P3-179: Isotoxic hypofractionated radical radiotherapy for non-small cell lung cancer (NSCLC)

Abstract

Many patients presenting with localized/locally advanced NSCLC are considered unsuitable for surgery; a significant proportion of these may be suitable for radical radiotherapy (radiotherapy given with curative intent). Clinical data suggest that: 1.There is a dose-response effect with radiotherapy (RT) in NSCLC.2.Shortening overall treatment time (acceleration) may limit tumour repopulation and improve outcomes. Accelerated hypofractionated RT shortens overall treatment time by using fewer, larger fractions compared to conventional RT and the regimen most commonly used in the UK for NSCLC is the hypofractionated schedule of 55Gy in 20 fractions over 4 weeks. Dose escalation can be achieved by keeping fraction number the same and increasing the dose per fraction. This maintains the benefits of acceleration, potentially maximizing the benefit of any increase in total dose. There are three dose-limiting organs at risk (OARs) in radical RT for NSCLC and treatment is designed to keep the dose to these within pre-defined limits; for lungs, mean normalized tissue dose <17Gy (NTDmean), spinal cord <44Gy and oesophagus <58.5Gy. Conventionally, the dose prescribed for all patients in a particular department is the same as long as OAR limits are not exceeded. Doses received by OARs vary greatly between patients depending on the size and position of the tumour. Therefore, in some patients it should be pos-sible to increase the dose prescribed whilst staying within OAR dose limits. In others, where OARs lie adjacent to the tumour within the high dose planning target volume (PTV), dose escalation will be limited. We conducted a retrospective review of 10 consecutive radical RT plans in patients prescribed 50-55Gy in 20 fractions delivered using 3DCRT to assess the feasibility of a prospective isotoxic dose escalation study. Doses were increased until the first OAR dose limit was reached. None of the treatment plans were changed. In 5/10 patients, lung NTDmean was the first dose limit reached, and theoretical prescribed doses ranged from 77-166Gy in 20 fractions. In 2/10 patients oesophagus was the dose limit first reached (theoretical prescribed doses 62Gy and 129Gy). In 1/10 patients, the cord limited the theoretical prescribed dose to 81Gy. 2/10 patients had oesophagus in the PTV, and no escalation was possible. The dose to the heart was insig-nificant in all cases. Individualised dose escalated isotoxic radical RT to pre-defined safe OAR limits may improve outcomes. We will shortly be assessing the feasibility of this concept a prospective study.