The in vitro radiosensitivity of human head and neck cancers

Towards Data Driven RT Prescription: Integrating Genomics into RT Clinical Practice

The genomic era has significantly changed the practice of clinical oncology. The use of genomic-based molecular diagnostics including prognostic genomic signatures and new-generation sequencing has become routine for clinical decisions regarding cytotoxic chemotherapy, targeted agents and immunotherapy. In contrast, clinical decisions regarding radiation therapy (RT) remain uninformed about the genomic heterogeneity of tumors. In this review, we discuss the clinical opportunity to utilize genomics to optimize RT dose. Although from the technical perspective, RT has been moving towards a data-driven approach, RT prescription dose is still based on a one-size-fits all approach, with most RT dose based on cancer diagnosis and stage. This approach is in direct conflict with the realization that tumors are biologically heterogeneous, and that cancer is not a single disease. