tumour center equal to 169Gy, which was approximately twice as large as the maximal dose found within the conventional plan. Despite escalated tumour dose levels, normal lung dose was still decreased. For example, the mean lung dose and V20 decreased by ~ 0.5 Gy and 1.1%, respectively, comparing the LED-optimized plan to the clinical standard.

This article introduces a novel SBRT technique, LED-SBRT, which exploits the LED phenomenon to reduce normal lung dose levels and permit tumour dose escalation.  

These results are significant as tumour dose escalation correlates well with tumour control (and overall survival), while lower lung dose metrics reduce the risk of lung toxicities (e.g. pneumonitis).

Disher, B, Hajdok, G, Gaede, S, Mulligan, M, Battista, J, Forcing Lateral Electron Disequilibrium to Spare Lung Tissue: A Novel Technique for Stereotactic Body Radiation Therapy of Lung Cancer.  Radiological Society of North America 2013 Scientific Assembly and Annual Meeting, December 1 - December 6, 2013 ,Chicago IL. http://archive.rsna.org/2013/13020207.html