We propose a novel mechanism by which cancer cells can modulate the oxygen concentration within the nucleus, potentially creating low nuclear oxygen conditions without the need of an hypoxic micro-environment and suited for allowing cancer cells to resist chemo- and radio-therapy. The cells ability to alter intra-cellular oxygen conditions depends on the amount of cholesterol present within the cellular membranes, where high levels of cholesterol can yield rigid membranes that slow oxygen diffusion. The proposed mechanism centers on the competition between (1) the diffusion of oxygen within the cell and across cellular membranes that replenishes any consumed oxygen and (2) the consumption of oxygen in the mitochondria, peroxisomes, endoplasmic reticulum (ER), etc. The novelty of our work centers around the assumption that the cholesterol content of a membrane can affect the oxygen diffusion across the membrane, reducing the cell ability to replenish the oxygen consumed within the cell. For these conditions, the effective diffusion rate of oxygen becomes of the same order as the oxygen consumption rate, allowing the cell to reduce the oxygen concentration of the nucleus, with implications to the Warburg Effect. The cellular and nucleus oxygen content is indirectly evaluated experimentally for bladder (T24) cancer cells and during the cell cycle, where the cells are initially synchronized using hydroxeaurea (HU) at the late G1-phase/early S-phase. The analysis of cellular and nucleus oxygen concentration during cell cycle is performed via (i) RT-qPCR gene analysis of hypoxia inducible transcription factors (HIF) and prolyl hydroxylases (PHD) and (ii) radiation clonogenic assay every 2 h, after release from synchronization. The HIF/PHD genes allowed us to correlate cellular oxygen with oxygen concentration in the nucleus that is obtained from the cells radiation response, where the amount DNA damage due to radiation is directly related to the amount of oxygen present in the nucleus. We demonstrate that during the S-phase cells can become hypoxic in the late S-phase/early G2-phase and therefore the radiation resistance increases 2- to 3-fold.

We present a rigorously defined dosimetric Monte Carlo (MC) model of the MultiRad225 kilovoltage X-ray irradiator, which is validated using experimental techniques. Previous MC studies performed with the MultiRad225 lacked rigorous dosimetric validation. Experimental measurements are conducted with an ionization chamber and radiochromic film with beam energies of 119, 160, and 200 kV. MC simulations are conducted mimicking each experiment. Both beam quality (half-value layer, HVL) and quantity (dose rate) are assessed with these methods. MC simulated dose rates and HVL values show close agreement with experimental results across varying source-to-surface distances (SSD) for each beam energy with most dose rate simulations falling within 5% of the experimentally measured values. This work provides a validated MC model as a foundation for future studies with the MultiRad225. The MC model utilizes a comprehensive geometry which accurately captures relevant physical phenomena.