ESTRO 2024 - Abstract Book

S1548

Clinical - Lower GI

ESTRO 2024

In this thus study the aim was to examine whether imaging parameters derived from diffusion weighted (DW) MR and FDG-PET images can be used to select patients that are not cured by todays treatment and therefor are candidates for treatment intensification.

Material/Methods:

A total of 48 anal cancer patients (T-stage 2-4) recruited to the Anal Cancer Radiotherapy study (ANCARAD, NCT01937780) were included in this study. Tumours were delivered a total dose of 54-58 Gy in 2 Gy fractions. Patients underwent medical imaging including MR and FDG-PET prior to treatment (MR n=48, PET n=45), and after receiving 20 Gy (MR n=27, PET n=20). The MR imaging protocol included multi b-value DW-MR and from these the parameters ADC, reflecting cell density, and fBV, reflecting blood volume, were derived using the simplified IVIM model. The entire tumour was delineated in the DW images and median ADC and fBV as well as tumour volume was extracted for each patient. From FDG-PET images the standardised uptake value (SUV) was calculated for each pixel in the tumour, and median values were calculated and used for further analysis. The potential of imaging and clinical parameters to predict treatment outcome was tested by performing a t-test between the parameters in patients with and without relapse after 4 years. 8 patients presented with relapse during the 4 years follow up. 39 of the tumours were found to be HPV related. None of the clinical parameters T-stage, lymph node status, HPV status or longest axis were significantly associated with outcome (p>0.05). MR and PET were mostly unrelated to clinical parameters except for an association between high SUV and lymph node positivity and larger diameter (p<0.05). Furthermore, higher tumour volume was associated with higher T-stage, longer diameter and positive lymph node status (p<0.05). DW MR and PET parameters were largely unrelated, with only a weak correlation between median ADC and median SUV (P<0.01, R=-0.6). None of the imaging parameters obtained at baseline provided information on patient outcome. After 20 Gy most patients observed a reduction in tumour volume (p<0.001) and SUV (p<0.001) and an increase in ADC (p<0.001) and fBV (p<0.05). These responses were heterogeneous between patients, and a large increase in ADC and large decrease in tumour volume were both associated with patient outcome (p<0.01). Furthermore, by combining information on relative change in ADC and change in tumour volume a stronger outcome prediction was achieved (p<0.001) indicating that they provide independent information. HPV related tumours had a smaller volume reduction and smaller increase in ADC than HPV unrelated tumours indicating a poorer response to radiation therapy. This is in line with previous findings that HPV negative tumours are more radioresistant than HPV positive tumours. Results:

Conclusion:

Image parameters obtained prior to treatment did not reflect treatment outcome in this anal cancer cohort. A possible explanation for this may be due to the heterogeneous patient population including both HPV positive and negative tumours which often presents with very different intrinsic radiation resistance. Relative change in ADC and tumour volume early into the treatment were both associated with outcome. The combination of these two parameters may be a promising marker of radioresistance early into treatment and may provide a window of opportunity for dose escalation in patients with radioresistant tumours.

Keywords: DW-MRI, FDG-PET, radioresistance