However, the presence of the margins around the clinical target accounting for possible infiltrations but most importantly for tumour motion is supposed to reduce the impact of this factor on treatment outcome in SBRT. Instead, one has to be concerned with the fractionation pattern in which the prescribed dose is delivered in order to ensure the clinical gain or at least the equivalence of different combinations between the number of fractions and the dose per fraction. As the positive clinical experience of treating lung and liver cancer patients as well as oligometastatic patients with stereotactic body radiation therapy using only few fractions of high doses is giving confidence in the technique to the clinicians, there is a trend towards reducing the number of fractions even further provided that the toxicity of the normal tissue is not escalated.

Under controlled normal tissue complication probability, a reduction of the number of fractions in which the treatment is delivered appears to be an advantage due to several reasons, not only economical and practical, but also with regard to the radiobiology of the tumour. Three of the classic 4 R’s of radiobiology, Repair, Redistribution and Repopulation, will act in a beneficial manner with respect to tumour control in case of a reduction of the number of fractions. However, the fourth R of tumour radiobiology, Reoxygenation, also has a role to play in the success of the SBRT treatment. If the number of fractions is very low, the chances for a hypoxic tumour to reoxygenate are also low, and therefore the positive outcome of the treatment might be dramatically decreased.

The question though is how efficient is this approach particularly for the case of tumours with hypoxic regions. The optimal number of fractions in which the SBRT has to be delivered must therefore be customised for each individual patient depending not only on the intrinsic sensitivity of the cells to radiation but also on the oxygenation of the tumour. This is therefore an issue of key importance for SBRT which has to be acknowledged as such, thoroughly analysed and evaluated. Thus, the specific aim of this project is to analyse of the impact of the fractionation schedule in which the prescribed dose is delivered in SBRT on the tumour control probability for non-small cell lung cancer (NSCLC) depending on the intrinsic sensitivity of the cells to radiation and the oxygenation of the tumour and to explore the possibilities for the clinical validation of the theoretical findings.

For more information contact Iuliana Toma-Dasu