Background: There is an increasing interest in developing predictive biomarkers of tissue hypoxia using functional imaging for personalised radiotherapy in patients with rectal cancer that are considered for neoadjuvant chemoradiotherapy (CRT). The study explores [

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The purpose of this study was to establish the dose-response relationship of selective internal radiation therapy (SIRT) in patients with metastatic colorectal cancer (mCRC), when informed by radiobiological sensitivity parameters derived from mCRC cell lines exposed to yttrium-90 (90 Y). Methods: 23 mCRC patients with liver metastases refractory to chemotherapy were included. 90 Y bremsstrahlung SPECT images were transformed into dose maps assuming the local dose deposition method. Baseline and follow-up CT scans were segmented to derive liver and tumor volumes. Mean, median, and D 70 (minimum dose to 70% of tumor volume) values determined from dose maps were correlated with change in tumor volume and vRECIST response using linear and logistic regression, respectively. Radiosensitivity parameters determined by clonogenic assays of mCRC cell lines HT-29 and DLD-1 after exposure to 90 Y or external beam radiotherapy (EBRT; 6MV photons) were used in biological effective dose (BED) calculations. Results: Mean administered radioactivity was 1469±428 MBq (847-2185 MBq), achieving a mean radiation absorbed tumor dose of 35.5±9.4 Gy and mean normal liver dose of 26.4±6.8 Gy. A 1.0 Gy increase in mean, median, and D 70 absorbed dose was associated with reduction in tumor volume of 1.8%, 1.8%, and 1.5%, respectively, and increased probability of vRECIST response (odds ratio: 1.09, 1.09, and 1.10 respectively). Threshold mean, median and D 70 doses for response were 48.3, 48.8, and 41.8 Gy respectively. EBRT-equivalent BEDs for 90 Y are up to 50% smaller than those calculated by applying protraction-corrected radiobiological parameters derived from EBRT alone. Conclusion: Dosimetric studies have assumed equivalence between 90 Y SIRT and EBRT, leading to inflation of BED for SIRT and possible under-treatment. Radiobiological parameters for 90 Y were applied to a BED model, providing a calculation method that has the potential to improve assessment of tumor control.

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