American Board of Surgery Qualifying Exam (ABS QE) Practice Test

During which phase of the cell cycle is a cell most vulnerable to radiation therapy?

1. G1 phase

2. S phase

3. M phase

4. G2 phase

The correct answer is: M phase

During the M phase of the cell cycle, which includes both mitosis and cytokinesis, the cell is particularly vulnerable to radiation therapy. This vulnerability arises because the M phase is characterized by the highest turnover of DNA and the active separation of chromosomes. Radiation primarily causes damage to DNA, and since the M phase involves the stages where the chromosomal DNA is condensed and aligned for division, any damage inflicted during this phase can lead to significant genomic instability or cell death. Cells in the M phase are less able to repair damage caused by radiation because they are actively dividing. Once radiation inflicts damage, the likelihood of catastrophic outcomes—such as apoptosis or mitotic catastrophe—increases, making it the most critical phase for the efficacy of radiation therapy. While other phases such as G1, S, and G2 do experience some level of sensitivity to radiation, particularly when the cells are replicating their DNA or preparing for division, none have the same level of acute vulnerability seen in the M phase. The S phase has ongoing DNA synthesis, and while it is also significant, it's during mitosis that the consequences of radiation damage can be most pronounced.