Lack of predictive value of potential doubling time and iododeoxyuridine labelling index in radiotherapy of squamous cell carcinoma of the head and neck

Where Do We Look for Markers of Radiotherapy Fraction Size Sensitivity?

The response of human normal tissues to radiotherapy fraction size is often described in terms of cellular recovery, but the causal links between cellular and tissue responses to ionising radiation are not necessarily straightforward. This article reviews the evidence for a cellular basis to clinical fractionation sensitivity in normal tissues and discusses the significance of a long-established inverse association between fractionation sensitivity and proliferative indices. Molecular mechanisms of fractionation sensitivity involving DNA damage repair and cell cycle control are proposed that will probably require modification before being applicable to human cancer. The article concludes by discussing the kind of correlative research needed to test for and validate predictive biomarkers of tumour fractionation sensitivity.

Importance of tumor volume in supraglottic and glottic laryngeal carcinoma

PURPOSE

The aim of our study was to assess the prognostic value of tumor volume compared to and in addition to T-stage on local control (LC), disease-free survival (DFS), and overall survival (OS) in glottic and supraglottic laryngeal carcinoma patients.

PATIENTS AND METHODS

In 150 patients, we determined tumor volume on diagnostic CT scans. We applied crude and multivariable Cox regression analysis to relate volume (continuous), T-stage and the combination to 5-year DFS, OS, and LC. Before, we examined the linearity assumption of the association between volume and outcome with restricted cubic spline functions. Prognostic performance of the models was examined by determination of the model's discrimination. Discriminative ability was determined with the C statistic referring to the ability to discriminate between patients who do and do not develop an event during follow-up.

RESULTS

A strong association between tumor volume and DFS and OS was found. The restricted cubic spline plot did not indicate a non-linear relationship between tumor volume and DFS and local control. Tumor volume demonstrated a better discriminative ability to predict DFS and OS compared to T-stage (0.68 and 0.57 vs. 0.59 and 0.54, respectively). For local control, T-stage showed a higher discriminative ability than tumor volume (0.63 vs. 0.61). The combined model increased discriminative power (0.69).

CONCLUSION

Volume seems to be more important than T-stage in prediction of DFS or OS in laryngeal squamous cell carcinoma patients. Perhaps prediction of DFS, OS, and LC could be improved by including tumor volume into the staging process.

Radiotherapy treatment delays and their influence on tumour control achieved by various fractionation schedules

There is often a considerable delay from initial tumour diagnosis to the start of radiotherapy treatment. This paper extends the calculations of a previous paper on the effects of delays before the initiation of radiotherapy treatment to include results from a variety of practical fractionation regimes for three different types of tumour: squamous cell carcinoma (head and neck), breast and prostate. The linear quadratic model of radiation effect, logarithmic tumour growth (coupled with delay times where relevant) and the Poisson model for tumour control probability (TCP) are used to calculate the change in TCP for delays between diagnosis and treatment. Within the limitations of radiobiological modelling, these data can be used to tentatively assess the interactions between delays, dose fractionation and TCP. The results show that delays in the start of radiotherapy treatment do have an adverse effect on tumour control for fast-growing tumours. For example, calculations predict a reduction in local tumour control of up to 1.5% per week's delay for head and neck cancers treated following surgery. In addition, there may be a variety of fractionation regimes that will yield very similar clinical results for each tumour type. It is shown theoretically that, for the tumour types considered here, it is possible to increase the dose per fraction and decrease the number of fractions while maintaining or increasing TCP relative to standard 2 Gy fractionation regimes, although there may be some advantage to using hyperfractionated regimes for head and neck cancers in order to reduce normal tissue effects.

Cell kinetics

Cell kinetic concepts have pervaded radiation therapy since the early part of the 20th century and have been instrumental in the development of modern radiotherapy. In this review, the fundamental radiobiological concepts that have been developed based on cell kinetic knowledge will be revisited and discussed in the context of contemporary radiation therapy. This will include how the proliferation characteristics, variation in sensitivity during the cell cycle and the extent of radiation-induced cell cycle delay translate into a variable time for the expression of damage, how cell kinetics interacts with hypoxia and how the response to fractionated radiation schedules is influenced by cell kinetics in terms of repair, redistribution, reoxygenation and repopulation. The promise of combining radiation with new biologically targeted agents and the potential of non-invasive positron emission tomography imaging of proliferation are areas where cell kinetics will continue to influence radiotherapy practice.

