ESTRO 2022 - Abstract Book

S1555

Abstract book

ESTRO 2022

Conclusion Daily DWI measurements on the MRL are feasible. Further optimisation of DWI image resolution and contrast is needed to help distinguish tumour from prostate tissue. There was no significant change in ADC between baseline and end of treatment with either conventional radiotherapy or SABR. Limitations included: the low numbers of patients; men treated with neo- adjuvant hormone therapy as well as radiotherapy, which has been shown to reduce changes in pADC due to radiotherapy, and only being able to assess the whole prostate, rather than having ROIs for the tumour, due to current workflow limitations.

PO-1753 Comparison of low distortion methods to calculate ADC in metastatic brain tumours and normal tissue

M. Riveira 1 , A. Lopez Medina 2 , A. Gonzalez Pose 3 , A. Fernandez Gonzalez 3 , F. Salvador Gomez 2 , O.M. Vila Nieto 4 , I. Nieto Regueira 5 , V. Ochagavia Galilea 5 , V.M. Muñoz Garzon 5 1 Galicia Sur Health Research Institute, Medical Physics and RP Department, Vigo, Spain; 2 Hospital do Meixoeiro (GALARIA), Medical Physics and RP Department, Vigo, Spain; 3 Hospital do Meixoeiro (SERGAS), Medical Physics and RP Department, Vigo, Spain; 4 Alvaro Cunqueiro Hospital (SERGAS), Radiology Department, Vigo, Spain; 5 Hospital do Meixoeiro (GALARIA), Radiation Oncology Department, Vigo, Spain Purpose or Objective The Apparent Diffusion Coefficient (ADC) derived from diffusion-weighted imaging (DWI) in MRI for adaptative radiotherapy is a promising method to quantify early tumour response and thus to optimize dose distributions. We aim to compare several methods to obtain the ADC in cerebral structures and brain metastases. Materials and Methods MRI series were performed on a Philips 3T Ingenia magnet. Regions of interest were delineated with ARIA (Varian, v. 15.1) and ADC values were obtained with IntelliSpace Portal (Philips, v. 11.1) according to the monoexponential approach. The mean ADC values of three brain structures (brainstem, ventricles and vitreous humour) from a cohort of four brain tumour patients were calculated from DWI using turbo spin-echo (TSE) (b = 0, 500, 1000 s/mm2) and echo-planar imaging (EPI), the latter with two phase-encoding directions (antero-posterior - AP, and postero-anterior - PA) and applying the reversed gradient (RG) algorithm to reduce geometric distortion. For EPI sequences, b was 0, 50, 100, 200, 500, 1000, 3000 s/mm ² , but we calculated ADC maps with 3 and 7 values (Fig.1). Seven brain metastatic tumours were analysed from the four patients. Using dynamic studies (Ktrans), we differentiated between heterogeneously (n = 2) and homogeneously vascularised (n = 5) tumours, dividing the former into well and poorly vascularised areas, where mean ADC values were calculated separately. Results ADC values from DWI-EPI with 7 b-values differ from those obtained with 3 b-values and DWI-TSE, which are closer to the expected ones (Fig. 1). This is due to the monoexponential approach, since lower and higher b-values are better adjusted to a biexponential curve. When distortion is not corrected, ADC values barely change for large structures (brainstem and ventricles), but it does for smaller ones (eyes), as wider dispersion reflects. RG correction reduces this dispersion. In previous works, we obtained similar results on a specific phantom made of water, ethanol and propanol.