R7.3: Repopulation

This is one of the topics that I think Joiner and van der Kogel cover well (chapter 11). A good reference is:
Wolfgang Dorr, Christopher S. Hamilton, Teresa Boyd, Barry Reed, James W. Denham, Radiation-induced changes in cellularity and proliferation in human oral mucosa, International Journal of Radiation Oncology*Biology*Physics, Volume 52, Issue 4, 15 March 2002

Importance and Timing of Repopulation

Repopulation is the regrowth of normal and malignant cells that occurs during fractionation of radiotherapy, counteracting the cell loss caused by ongoing treatment. In some cases, repopulation can exceed the cell kill induced by radiation, leading to improvement of early effects during treatment or, alternatively, inability to cure a malignancy.
Repopulation does not begin immediately after radiation exposure, and is tissue dependent. In the mucosa of the head and neck in mice, repopulation begins after about 1 week.

Most information on repopulation has been derived from studies of oral mucosa and squamous cell carcinomas of the head and neck region.

Radiation Effects on the Oropharyngeal Mucosa

The oropharynx is lined with a thin, non-keratinising stratified squamous epithelium. The basal layer of cells contains the stem cells of the epithlium, which progressively mature as they ascend towards the surface. The basal layer rests on the basement membrane, below which are the supportive lamina propria containing vessels, nerves and connective tissues.

Maintenance of oropharyngeal mucosa

The time taken for newly created parenchymal cells to ascend from the basement membrane to the surface to be lost through shedding is about 5 days. Germinal cells have a cell cycle time of about 3.5 days.

Response of oropharyngeal mucosa to radiation

Radiation affects the basal stem cell population significantly, with differentiated cells suffering minimal impact. The loss of proliferative capacity leads a reduction in the cells of the mucosal layer, which may lead to mucousitis (complete loss of the mucosal layer).
The EORTC and RTOG scoring scale for acute oropharyngeal reactions indicates the severity of the reaction:

Grade Description ~ Onset after conventional fractionation
0 No observable change, no pain N/A
1 Injected, not requiring analgesia for pain control 6 - 9 days
2 Patchy mucousitis requiring analgesia 9 - 16 days
3 Confluent mucousitis requiring admission or narcotic analgesia 11 - 23 days
4 Ulceration, haemorrhage or necrosis N/A
5 Death due to side effects N/A

Development and resolution of early effects in the oropharynx

Development of early effects

Early effects occur due to depletion of the stem cell component of the oropharyngeal mucosa. Differentiated cells continue to ascend through the mucosa and are shed from the surface, but are not replaced in sufficient quantities by the basal cells. This leads to loss of the protective functions of the mucosal layer with responding inflammation, known as mucousitis. The extent of mucousitis varies from small patches to confluent.
As demonstrated in the table above, there is a time lag until early effects become apparent. This depends on the grade of effect being observed and the rate of dose delivery, but is in the order of 6 - 9 days for grade 1 reactions and about two weeks for grade 2 reactions. This reflects the gradual loss of cells from the stem cell compartment but the ongoing survival of the differentiating parenchymal cells.

Recovery from early effects

During the latter parts of a conventionally fractionated treatment, mucousitis may begin to heal despite ongoing treatment. This is due to repopulation - an increase in the number of dividing cells to compensate for loss of stem cells and tissue hypoplasia. Recovery is not seen in more rapidly delivered treatments (such as CHART or in EORTC 22851) until treatment is completed, as repopulation does not begin for some time after treatment has begun.

Cellular responses following radiation

A single week of conventional fractionation to the oropharyngeal mucosa results in death of 90% of stem cells. The remaining 10% of stem cells must then try to maintain the mucosa, an impossible task using their normal speed of cell cycling.
Observations have shown that several processes are responsible for the recovery of normal tissue, including the ability of the mucosa to completely compensate for radiation exposure.

  • Loss of stem cells leads to loss of assymetrical divisions of surviving stem cells. Stem cells, instead of producing a new stem cell and a differentiating cell, produce two new stem cells. This allows the stem cells to proliferate more rapidly.
  • Tissue hypoplasia leads to a reduction in the cell cycle time of stem cells, to about 1.4 days (from 3.5).
  • Stem cells which have received lethal damage may still be able to undergo several divisions to produce differentiated cells.

The proliferative layer of the mucosa, which is normally just the basal layer, becomes larger as more cells become involved in mitosis.

Clinical Implications of Oropharyngeal Repopulation

The exact mechanisms behind loss of stem cells and subsequent repopulation of normal tissues are not fully understood, but have been studied extensively in the oropharynx of humans and other mammals. The primary clinical implications in oropharyngeal radiotherapy:

  • Conventionally fractionated radiotherapy will usually result in rapid development (over 1 - 3 weeks) of significant mucousitis, with difficulties swallowing and pain. Once developed, it is unlikely that these reactions will progress to grade IV in most patients due to repopulation of the mucosal cells. Some patients may begin healing from their mucousitis during radiotherapy.
  • Accelerated radiation will result in more rapid development of mucousitis (1 - 2 weeks). These changes are unlikely to resolve by the end of treatment as repopulation takes several weeks to reach a maximum effect. The increased killing of stem cells by accelerated treatment may result in extended time for tissue healing as well as an increased risk of consequential late effects.

Repopulation of Tumours

Tumours may enter accelerated repopulation in response to radiation or other stimuli (possibly surgery and chemotherapy). The growth fraction increases, the cell cycle time decreases, and the cell loss factor decreases; this leads to a more rapid tumour doubling time (both potential and actual). Accelerated repopulation was demonstrated well by Withers et al in 1975; tumour control required increasing dose if treatment time extended beyond 4 weeks.
Accelerated repopulation is thought to exist for squamous cell carcinomas of the head and neck mucosa, as well as of the cervix. Although lengthening treatment time is one method of reducing normal tissue effects, this runs the risk of reducing tumour control.