Colocalization of Carbonic Anhydrase 9 Expression and Cell Proliferation in Human Head and Neck Squamous Cell Carcinoma

Purpose: Tumor cells undergo a variety of biological changes under sustained hypoxic conditions, allowing cells to survive and retain their clonogenic potential. The purpose of this study is to relate the expression of the hypoxia marker carbonic anhydrase 9 (CA9) to the uptake of iododeoxyuridine (IdUrd), a marker of proliferation, in head and neck squamous cell carcinomas. Colocalization of IdUrd and CA9 may identify an important subpopulation of tumor cells that might be responsible for repopulation and disease progression.

Experimental Design: Expression of CA9, IdUrd labeling, and colocalization between IdUrd and CA9 was examined by immunohistochemistry in biopsies of head and neck squamous cell carcinomas. Biopsies were taken from 51 patients recruited between 1998 and 2001 after administration of the proliferation marker IdUrd.

Results: A large variation was observed between the tumors in CA9 expression (range 0-39%), IdUrd labeling (range 0-81%), and colocalization between IdUrd and CA9 [FId(CA9); range 0-53%]. FId(CA9), the fraction of IdUrd-labeled cells positive for CA9, was highest at an intermediate distance from the blood vessels (100-150 μm). IdUrd labeling was higher in T4 carcinomas relative to lower stage tumors (P = 0.04). High FId(CA9) correlated with the worst disease-free survival rates (P = 0.04).

Conclusions: Colocalization between IdUrd labeling and CA9 expression was observed in head and neck squamous cell carcinomas, suggesting the presence of a population of tumor cells under intermediate hypoxic conditions which still has proliferative capacity. The size of this subpopulation may be indicative of tumor aggressiveness and is associated with the worst disease-free survival rates.

Modeling and computer simulations of tumor growth and tumor response to radiotherapy

A model of tumor growth and tumor response to radiation is introduced in which each tumor cell is taken into account individually. Each cell is assigned a set of radiobiological parameters, and the status of each cell is checked in discrete intervals. Tumor proliferation is governed by the cell cycle times of tumor cells, the growth fraction, the apoptotic capacity of the tumor, and the degree of tumor angiogenesis. The response of tumor cells to radiation is determined by the radiosensitivities and the oxygenation status. Computer simulation is performed on a 3D rigid cubic lattice, starting out from a single tumor cell. Random processes are simulated by Monte Carlo methods. Short cell cycle time, high growth fraction, and tumor angiogenesis all increase tumor proliferation rates. Accelerated time-dose patterns result in lower total doses needed for tumor control, but the extent of dose reduction depends on the kinetics and the radiosensitivities of tumor cells. Tumor angiogenesis alters fully oxygenated and hypoxic fractions within the tumor and subsequently affects the radiation response. It is demonstrated for selected radiobiological parameters that the simulation tools are suitable to quantitatively assess the total doses needed for tumor control. Using the simulation tools, it is feasible to simulate time-dependent effects during fractionated radiotherapy and to compare different time-dose patterns in terms of their tumor control.

A prospective study of pre-treatment cell kinetics and clinical outcome in nasopharyngeal carcinoma

BACKGROUND AND PURPOSE

To study pre-treatment cell kinetics and their clinical correlations in nasopharyngeal carcinoma (NPC).

MATERIALS AND METHODS

Ninety newly diagnosed NPC patients were studied using in vivo Bromodeoxyuridine (BrdU) labeling and flow cytometric analysis. Immunohistochemical staining for BrdU and Ki 67 was also performed.

