Tumour cell repopulation during fractionated radiotherapy: correlation between flow cytometric and radiobiological data in three murine tumours

The Impact of Surgical Timing on Pathologic Tumor Response after Short Course and Long Course Preoperative Chemoradiation for Locally Advanced Rectal Adenocarcinoma

Purpose

A pooled analysis of multi-institutional trials was performed to analyze the effect of surgical timing on tumor response by comparing short course concurrent chemoradiotherapy (CCRT) with long course CCRT followed by delayed surgery in locally advanced rectal cancer.

Materials and Methods

Three hundred patients with cT3-4N0-2 rectal adenocarcinoma were included. Long course patients from KROG 14-12 (n=150) were matched 1:1 to 150 short course patients from KROG 10-01 (NCT01129700) and KROG 11-02 (NCT01431599) according to stage, age, and other risk factors. The primary endpoint was to determine the interval between surgery and the last day of neoadjuvant CCRT which yields the best tumor response after the short course and long course CCRT. Downstaging was defined as ypT0-2N0M0 and pathologic complete response (ypCR) was defined as ypT0N0M0, respectively.

Results

Both the long and short course groups achieved lowest downstaging rates at < 6 weeks (long 20% vs. short 8%) and highest downstaging rates at 6-7 weeks (long 44% vs. short 40%). The ypCR rates were lowest at < 6 weeks (both long and short 0%) and highest at 6-7 weeks (long 21% vs. short 11%) in both the short and long course arms. The downstaging and ypCR rates of long course group gradually declined after the peak at 6-7 weeks and those of the short course group trend to constantly increase afterwards.

Conclusion

It is optimal to perform surgery at least 6 weeks after both the short course and long course CCRT to obtain maximal tumor regression in locally advanced rectal adenocarcinoma.

Cyclooxygenase-2: A Role in Cancer Stem Cell Survival and Repopulation of Cancer Cells during Therapy

Cyclooxygenase-2 (COX-2) is an inducible form of the enzyme that catalyses the synthesis of prostanoids, including prostaglandin E2 (PGE₂), a major mediator of inflammation and angiogenesis. COX-2 is overexpressed in cancer cells and is associated with progressive tumour growth, as well as resistance of cancer cells to conventional chemotherapy and radiotherapy. These therapies are often delivered in multiple doses, which are spaced out to allow the recovery of normal tissues between treatments. However, surviving cancer cells also proliferate during treatment intervals, leading to repopulation of the tumour and limiting the effectiveness of the treatment. Tumour cell repopulation is a major cause of treatment failure. The central dogma is that conventional chemotherapy and radiotherapy selects resistant cancer cells that are able to reinitiate tumour growth. However, there is compelling evidence of an active proliferative response, driven by increased COX-2 expression and downstream PGE₂ release, which contribute to the repopulation of tumours and poor patient outcome. In this review, we will examine the evidence for a role of COX-2 in cancer stem cell biology and as a mediator of tumour repopulation that can be molecularly targeted to overcome resistance to therapy.

Involvement of UTR-dependent gene expression in the maintenance of cancer stem cell like phenotypes

The present study demonstrated the acquisition of additional malignant characteristics in irradiated mouse fibrosarcoma cells compared with the parent cells. Several reporter assays indicated that hypoxia-inducible factor (HIF)-1α, activator protein-1 and Ets-dependent transcription were activated in irradiated cells. The cis-elements in the 5'-untranslated region (UTR) of these transcription factors plays a major role in their expression in surviving irradiated cancer cells. By contrast, there were no evident differences between the 3'-UTR-dependent repression demonstrated by parent cells and irradiated cells. A small population of parental fibrosarcoma cells was also found to exhibit the same enhanced 5'-UTR-dependent HIF-1α expression as that demonstrated by irradiated cells. These observations may indicate that high-dose X-ray irradiation affects the majority of proliferating cancer cells, but not the cancer stem cells (CSCs), and an increased CSC population may explain the progressive phenotypes of the irradiated cells. It appears likely that the transcription factors that maintain stemness are regulated by the same 5'-UTR-dependent mechanism.

Cell death-stimulated cell proliferation: a tissue regeneration mechanism usurped by tumors during radiotherapy

The death of all the cancer cells in a tumor is the ultimate goal of cancer therapy. Therefore, much of the current effort in cancer research is focused on activating cellular machinery that facilitates cell death such as factors involved in causing apoptosis. However, recently, a number of studies point to some counterintuitive roles for apoptotic caspases in radiation therapy as well as in tissue regeneration. It appears that a major function of apoptotic caspases is to facilitate tissue regeneration and tumor cell repopulation during cancer therapy. Because tumor cell repopulation has been shown to be important for local tumor relapse, understanding the molecular mechanisms behind tumor repopulation would be important to enhance cancer radiotherapy. In this review, we discuss our current knowledge of these potentially paradigm-changing phenomena and mechanisms in various organisms and their implications on the development of novel cancer therapeutics and strategies.

The HYP-RT hypoxic tumour radiotherapy algorithm and accelerated repopulation dose per fraction study

The HYP-RT model simulates hypoxic tumour growth for head and neck cancer as well as radiotherapy and the effects of accelerated repopulation and reoxygenation. This report outlines algorithm design, parameterisation and the impact of accelerated repopulation on the increase in dose/fraction needed to control the extra cell propagation during accelerated repopulation. Cell kill probabilities are based on Linear Quadratic theory, with oxygenation levels and proliferative capacity influencing cell death. Hypoxia is modelled through oxygen level allocation based on pO₂ histograms. Accelerated repopulation is modelled by increasing the stem cell symmetrical division probability, while the process of reoxygenation utilises randomised pO₂ increments to the cell population after each treatment fraction. Propagation of 10⁸ tumour cells requires 5–30 minutes. Controlling the extra cell growth induced by accelerated repopulation requires a dose/fraction increase of 0.5–1.0 Gy, in agreement with published reports. The average reoxygenation pO₂ increment of 3 mmHg per fraction results in full tumour reoxygenation after shrinkage to approximately 1 mm. HYP-RT is a computationally efficient model simulating tumour growth and radiotherapy, incorporating accelerated repopulation and reoxygenation. It may be used to explore cell kill outcomes during radiotherapy while varying key radiobiological and tumour specific parameters, such as the degree of hypoxia.

