Radiosensitivity of colorectal cancer to Y and the radiobiological implications for radioembolisation therapy.

Approximately 50% of all colorectal cancer (CRC) patients will develop metastasis to the liver. Y selective internal radiation therapy (SIRT) is an established treatment for metastatic CRC. There is still a fundamental lack of understanding regarding the radiobiology underlying the dose response. This study was designed to determine the radiosensitivity of two CRC cell lines (DLD-1 and HT-29) to Y β radiation exposure, and thus the relative effectiveness of Y SIRT in relation to external beam radiotherapy (EBRT). A Y-source dish was sandwiched between culture dishes to irradiate DLD-1 or HT-29 cells for a period of 6 d. Cell survival was determined by clonogenic assay. Dose absorbed per Y disintegration was calculated using the PENELOPE Monte Carlo code. PENELOPE simulations were benchmarked against relative dose measurements using EBT3 GAFchromic film. Statistical regression based on the linear-quadratic model was used to determine the radiosensitivity parameters [Formula: see text] and [Formula: see text] using R. These results were compared to radiosensitivity parameters determined for 6 MV clinical x-rays and Cs γ-ray exposure. Equivalent dose of EBRT in 2 Gy ([Formula: see text]) and 10 Gy ([Formula: see text]) fractions were derived for Y dose. HT-29 cells were more radioresistant than DLD-1 for all treatment modalities. Radiosensitivity parameters determined for 6 MV x-rays and Cs γ-ray were equivalent for both cell lines. The [Formula: see text] ratio for Y β -particle exposure was over an order of magnitude higher than the other two modalities due to protraction of dose delivery. Consequently, an Y SIRT absorbed dose of 60 Gy equates to an [Formula: see text] of 28.7 and 54.5 Gy and an [Formula: see text] of 17.6 and 19.3 Gy for DLD-1 and HT-29 cell lines, respectively. We derived radiosensitivity parameters for two CRC cell lines exposed to Y β -particles, 6 MV x-rays, and Cs γ-ray irradiation. These radiobiological parameters are critical to understanding the dose response of CRC lesions and ultimately informs the efficacy of Y SIRT relative to other radiation therapy modalities.