RESULTS

The median S-phase duration (Ts) was 6.2 h (range 3.5-18.7 h), median flow cytometric labeling index (FCM-LI) was 7.4% (1.3-37.6%), and median potential doubling time (Tpot) was 3.6 days (0.5-19.9 days). The median histologic labeling index (H-LI) was 12.4% (1.2-43.3%), and median histologic Tpot (H-Tpot) was 2.1 days (0.5-33.3 days). FCM-LI and H-LI were both positively correlated with Ki67 whereas Tpot and H-Tpot were both negatively correlated with Ki67 and N-stage. In univariate analysis, Tpot and H-Tpot showed a trend for progression free survival. Tpot was significantly associated with local relapse free survival, but lost its significance in multivariate analysis. N-stage was the only significant prognostic factor for all radiotherapy outcomes in both univariate and multivariate analyses.

CONCLUSIONS

Tpot was the only pre-treatment cell kinetic parameter for which some evidence was found for an association with survival in NPC patients. Future studies should aim to combine cell kinetic parameters together with other biological markers and clinical parameters to provide more useful prognostic information to guide individual patient's therapy.

In vivo cell kinetic measurements in a randomized trial of continuous hyperfractionated accelerated radiotherapy with or without mitomycin C in head-and-neck cancer

PURPOSE

Tumor cell repopulation is still considered to be a major cause of failure in radiotherapy. In this study, we investigated the influence of cell kinetic parameters on the outcome of patients treated in a randomized trial of accelerated fractionation, with or without mitomycin C, vs. conventional fractionation.

METHODS AND MATERIALS

Sixty-two patients were studied using administration of bromodeoxyuridine (BrdUrd), and cell kinetic parameters were measured using flow cytometry. The patients were treated with either 70 Gy for 7 weeks or 55.3 Gy for 17 continuous days (V-CHART) with or without 20 mg/m(2) mitomycin C on day 5.

RESULTS

The potential doubling time (Tpot) and labeling index (LI) failed to provide any prognostic information with regard to local control or survival. However, the duration of the S phase (Ts) revealed patients whose tumors had a long Ts had significantly worse local control (p = 0.028) and survival (p = 0.034) irrespective of treatment. A similar trend was evident within the different treatment arms particularly associated with overall survival.

CONCLUSIONS

The Ts values of head-and-neck squamous cell cancers provided prognostic information that predicted clinical outcome irrespective of treatment schedule in this study. This neglected parameter of the Tpot method might provide information related to redistribution of cells during fractionated radiotherapy.

Accelerated hyperfractionation (AHF) compared to conventional fractionation (CF) in the postoperative radiotherapy of locally advanced head and neck cancer: influence of proliferation

Based on the assumption that an accelerated proliferation process prevails in tumour cell residues after surgery, the possibility that treatment acceleration would offer a therapeutic advantage in postoperative radiotherapy of locally advanced head and neck cancer was investigated. The value of T(pot) in predicting the treatment outcome and in selecting patients for accelerated fractionation was tested. Seventy patients with (T2/N1-N2) or (T3-4/any N) squamous cell carcinoma of the oral cavity, larynx and hypopharynx who underwent radical surgery, were randomized to either (a) accelerated hyperfractionation: 46.2 Gy per 12 days, 1.4 Gy per fraction, three fractions per day with 6 h interfraction interval, treating 6 days per week or (b) Conventional fractionation: 60 Gy per 6 weeks, 2 Gy per fraction, treating 5 days per week. The 3-year locoregional control rate was significantly better in the accelerated hyperfractionation (88 +/- 4%) than in the CF (57+/- 9%) group, P=0.01 (and this was confirmed by multivariate analysis), but the difference in survival (60 +/- 10% vs 46 +/- 9%) was not significant (P=0.29). The favourable influence of a short treatment time was further substantiated by demonstrating the importance of the gap between surgery and radiotherapy and the overall treatment time between surgery and end of radiotherapy. Early mucositis progressed more rapidly and was more severe in the accelerated hyperfractionation group; reflecting a faster rate of dose accumulation. Xerostomia was experienced by all patients with a tendency to be more severe after accelerated hyperfractionation. Fibrosis and oedema also tended to be more frequent after accelerated hyperfractionation and probably represent consequential reactions. T(pot) showed a correlation with disease-free survival in a univariate analysis but did not prove to be an independent factor. Moreover, the use of the minimum and corrected P-values did not identify a significant cut-off. Compared to conventional fractionation, accelerated hyperfractionation did not seem to offer a survival advantage in fast tumours though a better local control rate was noted. This limits the use of T(pot) as a guide for selecting patients for accelerated hyperfractionation. For slowly growing tumours, tumour control and survival probabilities were not significantly different in the conventional fractionation and accelerated hyperfractionation groups. A rapid tumour growth was associated with a higher risk of distant metastases (P=0.01). In conclusion, tumour cell repopulation seems to be an important determinant of postoperative radiotherapy of locally advanced head and neck cancer despite lack of a definite association between T(pot) and treatment outcome. In fast growing tumours accelerated hyperfractionation provided an improved local control but without a survival advantage. To gain a full benefit from treatment acceleration, the surgery-radiotherapy gap and the overall treatment time should not exceed 6 and 10 weeks respectively.