Monte Carlo radiotherapy simulations of accelerated repopulation and reoxygenation for hypoxic head and neck cancer

OBJECTIVE

A temporal Monte Carlo tumour growth and radiotherapy effect model (HYP-RT) simulating hypoxia in head and neck cancer has been developed and used to analyse parameters influencing cell kill during conventionally fractionated radiotherapy. The model was designed to simulate individual cell division up to 10(8) cells, while incorporating radiobiological effects, including accelerated repopulation and reoxygenation during treatment.

METHOD

Reoxygenation of hypoxic tumours has been modelled using randomised increments of oxygen to tumour cells after each treatment fraction. The process of accelerated repopulation has been modelled by increasing the symmetrical stem cell division probability. Both phenomena were onset immediately or after a number of weeks of simulated treatment.

RESULTS

The extra dose required to control (total cell kill) hypoxic vs oxic tumours was 15-25% (8-20 Gy for 5 × 2 Gy per week) depending on the timing of accelerated repopulation onset. Reoxygenation of hypoxic tumours resulted in resensitisation and reduction in total dose required by approximately 10%, depending on the time of onset. When modelled simultaneously, accelerated repopulation and reoxygenation affected cell kill in hypoxic tumours in a similar manner to when the phenomena were modelled individually; however, the degree was altered, with non-additive results. Simulation results were in good agreement with standard linear quadratic theory; however, differed for more complex comparisons where hypoxia, reoxygenation as well as accelerated repopulation effects were considered.

CONCLUSION

Simulations have quantitatively confirmed the need for patient individualisation in radiotherapy for hypoxic head and neck tumours, and have shown the benefits of modelling complex and dynamic processes using Monte Carlo methods.

Impact of overall treatment time on survival and local control in patients with anal cancer: a pooled data analysis of Radiation Therapy Oncology Group trials 87-04 and 98-11

PURPOSE

To determine whether increased duration of radiation therapy (RT) and overall treatment (RX) time has a detrimental effect in anal cancer.

PATIENTS AND METHODS

Data from Radiation Therapy Oncology Group (RTOG) 87-04 and RTOG 98-11 trials were combined to form three treatment groups: RT/fluorouracil (FU)/mitomycin (n = 472), RT/FU/cisplatin (n = 320), and RT/FU (n = 145). Cox proportional hazards models were used with the following variables: RT duration, RT intensity, RX duration, treatment group, age, sex, Karnofsky performance score (KPS), T stage, N stage, and RT dose.

RESULTS

In the univariate analysis, there was a significant association between RX duration and colostomy failure (CF; hazard ratio [HR] = 1.51; 95% CI, 1.07 to 2.14; P = .02), local failure (HR = 1.52; 95% CI, 1.14 to 2.03; P = .005), locoregional failure (HR = 1.51; 95% CI, 1.15 to 1.98; P = .003), and time to failure (HR = 1.40; 95% CI, 1.10 to 1.79; P = .007). The significance of RX duration was maintained after adjusting for treatment group. In multivariate modeling there was a trend toward an association between RX duration and CF (HR = 1.57; 95% CI, 0.98 to 2.50; P = .06) and a statistically significant association with local failure (HR = 1.96; 95% CI, 1.34 to 2.87; P = .0006). Age, sex, KPS, T stage, N stage, and RT dose, but not RT duration, RT intensity, or RX duration, were found to be statistically significant predictors of OS and colostomy-free survival.

CONCLUSION

Total treatment time, but not duration of radiation therapy, seems to have a detrimental effect on local failure and colostomy rate in anal cancer. Induction chemotherapy may contribute to local failure by increasing total treatment time.

Growth rate of newly developed metastatic brain tumors after thoracotomy in patients with non-small cell lung cancer

Among 1372 lung cancer patients without brain metastasis that underwent resection of lung cancer at our center from 2001 to 2007, brain metastases developed in 72 patients (5.2%) during their hospital course. We hypothesized that there were micro-metastases in the brain at the time of lung surgery in these patients, even though there were no detectable brain metastases on the MRI. The purpose of this study was to evaluate the growth rates of metastatic brain tumors in this unique subset of patients, and to compare the findings with our previous study that calculated the growth rate of brain metastases during chemotherapy. Among 72 patients, 23 with cystic or hemorrhagic metastases were excluded. Seventy-six metastatic brain tumors in 49 patients were reviewed. Twenty-five patients underwent adjuvant or neoadjuvant chemotherapy; however, for the rest of the patients, chemotherapy was not added after lung cancer surgery. The tumor volume was determined using V-works software (v. 4.0) (Cybermed, Seoul, Korea) and T1 gadolinium enhanced MR images. The overall median tumor growth rate was 11.7 mm³/day (interquartile range, 4.9-26.8). There were no statistically significant differences in the tumor growth among the lung cancer stages and the growth rate was similar regardless of the use of chemotherapy. The growth rate reported in this study shows consistency with that of our previous report (12.1 mm³/day). These findings may help optimize patient management during follow up.

Growth rates of metastatic brain tumors in nonsmall cell lung cancer

BACKGROUND

The purpose of the study was to evaluate the growth kinetics of metastatic brain tumors during chemotherapy and to analyze growth rates and volumetric doubling time of metastatic brain tumors in patients with nonsmall cell lung cancer (NSCLC) with tumor regrowth.

METHODS

NSCLC patients with minimally symptomatic brain metastases who were not treated previously were enrolled. Serial magnetic resonance images (MRI) of 30 metastatic brain tumors in 19 patients were reviewed. Tumor growth rates and volumetric tumor doubling time of tumor regrowth were estimated. All patients were treated with front-line chemotherapy until disease progression.

RESULTS

The median tumor growth rate was 12.10 mm(3)/day (interquartile range [IQR], 3.09-36.75). The volume percentage increase/day was 1.67 (IQR, 0.69-4.59). The median volumetric tumor doubling time was 58.48 days (IQR, 32.33-98.48).

