Radiation therapy (RT) uses high-energy beams to damage the DNA of cancer cells, shrinking tumors and preventing cell division. RT is delivered in a fractional manner, meaning the total dose is divided into small daily doses over several weeks. The precise scheduling is a deliberate part of the treatment plan, but interruptions can occur, which is a major concern for patients and care teams.
Reasons Why Treatment Might Pause
Radiation therapy interruptions stem from either patient-centric issues or logistical difficulties within the facility. Patient-related reasons often involve managing severe side effects that develop acutely during treatment. For example, patients may experience significant skin reactions (dermatitis) or inflammation of the mouth and throat (mucositis), especially when undergoing head and neck cancers treatment.
When these adverse reactions become too severe, a temporary break is required to allow healthy tissues time to heal before the next dose can be safely delivered. Other patient-related causes include concurrent illnesses, such as the flu or an infection, or a general deterioration of the patient’s performance status. Patients may also miss sessions due to personal reasons or unexpected scheduling conflicts.
On the technical and logistical side, interruptions frequently occur due to issues outside the patient’s control. These include equipment malfunction or the need for scheduled maintenance of the linear accelerator (LINAC). One study found that equipment maintenance was the most frequent cause of interruption, accounting for over half of all pauses in some centers. Holidays or other unforeseen scheduling difficulties also contribute to unplanned gaps.
The Biological Impact of Treatment Pauses
The fractional delivery of radiation is based on biological principles, known as the “Four Rs” of radiobiology. This strategy aims to maximize tumor cell death while allowing normal cells to recover. Healthy cells possess a greater ability to repair sublethal damage in the time between fractions, allowing the total prescribed dose to be safely delivered.
The primary concern with any unplanned break is tumor repopulation, which is the rapid proliferation of surviving cancer cells during the treatment interval. When radiation stops, cancer cells that survived previous fractions begin to divide quickly, making subsequent doses less effective. This repopulation accelerates after a lag period, which is why extending the overall treatment time (OTT) is avoided.
For certain aggressive cancers, particularly squamous cell carcinomas of the head and neck, the consequences of an interruption are substantial. A prolonged OTT is strongly correlated with a decrease in the probability of local control for these cancers. Studies suggest that in head and neck or cervical cancers, each day of prolongation can result in an absolute decrease in local control of around one percent. Even short breaks can compromise the final curative outcome.
Compensating for Missed Sessions
When a treatment interruption occurs, the oncology team must quickly develop a strategy to counteract tumor repopulation and maintain the planned total dose effectiveness. The goal of any compensation method is to minimize the extension of the overall treatment time. The most preferred method is often to treat on days that were not originally scheduled, such as adding treatments on a Saturday or Sunday, to quickly close the gap.
Another common strategy is to employ accelerated fractionation for the remaining course. This involves delivering the remaining fractions in a shorter time frame, such as administering two fractions on a single day, while maintaining the original dose per fraction. When using two fractions in one day, a mandatory minimum gap, usually six hours, is required between the two sessions for normal tissue repair.
In some scenarios, if the break is prolonged and significant tumor repopulation is suspected, the remaining dose per fraction might be slightly increased. This dose escalation is calculated carefully using radiobiological models. The goal is to ensure the increased dose damages the tumor without significantly increasing the risk of late side effects. The care team notes that not all short pauses require aggressive compensation, especially in palliative settings or for tumors with slow growth rates.