Patterns of proliferation related to vasculature in human head-and-neck carcinomas before and after transplantation in nude mice

PURPOSE

The predictive potential of tumor cell kinetic parameters may be improved when they are studied in relation to other microenvironmental parameters. The purpose of this investigation was to quantitatively categorize human tumor samples according to proliferation patterns. Second, it was examined whether these characteristics are retained after xenotransplantation.

METHODS AND MATERIALS

Fifty tumor samples from head-and-neck cancer patients were immunohistochemically stained for Ki-67 and vessels. Also, parts of the samples were transplanted into nude mice. Tumors were categorized according to previously described patterns of proliferation. Vascular and proliferation patterns were analyzed using an image processing system.

RESULTS

The 50 tumors were categorized into four patterns of proliferation by visual assessment: marginal (6), intermediate (10), random (21), and mixed (12). One tumor could not be classified. These patterns were quantified by calculating the Ki-67 labeling index in distinct zones at increasing distance from vessels yielding good discrimination and significant differences between patterns. The probability of growth after xenotransplantation was significantly higher for tumors with a labeling index and vascular density above the median value compared to tumors with both parameters below the median (82% vs. 35%). Fifty percent of the tumors retained their proliferation patterns after xenotransplantation.

CONCLUSION

The categorization by proliferation pattern previously described by others was reproduced quantitatively and spatially related to the vascular network using a computerized image processing system. The combination of quantitative and architectural information of multiple microenvironmental parameters adds a new dimension to the study of treatment resistance mechanisms. Tumor models representative of the various patterns can be used to further investigate the relevance of these architectural patterns.

Interaction between potential doubling time and TP53 mutation: predicting radiotherapy outcome in squamous cell carcinoma of the head and neck

PURPOSE

To investigate the correlation between tumor potential doubling time, Tpot, and mutations in the p53 gene, TP53, and the potential of these parameters to predict outcome of head and neck cancer patients treated with radiotherapy.

METHODS AND MATERIALS

Data from two independent studies on Tpot and TP53 mutations were combined, including 58 patients with squamous cell carcinoma of the head and neck. Tpot was estimated on biopsies obtained 6-9 h after infusion of iododeoxyuridine by combined flow cytometry and immunohistology. TP53 mutations were detected using DGGE and sequenced. All patients received primary radiotherapy alone.

RESULTS

The predictive value of Tpot alone was of borderline significance. However, in TP53 wild-type tumors, Tpot was a strong predictor of outcome, whereas Tpot in TP53 mutant tumors failed to provide any information. Tpot and TP53 were not associated with nodal control; however, there was a strong relationship with control in the T-position, disease-specific survival, and overall survival.

CONCLUSION

Tpot can to be a relevant parameter for predicting outcome of radiotherapy in head and neck cancer but only in the subset of patients without mutations in the p53 gene.

Genome-wide screening for radiation response factors in head and neck cancer

INTRODUCTION

Radiation therapy is an integral part of the treatment of head and neck cancer. Factors predicting radiation response are ill defined. The aim of this study was to identify genetic aberrations associated with radiation response in cell lines derived from head and neck squamous cell carcinomas (HNSCC) using comparative genomic hybridization (CGH) for genome-wide screening.