CONCLUSIONS

These findings may help optimize patient management during follow-up. Study results indicated that brain MRI should be obtained at a minimum of 2-month intervals to screen for metastatic brain tumors.

NF-kappa B-mediated adaptive resistance to ionizing radiation

Ionizing radiation (IR) began to be a powerful medical modality soon after Wilhelm Röntgen's discovery of X-rays in 1895. Today, more than 50% of cancer patients receive radiotherapy at some time during the course of their disease. Recent technical developments have significantly increased the precision of dose delivery to the target tumor, making radiotherapy more efficient in cancer treatment. However, tumor cells have been shown to acquire a radioresistance that has been linked to increased recurrence and failure in many patients. The exact mechanisms by which tumor cells develop an adaptive resistance to therapeutic fractional irradiation are unknown, although low-dose IR has been well defined for radioadaptive protection of normal cells. This review will address the radioadaptive response, emphasizing recent studies of molecular-level reactions. A prosurvival signaling network initiated by the transcription factor NF-kappa B, DNA-damage sensor ATM, oncoprotein HER-2, cell cyclin elements (cyclin B1), and mitochondrial functions in radioadaptive resistance is discussed. Further elucidation of the key elements in this prosurvival network may generate novel targets for resensitizing the radioresistant tumor cells.

Effect of the Selective Estrogen Receptor Modulator Arzoxifene on Repopulation of Hormone-Responsive Breast Cancer Xenografts between Courses of Chemotherapy

Selective inhibition of repopulation of clonogenic tumor cells between courses of chemotherapy has potential to improve the effectiveness of treatment. Here we study arzoxifene, a short-acting selective estrogen receptor modulator, for its potential to inhibit repopulation in estrogen-dependent human breast cancer MCF-7 xenografts between courses of chemotherapy. Proliferation of tumor cells was evaluated by cyclin D1 expression and uptake of 5-bromo-2'-deoxyuridine. Arzoxifene decreased cell proliferation in xenografts. To model adjuvant treatment of human breast cancer, MCF-7 cells were injected s.c. into nude mice and four groups of mice received the following treatments beginning after implantation: (a) control (vehicle solution); (b) arzoxifene alone, 5 days per week by oral gavage for 3 weeks; (c) 5-fluorouracil (5-FU) or paclitaxel i.p. weekly, for 3 doses; and (d) arzoxifene following each cycle of chemotherapy. The incidence of tumors with volume > or =50 mm(3) was determined as a function of time. MCF-7 xenografts developed in 100% of control mice by 4 weeks after implantation. Paclitaxel or 5-FU alone had minor effects to delay the appearance of xenografts whereas arzoxifene alone caused longer delay. Combined treatment with arzoxifene given between cycles of 5-FU or paclitaxel had substantial effects, with approximately 50% tumor incidence by 5 weeks. Our results indicate that arzoxifene can inhibit repopulation of hormone-responsive MCF-7 breast cancer xenografts when given between courses of chemotherapy. The scheduling of short-acting hormonal agents between courses of adjuvant chemotherapy for human breast cancer has potential to improve the outcome of treatment.

Repopulation of cancer cells during therapy: an important cause of treatment failure

Radiotherapy and chemotherapy are given in multiple doses, which are spaced out to allow the recovery of normal tissues between treatments. However, surviving cancer cells also proliferate during the intervals between treatments and this process of repopulation is an important cause of treatment failure. Strategies developed to overcome repopulation have improved clinical outcomes, and now new strategies to inhibit repopulation are emerging in parallel with advances in the understanding of underlying biological mechanisms.

Accelerated regrowth of non-small-cell lung tumours after induction chemotherapy

Induction chemotherapy of non-small-cell lung cancer (NSCLC) stage III with gemcitabine and cisplatin for downstaging of the tumour with the aim for further treatment with ionising radiation is one of the treatments for lung cancer patients. The purpose of this study was to investigate the influence of the waiting time for radiotherapy, that is, the interval between induction chemotherapy and radiotherapy, on the rate of tumour growth for patients with NSCLC. Interval times between the end of induction chemotherapy and date of diagnostic CT, planning CT and first day of radiotherapy were determined for 23 patients with NSCLC. Increase in gross tumour volume was measured for 18 patients by measuring the dimensions of the primary tumour and lymph node metastases on the diagnostic CT after induction chemotherapy and on the CT used for radiotherapy planning. For each patient, the volume doubling time was calculated from the time interval between the two CTs and ratio of the gross volumes on planning CT and diagnostic CT. The mean time interval between end of chemotherapy and day of diagnostic CT was 16 days, and till first day of radiotherapy 80.3 (range 29 – 141) days. In all, 41% of potentially curable patients became incurable in the waiting period. The ratio of gross tumour volumes of the two CTs ranged from 1.1 to 81.8 and the tumour doubling times ranged from 8.3 to 171 days, with a mean value of 46 days and median value of 29 days. This is far less than the mean doubling time of NSCLC in untreated patients found in the literature. This study shows that in the time interval between the end of induction chemotherapy and the start of radiotherapy rapid tumour progression occurs as a result of accelerated tumour cell proliferation: mean tumour doubling times are much shorter than those in not treated tumours. As a consequence, the gain obtained with induction chemotherapy with regard to volume reduction was lost in the waiting time for radiotherapy. We recommend diminishing the time interval between chemo- and radiotherapy to as short as possible.

Selection of genetically distinct, rapidly proliferating clones does not contribute to repopulation during fractionated irradiation in FaDu squamous cell carcinoma

Acceleration of clonogen repopulation during fractionated irradiation after about 3 weeks has been demonstrated previously in FaDu human squamous cell carcinoma in nude mice (Petersen et al., Int. J. Radiat. Oncol. Biol. Phys. 51, 483-493, 2001). Selection of genetically distinct, rapidly proliferating clones might contribute to this phenomenon. To address this question, three sublines (R1-R3) were established from FaDu tumors that recurred locally after fractionated irradiation. The tumors were retransplanted and irradiated under clamp hypoxia with single doses or with 18 x 3 Gy within 18 days or 36 days, followed by graded top-up doses. The results were compared with data obtained after the same treatment schedules in the parental tumor line. Histologies, tumor volume doubling times, and potential doubling times of FaDu sublines R1-R3 were not different from those of the parental line. The radiation dose required to control 50% of the tumors (TCD(50)) after single-dose irradiation of 37-38 Gy was the same for the FaDu sublines R1-R3 and the parental tumor. The top-up TCD(50) values for the FaDu sublines R1-R3 after 18 fractions within 36 days were 14-17 Gy higher than those after 18 fractions within 18 days, indicating significant repopulation. The magnitude of this effect was not significantly different between the sublines R1-R3 or between these sublines and the parental FaDu tumors. The results indicate that selection of genetically distinct, rapidly proliferating clones does not contribute to the acceleration of repopulation during fractionated irradiation in poorly differentiated FaDu tumors.

Cytostatic potential of novel agents that inhibit the regulation of intracellular pH

Cells within the acidic extracellular environment of solid tumours maintain their intracellular pH (pHi) through the activity of membrane-based ion exchange mechanisms including the Na(+)/H(+) antiport and the Na(+)-dependent Cl(-)/HCO(3)(-) exchanger. Inhibition of these regulatory mechanisms has been proposed as an approach to tumour therapy. Previously available inhibitors of these exchangers were toxic (e.g. 4,4-diisothiocyanstilbene-2,2-disulphonic acid), and/or non-specific (e.g. 5-N-ethyl-N-isopropyl amiloride). Using two human (MCF7, MDA-MB231) and one murine (EMT6) breast cancer cell lines, we evaluated the influence of two new agents, cariporide (an inhibitor of the Na(+)/H(+) antiport) and S3705 (an inhibitor of the Na(+)-dependent Cl(-)/HCO(3)(-) exchanger) on the regulation of intracellular pH (pHi). The cytotoxicity of the two agents was assessed by using clonogenic assays. Our results suggest that cariporide has similar efficacy and potency to 5-N-ethyl-N-isopropyl amiloride for inhibition of Na(+)/H(+) exchange while S3705 is more potent and efficient than 4,4-diisothiocyanstilbene-2,2-disulphonic acid in inhibiting Na+-dependent Cl(-)/HCO3(-) exchange. The agents inhibited the growth of tumour cells when they were incubated at low pHe (7.0-6.8), but were non-toxic to cells grown at doses that inhibited the regulation of pHi. Our results indicate that cariporide and S3705 are selective cytostatic agents under in vitro conditions that reflect the slightly acidic microenvironment found in solid tumours.

Is there a future for cell kinetic measurements using IdUrd or BdUrd?

PURPOSE

The analysis of causes of radiation failure both in retrospective series of patients with head and neck cancer and in several randomized clinical trials suggests a loss of local control as the overall treatment time increases for the same total dose. This is attributed to tumor cell proliferation during fractionated radiotherapy. As longer treatment times lead to loss of local control, it has been suggested that shorter treatment times could lead to an increase in local control. For this reason accelerated treatment regimens have been and are being designed. However, these treatments may cause severe acute reactions. Due to this, lower total doses are sometimes given. Slowly proliferating tumors, therefore, may do worse when treated with accelerated schedules compared with conventional schedules. In addition, it is not desirable to subject all patients to the more intense acute reactions of accelerated schedules. It would thus be useful to predict which tumors will rapidly proliferate during treatment and are likely to benefit from accelerated radiotherapy. The potential doubling time (Tpot) is defined as the time within which the cell population of a tumor would double if there were no cell loss. The hypothesis is that the median Tpot measured before treatment might correlate with the effective doubling time (Tp) during treatment.

CONCLUSION

Tpot can be calculated knowing the labeling index (LI; proportion of cells incorporating the DNA precursor IdUrd or BdUrd) and Ts (the DNA synthesis time) measured by flow cytometry. A recent multicenter analysis has shown that the only pretreatment kinetic parameter for which some evidence is found for an association with local control is LI, not Tpot. Pitfalls associated with cell kinetic measurements such as assay variability, intratumor and intertumor variability, interlaboratory variability and the problem of an admixture of normal and malignant cells make Tpot not accurate and reproducible enough for a robust predictive assay. It therefore appears that pretreatment Tpot measurements using flow cytometry, provide only a relatively weak predictor of outcome after radiotherapy in head and neck cancer. Immunohistochemistry allows a simple measure of LI and may give additional independent information from labeling patterns, suggesting that this method is the (short term) future for clinical cell kinetic measurements using BdUrd or IdUrd.

Repopulation of tumour cells between cycles of chemotherapy: a neglected factor

Repopulation of clonogenic tumour cells during fractionated radiation treatment is recognised as an important factor affecting local control. Given the longer intervals between cycles and longer total duration of treatment, the impact of repopulation is likely to be greater following chemotherapy. Limited data from experimental models suggest that, after chemotherapy, there is a 'lag period', followed by variable but rapid rates of repopulation of tumour cells, possibly accelerating between cycles. Modelling of these properties indicates that after the initial response, accelerated repopulation between cycles can lead to tumour regrowth without any change in the drug sensitivity of the tumour cells. The importance of repopulation may be comparable with that of intrinsic or acquired cellular resistance in determining the effective resistance of tumours to chemotherapy. Biological agents with rapid onset and short duration of action, which can selectively inhibit tumour-cell repopulation, administered between cycles of chemotherapy, might improve the therapeutic index.

