B83: How is good and poor response to neoadjuvant therapy defined using histological and MRI regression scales in rectal cancer studies with reference to outcomes.

Muhammed Siddiqui1,2,Jemma Bhoday1,2,Nicholas Battersby1,Al-Mutaz Abulafi2,Paris Tekkis3,Gina Brown0

1Royal Marsden Hospital, Surrey, UK,2Croydon university Hospital, Surrey, UK,3Royal Marsden Hospital, London, UK

Presenting date: Tuesday 3 November
Presenting time: 12.20-13.10

Background

There is a range of histological and one MRI regression scales used in clinical studies to define good and poor response to neoadjuvant therapy for rectal cancer. Our hypothesis is that there is no single histological regression scale which distinguishes between good and poor responders. Our secondary hypothesis is that MRI regression scale effectively differentiates.

Method

All clinical studies that reported a histological or MRI regression scale were chosen for inclusion. Studies that used these regression scales to define good and poor responders were included for analysis in relation to outcomes.

Results

There are 19 histological and 1 MRI tumour regression scales for rectal cancer. The histological scales are used in different combinations producing 16 and 12 different definitions of poor and good response respectively.  The MRI  scale used one definition for poor response and 2 for good. The most common histological definition for good and poor response was the Mandard 1&2 and TRG 4&5. Using histologically defined poor response,, 5 year DFS ranged from 56% to 71% and 5-year OS was 60.7-75.8% compared to good response, 5 year DFS ranged from 78% to 90% and 5-year OS was 89.2-92.2%. MRI defined poor responders at 5 years had a DFS of 31-68% and OS of 27-68% compared to MRI defined good responders who at 5 years had a DFS of 64-83% and OS of 72-90%.

Conclusion

A range of histological TRG scales are used in clinical and pathological studies. Good and poor response is heterogeneously described, even when using the same histological regression scales. Current pathological TRG data are not distinguishing between types of response as reflected by survival outcomes of poor responders being quite high. A need for standardization, revision or adoption of complementary grades such as the MRI TRG scale is required to define poor response.