METHODS

Five cell lines derived from HNSCC were subjected to a single course of radiation (400 cGy) in parallel with a similarly handled, untreated control. Cellular response to radiation was determined on posttreatment days 1, 2, 3, 4, and 5 using a cell viability assay (MTT assay). Radiation response was defined as 35% or greater decrease in cell survival relative to control. Tumor doubling time was determined by cell counts performed at day 0 and 1 for each cell line. All experiments were done in quadruplicate. CGH analysis was performed by differentially labeling DNA from tumor and normal tissue with fluorescent agents. The labeled DNAs were simultaneously hybridized to normal metaphase chromosomes. Image analysis for fluorescence intensity along the entire length of each metaphase chromosome allowed generation of a color ratio, which was used to detect copy number changes.

RESULTS

Radioresistance was identified in two of five cell lines. The tumor doubling time was not a predictor of radiation response. CGH identified a complex pattern of aberrations, with gain of 3q common to all cell lines. The number of genetic aberration was higher in radiation-sensitive cell lines than in radiation-resistant ones. No recurrent aberrations were unique to the radiation-resistant cell lines. Recurrent gains at 7p and 17q and losses at 5q, 7q, and 18q were unique to the radiation-sensitive cell lines.

CONCLUSIONS

The number of aberrations identified by CGH analysis may be a predictor of radiation response. A large study of primary tumors is warranted to confirm this association and identify specific genetic aberrations associated with radiation response.

Cell production rates in human tissues and tumours and their significance. Part II: clinical data

This paper reviews the available data for cell production rates of human tissues and tumours, measured in vivo using halogenated pyrimidine labelling and laser cytometry. The technique has now been widely evaluated, and we draw general inferences from the proliferative data over a broad range of tumour and tissue types. Estimates of the S-phase duration, the time taken for DNA synthesis in cycling cells, are consistent over a narrow range with a median value of around 10 hours, notwithstanding the constraints of the experimental and statistical technique, in normal tissues and tumours. This suggests that Ts values may be a species-specific constant. The more easily measured labelled S-phase fraction, or labelling index, shows much greater intra and intertumour variation within any one tumour class. It may thus be a surrogate for time dependent measurements to a first order approximation. The cell production rate, described by the potential doubling time (Tpot), is remarkably rapid in most tumours, a median value of the order of 5 days, and much faster than clinical volume doubling times for most lesions. The rapid cell production rates in normal tissues and tumours highlight the importance of cell loss in the growth and modelling of biological structures. Cell production rate measurements do not adequately describe the biological aggressiveness of tumours. They may be used to refine adjuvant strategies for radiotherapy and chemotherapy in experimental research. Dynamic halogenated pyrimidine labelling has provided unique and valuable insights into the living biology of human tissues and tumours.

Accelerated radiotherapy for T1, 2 glottic carcinoma: analysis of results with KI-67 index

PURPOSE

Hyperfractionated and accelerated radiotherapy without a split was performed to improve the local control probability of early glottic carcinomas. We analyzed the results of this regimen by using the Ki-67 index.

METHODS AND MATERIALS

Over a 12-year period, 85 T1N0M0 glottic cancers and 50 T2N0M0 glottic cancers were treated with conventional fractionation (CF) from 1984 to 1989 and with accelerated fractionation (AF) since 1990. The CF program consisted of five daily fractions of 2 Gy per week, for a total of 64 Gy. The AF program consisted of 1.72 Gy per fraction, two fractions per day, 5 days a week, for a total of 55 or 58 Gy. The specimens, taken before radiotherapy, were immunohistochemically stained with anti-Ki-67 antibody.

RESULTS

The 5-year local control probability for T1 tumors was 79.6 +/- 6.9% with CF treatment, whereas with AF it was 86.9 +/- 5.6%. For T2 tumors it was 62.7 +/- 12.2% with CF, whereas it was 74.7 +/- 7.8% with AF. The difference between CF and AF did not reach the point of statistical significance. However, when T1 tumors had a Ki-67 index lower than 50%, the local control rate achieved with AF was significantly better than that with CF (p = 0.018). When the tumors had a Ki-67 index that was 50% or more, there was no difference in the local control rate between CF and AF, whether they were T1 or T2. The peak mucosal reactions at the larynx and/or hypopharynx were much more severe and appeared at smaller doses and earlier in AF than in CF. The patients with AF showed no severe late complications.