Changes in blood perfusion and hypoxia after irradiation of a human squamous cell carcinoma xenograft tumor line

The effect of irradiation depends on the oxygenation status of the tissue, while irradiation itself also changes the oxygenation and perfusion status of tissues. A better understanding of the changes in tumor oxygenation and perfusion over time after irradiation will allow a better planning of fractionated radiotherapy in combination with modifiers of blood flow and oxygenation. Vascular architecture (endothelial marker), perfusion (Hoechst 33342) and oxygenation (pimonidazole) were studied in a human laryngeal squamous cell carcinoma tumor line grown as xenografts in nude mice. The effect of a single dose of 10 Gy X rays on these parameters was evaluated from 2 h to 11 days after irradiation. Shortly after irradiation, there was an 8% increase in perfused blood vessels (from 57% to 65%) followed by a significant decrease, with a minimum value of 42% at 26 h after irradiation, and a subsequent increase to control levels at 7 to 11 days after irradiation. The hypoxic fraction showed a decrease at 7 h after treatment from 13% to 5% with an increase to 19% at 11 days after irradiation. These experiments show that irradiation causes rapid changes in oxygenation and perfusion which may have consequences for the optimal timing of radiotherapy schedules employing multiple fractions per day and the introduction of oxygenation- and perfusion-modifying drugs.

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.

Are hypoxic cells critical for the outcome of fractionated radiotherapy in a slow-growing mouse tumor?

PURPOSE

To investigate the significance of hypoxic cells, reoxygenation and repopulation for the outcome of fractionated radiotherapy of a slow-growing subline of a murine fibrosarcoma and to compare the results with those previously obtained from the original fast-growing tumor.

MATERIALS AND METHODS

A slow-growing subline, 457-O, was obtained among the tumors that recurred after a single irradiation to the third generation isotransplants of a mouse fibrosarcoma, FSa-II. The single cell suspensions were transplanted into the mouse foot and when the tumors reached an average diameter of 4 mm, they were subjected to one to 20 equal daily y-ray doses given in air (A) or under hypoxic conditions (H). The TCD50 (50% tumor control radiation dose) was calculated according to the tumor control frequency within 180 days. The linear-quadratic plus time model was fitted to these data by logistic regression analysis.

RESULTS

The volume doubling time of the 457-O tumors was approximately 2.2 times slower than that of the original FSa-II tumors. The TCD50(H) (single dose) was 52.3 Gy and increased with an increasing number of fractions to a TCD50(H) (20 doses) of 90.8 Gy. This increase of 38.5 Gy was much smaller than that of 149 Gy for the original FSa-II. The TCD50(A) (single dose) and TCD50(A) (20 doses) were 41.3 and 50.6 Gy, respectively. This small difference of 9.3 Gy contrasted with a significant increase of 52.9 Gy for the FSa-II.

DISCUSSION

These results suggested no repopulation of 457-O tumor clonogens during the course of up to 20 daily doses, while the original FSa-II tumor cells repopulated substantially. Hypoxic clonogens in the slow-growing tumor reoxygenated but some fractions remained critical.

CONCLUSION

The present data together with those obtained from the fast-growing FSa-II suggested that hypoxic clonogens were critical for the outcome of fractionated radiotherapy. Repopulation was insignificant in this slow-growing tumor during five to 20 daily doses.

CT-simulator based brachytherapy planner: seed localization and incorporation of biological considerations

Radiation dose prescription, interpretation, and planning can be problematic for brachytherapy due to high spatial heterogeneity, varying and various dose rates, absence of superimposed calculated isodose distributions onto affected tissues, and lack of dose volume histograms. A new treatment planner has been developed to reduce these limitations in brachytherapy planning. The PC-based planning system uses a CT-simulator to sequentially scan the patient to generate orthogonal images (to localize seed positions) and subsequently axially scan the patient. This sequential scanning procedure avoids using multiple independent patient scans, templates, external frames, or fiducial markers to register the reconstructed seed positions with patient contours. Dose is computed after assigning activity to (low dose rate) Ir192, linear Cs137, or I125 seeds or dwell times (high dose rate) to the Ir192 source. The planar isodose distribution is superimposed onto axial, coronal, or sagittal views of the tissues following image reconstruction. The treatment plan computes (1) direct and cumulative volume dose histograms for individual tissues, (2) the average, standard deviation, and coefficient of skewness of the dose distribution within individual tissues, (3) an average (over all tissue pixels) survival probability (S) and average survival dose DASD for a given radiation treatment, (4) normal tissue complication probability (NTCP) delivered to a given tissue. All four computed quantities account for dose heterogeneity. These estimates of the biological response to radiation from laboratory-based studies may help guide the evaluation of the prescribed low- or high-dose rate therapy in retrospective and prospective clinical studies at a number of treatment sites.

Cell kinetics and repopulation mechanisms during multifraction irradiation of spheroids

BACKGROUND AND PURPOSE

The objective of the present study was to evaluate the predictive potential of cell kinetic parameters and repopulation rates determined by flow cytometry during multifraction irradiation of spheroids, a system in which the fate of all cells can be determined with high precision. Ultimately, similar analytical techniques should provide a reproducible and prognostically significant clinical predictive assay.

MATERIALS AND METHODS

Multicellular spheroids of Chinese hamster V79 lung cells were irradiated with 2.5 Gy of 250 kVp X-rays twice daily to a total dose of 25 Gy. Repopulation parameters and cell kinetic parameters were followed throughout the irradiation period and for 5 days after completion of exposure.

RESULTS

(1) Regrowth (RG) took place early during multifraction irradiation. (2) Potential doubling time (Tpot) decreased steadily from the early part of treatment, remaining of short duration until the spheroids almost attained the pre-treatment number of clonogenic cells. (3) Accelerated repopulation was mainly due to a decreased cell loss factor (phi) and increased growth fraction (GF), although a modest decrease in cell cycle time (tc) was suggested. (4) Phi decreased during exponential RG. (5) Other parameters such as observed doubling time (td) and labelling index (LI) paralleled these findings.

CONCLUSIONS

Clonogen repopulation that began early in the irradiation scheme and accelerated rapidly is not consistent with the prevailing view that accelerated repopulation begins several weeks into clinical protocols. Also, pre-treatment Tpot did not adequately estimate the repopulation speed in the spheroids. Equivalent studies in animal tumour systems, and then in the clinic, are consequently indicated and of some urgency.

