Rapid repopulation in radiotherapy: a debate on mechanism

The start of chemotherapy until the end of radiotherapy in patients with limited-stage small cell lung cancer

BACKGROUND/AIMS

Chemotherapy combined with radiation therapy is the standard treatment for limited stage small cell lung cancer (LS-SCLC). Although numerous studies indicate that the overall duration of chemoradiotherapy is the most relevant predictor of outcome, the optimal chemotherapy and radiation schedule for LS-SCLC remains controversial. Therefore we analyzed the time from the start of any treatment until the end of radiotherapy (SER) in patients with LS-SCLC.

METHODS

We retrospectively analyzed 29 patients diagnosed histologically with LS-SCLC and divided them into two groups: a short SER group (< 60 days) and a long SER (> 60 days) group. Patients were treated with irinotecan-based chemotherapy and thoracic radiotherapy.

RESULTS

Sixteen patients were in the short SER group and 13 patients were in the long SER group. Short SER significantly prolonged survival rate (p = 0.03) compared with that of long SER. However, no significant differences in side effects were observed.

CONCLUSIONS

Short SER should be considered to improve the outcome of concurrent chemoradiotherapy for LS-SCLC.

Repopulation in the SCCVII squamous cell carcinoma assessed by an in vivo-in vitro excision assay

An in vivo-in vitro excision assay was used to study repopulation after a single dose of clamped irradiation (40 Gy) in the SCCVII tumour implanted in the foot of C3H/Km mice. The growth pattern of clonogenic cells was analysed by two different mathematical models: the logistic model and the Gompertz model. The logistic model described the data better than the Gompertz model. Accelerated repopulation was found when the regrowth rate after irradiation was compared to the growth rate at the time of treatment, and when it was compared to the growth rate in untreated tumours with a number of cells equivalent to the number that was found after irradiation. The clonogenic doubling time (cDT) was estimated at 15.1 h (95% c.i.: 14.2; 16.0) after irradiation, and 27.8 h (95% c.i.: 16.7; 43.5) in untreated controls of matching size. However, the estimate relies on the mathematical model chosen and on extrapolation below actually measured data. A small cDT points to shortening of the cell cycle time and recruitment of non-cycling clonogenic tumour cells to be the main mechanism behind the accelerated repopulation.

Cell loss factors and the linear-quadratic model

A method is described for incorporating a variable cell loss factor (phi) within the linear-quadratic (LQ) model. By allowing for a progressive reduction in phi as treatment progresses, the revised model behaves in a way which is consistent with the apparent presence of accelerated tumour repopulation during fractionated radiotherapy. Predictions based on a slowing of the cell loss process, rather than a true increase in clonogen proliferation rate, are consistent with the phenomenon of 'unmasking' of potential doubling time described by Fowler (Fowler, J.F. Rapid repopulation in radiotherapy: a debate on mechanism. Radiother Oncol 24: 125, 1992). An optional time delay factor may be included, to allow for the fact that progressive reduction in cell loss may not begin until part of the treatment has been delivered. The model provides a description of the manner in which tumour control doses are likely to increase as overall treatment time is increased.

The time-dependent LQ model applied to the matching line of adjacent fields in radiotherapy

The matching volume of adjacent therapeutic fields that are not given concomitantly is subject to a different fractionation schedule than that within the portals. The time-dependent LQ formalism is applied to this problem in general and to the case of Hodgkin's disease in particular. The fields should overlap but this is clinically infeasible.