CONCLUSIONS

AF could not obtain statistically significant improvement in local control probability of T1 or T2 glottic carcinomas.

Mathematical modeling of chronical hypoxia in tumors considering potential doubling time and hypoxic cell lifetime

PURPOSE

To model mathematically how potential doubling time and hypoxic cell lifetime affect the extent of chronical hypoxia in tumor tissue segments. Three capillary geometries were modeled under idealized steady state conditions.

MATERIALS AND METHODS

The capillary geometries are: tissue surrounding an axial capillary, tissue enclosed by a cylindrical capillary network, and tissue enclosed by a spherical capillary network. The tissue segments are modeled as three-compartment systems, where well nourished cells proliferate near the vasculature and, in so doing, displace 'older' cells into a quiescent compartment and, ultimately into a hypoxic region. The extent of the hypoxic zone is the distance traversed by cells during their hypoxic lifetime before becoming necrotic. The steady state situation, where the necrotic cell loss equals the cell gain caused by cell proliferation was investigated.

RESULTS

The hypoxic fraction, HF, was found to be inversely proportional to the potential doubling time of the tumor segment, T(pot), and proportional to the hypoxic cell lifetime, T(hypox). The extent of the oxygenated zone depends only on the capillary geometry, the capillary radius, the intracapillary oxygen tension, and the tissue respiration rate. The extent of the hypoxic zone in addition depends on T(pot) and T(hypox).

CONCLUSIONS

Mathematical modeling of idealized steady state conditions shows that the ratio of hypoxic cell lifetime and potential doubling time, T(hypox)/T(pot), determines the hypoxic fraction, HF, in tumor segments. The extents of the oxygenated and the hypoxic zones can be predicted from the models.

Surfing prognostic factors in head and neck cancer at the millennium

The ability to reliably predict cancer outcome could tailor therapy to the aggressiveness of the tumour to achieve the best results in terms of loco-regional control, overall survival and quality of life. Retrospective and prospective clinical trials involving large series of patients have validated some predictive clinical and pathological factors, whereas the utility of many other prognostic factors has not been established. This has led to some confusion in clinical practice. In order to clarify the significance, role and cost of these prognostic factors we carried out a Medline search of all papers published between 1993 and 1998 concerning the reliability and cost of markers with prognostic significance, in head and neck squamous cell carcinoma, and assessed the results according to a number of criteria relating to reliability and cost. Regarding reliability we classified prognostic factors into: (1) those with a proven significance based on the fact that they were unanimously reported as having an independent statistical correlation with outcome and prognosis; and (2) those for which results were not unanimous, and which significance is still controversial. Cost analysis showed a substantial difference between validated tests which are of low cost and experimental tests which are expensive. Based on these data regarding both the reliability and cost of each prognostic factor, we propose guidelines for their use in clinical practice in the year 2000.

Molecular markers as prognostic factors for local recurrence and radioresistance in head and neck squamous cell carcinoma

Squamous cell carcinoma of the head and neck affects more than 500,000 people worldwide each year. Local-regional recurrence of disease is a common and challenging oncological problem in patients affected by this disease. Identification of risk factors for local relapse after appropriate local therapy with surgery, radiation, or combination therapy remains an active area of clinical research. The recent development of novel molecular markers has resulted in numerous studies evaluating the prognostic significance and potential clinical utility of these markers in identifying patients at risk for local-regional relapse. This article reviews recent studies evaluating molecular markers, including p53, angiogenesis-related markers, cyclin D1, epidermal growth factor receptor, loss of heterozygosity, DNA ploidy, and cell kinetic markers. The potential clinical utility of these markers and future directions along this avenue of investigation are discussed.

Determination of tumor kinetics: strategies for the delivery of radiotherapy and chemotherapy

Both clinical and laboratory evidence indicates that local control rates for many experimental and clinical human tumors decrease with protraction of the overall duration of radiation therapy and that a likely basis for this decrease is tumor cell repopulation during treatment. Such observations have stimulated interest in tumor kinetics, and a number of techniques have been developed that increase the potential for meaningful clinical study of the proliferative behavior of tumors. This review discusses the clinical and experimental evidence for proliferation during treatment, describes two potential approaches-accelerated fractionation and concurrent chemotherapy and radiotherapy-that can be employed to counteract such intratreatment proliferation, explores methods available for measuring tumor cell kinetics, and discusses how kinetics information may be used in the future to tailor therapy to a tumor's individual characteristics.