Repopulation capacity during fractionated irradiation of squamous cell carcinomas and glioblastomas in vitro

PURPOSE

Determination of clonogenic cell proliferation of three highly malignant squamous cell carcinomas (SCC) and two glioblastoma cell lines during a 20-day course of fractionated irradiation under in vitro conditions.

METHODS AND MATERIALS

Tumor cells in exponential growth phase were plated in 24-well plastic flasks and irradiated 24 h after plating with 250 kV x-rays at room temperature. Six fractions with single doses between 0.6 and 9 Gy were administered in 1.67, 5, 10, 15, and 20 days. Colony growth was monitored for at least 60 days after completion of irradiation. Wells with confluent colonies were considered as "recurrences" and wells without colonies as "controlled." The dose required to control 50% of irradiated wells (WCD50) was estimated by a logistic regression for the different overall treatment times. The effective doubling time of clonogenic cells (T[eff]) was determined by a direct fit using the maximum likelihood method.

RESULTS

The increase of WCD50 within 18.3 days was highly significant for all tumor cell lines accounting for 7.9 and 12.0 Gy in the two glioblastoma cell lines and for 12.7, 14.0, and 21.7 Gy in the three SCC cell lines. The corresponding T(eff)s were 4.4 and 2.0 days for glioblastoma cell lines and 2.4, 4.2, and 1.8 days for SCC cell lines. Population doubling times (PDT) of untreated tumor cells ranged from 1.0 to 1.9 days, showing no correlation with T(eff)s. T(eff) was significantly longer than PDT in three of five tumor cell lines. No significant differences were observed comparing glioblastomas and SCC. Increase of WCD50 with time did not correlate with T(eff) but with T(eff) InSF2 (surviving fraction at 2 Gy).

CONCLUSION

The intrinsic ability of SCC and glioblastoma cells to repopulate during fractionated irradiation could be demonstrated. Repopulation induced dose loss per day depends on T(eff) and intrinsic radiation sensitivity. Proliferation during treatment was decelerated compared to pretreatment PDT in the majority of cell lines. Pretreatment cell kinetics did not predict for tumor cell proliferation during treatment.

Accelerated repopulation during fractionated irradiation of a murine ovarian carcinoma: downregulation of apoptosis as a possible mechanism

PURPOSE

To test whether accelerated tumor clonogen repopulation occurs during continuous fractionated radiotherapy of a slow-growing mouse ovarian tumor, and if so whether the accelerated rate of repopulation is predicted by the pretreatment potential doubling time, and whether changes in apoptotic response are a possible mechanism for this change.

METHODS AND MATERIALS

The rate of clonogen production during fractionated radiotherapy was followed using the tumor-control assay, with an independent determination of the sensitivity to repeated dose fractions in vivo in the absence of repopulation. The pretreatment potential doubling time was measured by bromodeoxyuridine (BrdUrd) labeling and fluorescence measurements. The apoptotic and mitotic indices at various times during treatment were scored histologically.

RESULTS

The slow-growing (pretreatment volume doubling time 6 days) ovarian tumor OCA responds to daily irradiation with 6 Gy under hypoxia by negligible tumor clonogen production in the first few days, followed by a change at about 9 days to accelerated repopulation, after which the effective clonogen doubling time Tclon was about 2 days, near the pretreatment Tpot of 1.7 days. Alternative interpretations of the data, such as a change in radiosensitivity vs. a change in the repopulation rate or acceleration at 3 days as opposed to 9 days, were shown to be unlikely. This change was accompanied by a reduced apoptotic response (measured morphometrically).

CONCLUSIONS

When sensitivity to fractionated doses has been corrected for in vivo, this slow-growing mouse tumor exhibits a change to accelerated clonogen production during a continuous radiotherapy regimen that is accompanied or preceded by a reduced histologic apoptotic response. Tclon during accelerated repopulation was slightly longer than the pretreatment Tpot.

Changes in cell proliferative parameters of SCCVII and EMT6 murine tumors after single-dose irradiation

To understand better the repopulation kinetics of tumor cells after radiotherapy, we investigated changes in cell proliferative parameters after single-dose irradiation of SCCVII tumors in C3H/He mice and EMT6 tumors in Balb/c mice. The following parameters were determined 0-15 days after single irradiation at 20 or 30 Gy; dividing fraction (DF), potential doubling time (Tpot), number of clonogenic cells per tumor (Ncln), and volume doubling time (Tvol). DF and Tpot were determined by in vivo-in vitro cytokinesis-block assay with cytochalasin B, Ncln was measured by in vivo-in vitro colony-forming assay, and Tvol was determined by growth delay assay. In both tumors, longer Tpot and lower DF and Ncln were obtained for 3-4 days after irradiation, but in SCCVII tumors these values returned to the pretreatment levels 9 days after irradiation. In EMT6 tumors, Tpot, DF, and Ncln did not return to the pretreatment levels even 12 days after irradiation. In the regrowth phase of both tumors following irradiation at 20 Gy, Tvol was longer than the pretreatment level, although Tpot was similar in SCCVII and only slightly longer in EMT6. Therefore, the cell loss factor in the regrowth phase was considered to be higher than the pretreatment level in both tumors. From the results, recruitment of previously quiescent cells into the proliferative pool in these tumors was suggested to contribute to repopulation after radiation.

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 effect of the overall treatment time of fractionated irradiation on the tumor control probability of a human soft tissue sarcoma xenograft in nude mice

PURPOSE

To study the impact of the overall treatment time of fractionated irradiation on the tumor control probability (TCP) of a human soft tissue sarcoma xenograft growing in nude mice, as well as to compare the pretreatment potential doubling time (Tpot) of this tumor to the effective doubling time (Teff) derived from three different schedules of irradiation using the same total number of fractions with different overall treatment times.

METHODS AND MATERIALS

The TCP was assessed using the TCD50 value (the 50% tumor control dose) as an end point. A total of 240 male nude mice, 7-8 weeks old were used in three experimental groups that received the same total number of fractions (30 fractions) with different overall treatment times. In group 1, the animals received three equal fractions/day for 10 consecutive days, in group 2 they received two equal fractions/day for 15 consecutive days, and in group 3 one fraction/day for 30 consecutive days. All irradiations were given under normal blood flow conditions to air breathing animals. The mean tumor diameter at the start of irradiation was 7-8 mm. The mean interfraction intervals were from 8-24 h. The Tpot was measured using Iododeoxyuridine (IudR) labeling and flow cytometry and was compared to Teff.