Cell kinetics and repopulation parameters of irradiated xenograft tumours in SCID mice: comparison of two dose-fractionation regimens

The extent and mechanism(s) of repopulation were assessed in SiHa (human cervical squamous cell carcinoma) xenografts in SCID mice for two fractionated irradiation regimens. Mice in one arm of the study received 50 Gy in 20 fractions over 23 days with a 14 day split between 10 fraction, 5 day courses. The other tumours were treated with 50 Gy in 20 fractions over 10 consecutive days. Cell kinetics and tumour regrowth parameters were monitored during and after treatment by measuring tumour volume and analysing cellular DNA content and proliferation parameters with flow cytometry. Repopulation occurred rapidly, beginning during irradiation and largely attributable to an increased growth fraction and decreased potential doubling time, apparently triggered by increased cell loss. Cell cycle time, in contrast, remained relatively constant throughout. Extrapolation of these results to humans suggests that treatment times should be minimised whenever possible, since regrowth rates exceeded those predicted from pretreatment Tpot measurements.

Repopulation characteristics and cell kinetic parameters resulting from multi-fraction irradiation of xenograft tumors in SCID mice

PURPOSE

Cell kinetics and repopulation rates during multifraction irradiation have previously been measured in SiHa human cervical carcinoma cells grown as spheroids. The current study applied similar techniques to SiHa tumor xenografts with the ultimate goal of assessing the clinical prognostic value of in situ cell kinetics.

METHODS AND MATERIALS

SiHa (human squamous cell cervical tumor) cells were inoculated subcutaneously in the flank or back of SCID mice. When tumors reached a size of 200-300 mg, they received 25 Gy in 10 fractions over 5 days. Tumor regrowth and cell kinetics parameters were followed during treatment, and for 10 days after completion by measuring tumor volume and analyzing cellular BrdUrd and IdUrd incorporation with flow cytometry.

RESULTS

Tumor volume was of limited use in assessing response to irradiation. The fraction of proliferating cells increased early during irradiation as did the labeling index; potential doubling time (Tpot) decreased during treatment and returned to the pre-irradiation value after treatment. Cell cycle time remained relatively constant throughout the experiments.

CONCLUSION

These results confirm the feasibility of evaluating cell cycle kinetics and repopulation parameters in a murine tumor model undergoing a fractionated course of irradiation. Repopulation of clonogenic tumor cells occurred more rapidly than predicted by pretreatment measurements, primarily due to an increased growth fraction and consequent decrease in Tpot.

The value of pretreatment cell kinetic parameters as predictors for radiotherapy outcome in head and neck cancer: a multicenter analysis

PURPOSE

The aim of this study was to assess the potential of pre-treatment cell kinetic parameters to predict outcome in head and neck cancer patients treated by conventional radiotherapy.

MATERIALS AND METHODS

Data from 11 different centers were pooled. Inclusion criteria were such that the patients received radiotherapy alone, and that the radiotherapy was given in an overall time of at least 6 weeks with a dose of at least 60 Gy. All patients received a tracer dose of either iododeoxyuridine (IdUrd) or bromodeoxyuridine (BrdUrd) intravenously prior to treatment and a tumor biopsy was taken several hours later. The cell kinetic parameters labeling index (LI), DNA synthesis time (Ts) and potential doubling time (Tpot) were subsequently calculated from flow cytometry data, obtained on the biopsies using antibodies against I/BrdUrd incorporated into DNA. Each center carried out their own flow cytometry analysis.

RESULTS

From the 11 centers, a total of 476 patients conforming to the inclusion criteria were analyzed. Median values for overall time and total dose were 49 days and 69 Gy, respectively. Fifty one percent of patients had local recurrences and 53% patients had died, the majority from their disease. Median follow-up was 20 months; being 30 months for surviving patients. Multivariate analysis revealed that T-stage, maximum tumor diameter, differentiation grade, N-stage, tumor localization and overall time correlated with locoregional control, in decreasing order of significance. For the cell kinetic parameters, univariate analysis showed that only LI was significantly associated with local control (P=0.02), with higher values correlating with a worse outcome. Ts showed some evidence that patients with longer values did worse, but this was not significant (P=0.06). Tpot showed no trend (P=0.8). When assessing survival in a univariate analysis, neither LI nor Tpot associated with outcome (P=0.4, 0.4, respectively). Surprisingly, Ts did correlate with survival, with longer values being worse (P=0.02). In the multivariate analysis of local control, LI lost its significance (P=0.16).

CONCLUSIONS

The only pretreatment kinetic parameter for which some evidence was found for an association with local control (the best end-point for testing the present hypothesis) was LI, not Tpot, and this evidence disappeared in a multivariate analysis. It therefore appears that pretreatment cell kinetic measurements carried out using flow cytometry, only provide a relatively weak predictor of outcome after radiotherapy in head and neck cancer.

The effect of heterogeneity in tumor cell kinetics on radiation dose-response. An exploratory investigation of a plateau effect

PURPOSE

To investigate the effect of heterogeneity in tumor cell kinetics on radiation dose-response curves for a population of patients.

MATERIALS AND METHODS

A series of exploratory calculations have been performed using an improved geometric-stochastic model of tumor cure.

RESULTS

Radiation therapy dose-response curves may plateau, or nearly so, at tumor control levels well below 100%, if a proportion of tumors would grow sufficiently fast to counterbalance the effect of fractionated radiotherapy. If the model assumptions of doubling time heterogeneity are correct, the difference between a short and protracted radiation regimen would be not only in the position and steepness of the radiation dose-response curve, but also in the level of the predicted plateau.

CONCLUSIONS

For a given rate of dose accumulation, the one-sided flattening in dose-response curves at high doses is predicted from the modeling, and determined by the proportion of most radioresistant and rapidly growing tumors. This shows that empirical models of tumor control probability which assume a symmetric sigmoid relationship from 0 to 100% have apparent limitations, seemingly not well acknowledged in the literature.

Relationship between tumour cell in vitro radiosensitivity and clinical outcome after curative radiotherapy for squamous cell carcinoma of the head and neck

BACKGROUND AND PURPOSE

Clinically, it is recognized that individual tumours respond differently to radiation treatment. Variation in tumour cell radiosensitivity is believed to be an important underlying factor. In the current study, cellular in vitro radiosensitivity was estimated as the fraction of surviving cells after a radiation dose of 2 Gy (SF2) and related to clinical outcome after curative radiotherapy.

PATIENTS AND METHODS

Thirty-eight patients with squamous cell carcinoma of the head and neck were treated with curative radiotherapy alone. Pre-treatment biopsies were disaggregated to form a single-cell suspension and cells were cultured in the modified Courtenay-Mills soft agar clonogenic assay. Directly from this assay and with no respect to cell type, overall SF2 was assessed. By collecting the obtained colonies on a preparation slide using a colony-filter technique, and with immunocytochemical staining, it was possible to measure the surviving fraction of tumour cells selectively as tumour cell SF2.

RESULTS

Experimentally, a broad inter-tumour variation was found for both tumour cell SF2 and overall SF2. Using weighted linear regression, it was demonstrated that tumour cell SF2 and overall SF2 were two independent measures of tumour radiosensitivity. In general, the measures of tumour radiosensitivity were independent of patient sex and age, T- and N-category, disease stage, tumour size and plating efficiency. Among the 38 patients grouped in loco-regional failures and patients with loco-regional control, respectively, sex, age, total radiation dose, overall treatment time and tumour grade were equally distributed. Advanced stage, lymph node involvement and tumour size correlated statistically significantly with poor loco-regional control. Neither tumour cell SF2, overall SF2, nor plating efficiency predicted loco-regional tumour control probability. In a multivariate analysis with respect to the risk of loco-regional tumour failure, only disease stage yielded independent prognostic significance. This significance suggests that this patient sample was representative for the patient population with head and neck cancer.

CONCLUSION

In 38 patients with squamous cell carcinoma of the head and neck, the estimated tumour radiosensitivities were not statistically related to clinical outcome after curative radiotherapy alone.