RESULTS

The TCD50 values of the three different treatment schedules were 58.8 Gy, 63.2 Gy, and 75.6 Gy for groups 1, 2, and 3, respectively. This difference in TCD50 values was significant (p < 0.05) between groups 1 and 2 (30 fractions/10 days and 30 fractions/15 days) vs. group 3 (30 fractions/30 days). The loss in TCP due to the prolongation of the overall treatment time from 10 days to 30 days was found to be 1.35-1.4 Gy/day. The pretreatment Tpot (2.4 days) was longer than the calculated Teff in groups 2 and 3 (1.35 days).

CONCLUSION

Our data show a significant loss in TCP with prolongation of the overall treatment time. This is most probably due to an accelerated repopulation of tumor clonogens. The pretreatment Tpot of this tumor model does not reflect the actual doubling of the clonogens in a protracted regimen.

The decreased influence of overall treatment time on the response of human breast tumor xenografts following prolongation of the potential doubling time (Tpot)

PURPOSE

Repopulation during fractionated radiotherapy has been postulated to result in a significant loss in local control in rapidly proliferating tumors. Clinical data suggest that accelerated fractionation schedules can overcome the influence of repopulation by limiting the overall treatment time. Unfortunately, accelerated therapy frequently leads to increased acute reactions, which may become dose limiting. An alternative to accelerated fractionation would be to decrease the rate of repopulation during therapy. To test the potential efficacy of this alternative, we examined the effect of reducing tumor proliferation rate on the response of MCF-7 human breast carcinoma xenografts treated with a short vs. a long course of fractionated therapy. To reduce the proliferation rate, we deprived nude mice transplanted with MCF-7 xenografts of the growth-stimulating hormone estradiol (E2). We have previously reported that E2 deprivation increases the potential doubling time (Tpot) for MCF-7 xenografts from a mean of 2.6 days to 5.3 days (p < 0.001).

METHODS AND MATERIALS

E2-stimulated and E2-deprived MCF-7 breast carcinoma xenografts were clamped hypoxically and irradiated with four fractions of 5 Gy each, using either a short (3-day) or long (9-day) treatment course. E2 stimulation was restored in all animals at the completion of irradiation. Radiation response was determined by regrowth time and regrowth delay of the irradiated tumors as compared to unirradiated controls.

RESULTS

Prolongation of therapy in rapidly proliferating, E2-stimulated tumors (Tpot approximately 2.6 days) resulted in a significant decrease in regrowth time in two identical experiments. With results pooled for analysis, the regrowth times for the short and long treatments were 62 and 32 days, respectively (combined p < 0.001). The shorter regrowth times suggest that there was less overall tumor damage with the longer fractionated radiotherapy course. No significant difference in regrowth time was observed in the more slowly proliferating, E2-deprived tumors (Tpot approximately 5.3 days) treated with either the short or long regimen. Median regrowth times were 48 and 54.5 days for the short and long treatments, respectively (combined p = 0.14). Similar changes were observed in regrowth delay.

CONCLUSIONS

Reduction in the rate of cell proliferation, induced by E2 deprivation in MCF-7 human breast xenografts during fractionated radiotherapy, resulted in a significantly decreased dependence on overall treatment time in comparison to the more rapidly proliferating E2-stimulated tumors. This model suggests that pharmacologically induced reduction in the rate of tumor cell proliferation during a course of fractionated radiotherapy may be a viable alternative to accelerated fractionation for the treatment of rapidly proliferating tumors.

Repopulation kinetics during fractionated irradiation and the relationship to the potential doubling time, Tpot

PURPOSE

Therapeutic outcome may be adversely affected by repopulation in solid tumors during fractionated irradiation. It has been proposed that the repopulation rate of the surviving cells may be reflected by the pretreatment potential doubling time (Tpot). This concept has been examined by comparing pretreatment Tpot measurements to repopulation monitored in five transplantable murine tumors during fractionated radiation treatment.

METHODS AND MATERIALS

Up to nine fractions of 2 Gy were given to clamped tumors on a 6 h schedule, which allowed adequate time for repair, or on a 24 h schedule, which incorporated more time for repopulation. Tumors were removed from treatment at various times and tumor cell survival was analyzed using an excision assay. The ratio of the cell survival in tumors treated with the same total dose on the two different fractionation schedules (24 h/6 h) was used to calculate an effective doubling time for repopulation during the treatment (Teff). Potential doubling time was assessed in untreated tumors by giving the tumor-bearing animals 5-bromodeoxyuridine (BrdUrd) and, at various times later, removing the tumors for flow cytometric analysis. Tpot values were calculated by two different widely used methods.

RESULTS

For four tumors (RIF-1, KHT-C, KHT-LP1, and B16-F1), the Teff was greater than Tpot indicating that repopulation was not as rapid as suggested by Tpot. For SCC-VII, the only carcinoma tested, Teff was smaller than Tpot indicating that repopulation was more rapid than predicted by Tpot. Individual estimates of Tpot made from single tumor samples taken at different times after BrdUrd administration varied by factors of 2 to 7 for the different tumors.

CONCLUSION

These findings indicate a need for caution in applying measurements of Tpot for prediction of regrowth rates in individual patients' tumors.

Radiation effects on S-phase duration, labelling index, potential doubling time and DNA distribution in head and neck cancer xenografts

The effect of irradiation on S-phase duration (Ts), labelling index (LI), potential doubling time (Tpot), and cell cycle phase distributions was determined by DNA flow cytometry in xenografted human squamous cell carcinoma of the head and neck (SCCHN). Tumours were treated with a single dose of 3 Gy, and excised at intervals over a 90-h period. Six hours before each excision the tumours were labelled in vivo with bromodeoxyuridine (BrdUrd). Although the growth rate of irradiated tumours was comparable with that of untreated controls, analysis of BrdUrd uptake revealed a transient reduction of LI and a prolongation of Ts in irradiated tumours. Maximum mean Tpot was 931 days in irradiated tumours as compared to 13 days in untreated controls. The variations in Ts, LI and Tpot all occurred within the first hours after irradiation; during the remainder of the observation time, the values of the variables did not differ from those of untreated controls. In irradiated tumours the distribution of cells according to DNA content changed significantly on three occasions during the observation period: 1) Parallel to the initial lowering of LI and prolongation of Ts there was a transient increase in the proportion of cells in G0/G1 and a decrease in the proportion of cells in S and G2; 2) At 18 h, the most pronounced cell cycle phase redistribution occurred when the G0/G1 fraction decreased and the S and G2 phase fractions increased; 3) At 66 h (i.e., approximately one cell cycle later), the pattern was the same as that after 18 h. The findings suggest that the transient prolongation of DNA replication seen in SCCHN cells immediately after a single radiation dose is a symptom of DNA damage inflicted during late G1 or early S-phase, and that this disturbance in DNA synthesis is associated with the subsequent accumulation of cells in G2 phase.

Tumor cell kinetics using two labels and flow cytometry

A flow cytometry based, single sampling method employing sequential bromodeoxyuridine (BrdUrd) and iododeoxyuridine (IdUrd) labeling with an intervening interval is examined. BrdUrd-specific and nonspecific monoclonal antibodies and flow cytometry are used to estimate the duration of DNA synthesis and the potential doubling time in tumor cell populations using a single sampling or biopsy. A correction for labeled, divided cells allows postlabel incubation intervals to exceed the length of the G2/M phase of the cell cycle. The method yields reliable results when tested in experimental tumor systems in vitro and in situ, as well as in nine human tumors labeled in situ. It uses a somewhat simpler analysis than the alternative relative movement method, requiring only labeling indices, rather than both a labeling index and relative movement, but doses require the administration of two, rather than one, label. It provides an independent verification of and a useful single sampling alternative to the relative movement method for the estimation of the length of S phase and, from this, the potential doubling time in experimental or clinical human tumors.

Potential doubling time in head and neck tumors treated by primary radiotherapy: preliminary evidence for a prognostic significance in local control

PURPOSE

The aim of the study was to determine preliminarily whether cell kinetic parameters evaluated using in vivo infusion of bromodeoxyuridine (BrdUrd) and flow cytometry, play a role as prognostic factors of loco-regional control in squamous cell head and neck carcinoma treated with radiotherapy.

METHODS AND MATERIALS

Between April 1989 and December 1991, 42 patients with unresectable Stage II-IV squamous cell carcinoma of the oral cavity, pharynx or larynx were given an infusion of BrdUrd solution prior to primary tumor biopsy sampling at 4-6 hr later. The simultaneous labeling S-phase fraction (LI) and duration (Ts) as well as the estimated potential doubling time (Tpot) were measured using flow cytometric analysis of BrdUrd and DNA content. Twenty-six patients received standard radiotherapy (70 Gy/35 fractions/7 weeks) whereas 15 patients were treated with the concomitant boost technique (75 Gy/40 fractions/6 weeks).

RESULTS

A complete set of flow cytometric data was available for 31 patients. The median value of LI, Ts, and Tpot were 9%, 9 hr and 5 days, respectively. Univariate analysis among the patients treated homogeneously by standard radiotherapy, indicated that local control was affected by Tpot value (p = 0.02). When the same analysis was performed for the patients treated with either standard radiotherapy or concomitant boost regimen, we found a p = 0.04. Thus, patients with a tumor Tpot value < or = 5 days had a significantly lower three-year local control than patients with Tpot > 5 days. Log-rank test univariate analysis showed, in addition, that nodal status was the strongest prognostic factor of local control (p = 0.005). Age, tumor stage, tumor site, performance status, grading, radiotherapy regimen, DNA ploidy and LI value were, instead, not significantly related to loco-regional control. Finally, when comparing the type of radiotherapy for tumors with Tpot < or = 5 days, we found a trend toward a better local control after concomitant boost regimen, with respect to standard regimen (p = 0.06).

CONCLUSION

The present preliminary results suggest that Tpot could play a role as additional prognostic factor influencing the disease outcome in head and neck carcinoma treated by radiotherapy.

New approaches to cell kinetics in human tumors

Both clinical and laboratory evidence indicate that local control rates for many experimental and clinical human tumors decrease with protraction of the overall duration of radiation therapy. A likely basis for this is tumor cell repopulation during treatment. Such observations have stimulated interest in tumor kinetics, and a number of techniques have evolved to 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 inhibition of proliferation--that could be employed in attempting to overcome such intratreatment proliferation, explores both past and newer, evolving methods available for measuring tumor cytokinetics, and discusses how such kinetic information could be used in the future to tailor therapy to a tumor's individual characteristics.

Hyperfractionation versus conventional fractionation in oropharyngeal carcinoma: final analysis of a randomized trial of the EORTC cooperative group of radiotherapy

EORTC protocol 22791 compared once daily fractionation (CF) of 70 Gy in 35-40 fractions in 7-8 weeks, to pure hyperfractionation (HF) of 80.5 Gy in 70 fractions in 7 weeks using 2 fractions of 1.15 Gy per day, in T2-T3 oropharyngeal carcinoma (excluding base of tongue), N0,N1 of less than 3 cm. From 1980 to 1987, 356 patients were entered. In the final analysis (June 1990), the local control was significantly higher (p = 0.02 log-rank) after HF compared with CF. At 5 years, 59% of patients are local disease-free in the HF arm compared to 40% in the CF arm. The superiority of HF was demonstrated in patients staged T3N0,T3N1 but not in T2. The Cox model confirmed that the treatment regimen was an independent significant prognostic factor for locoregional control (p = 0.007 log-rank). This improvement of locoregional control was responsible for a trend to an improved survival (p = 0.08 log-rank). There was no difference in late normal tissue damage between the two treatment modalities.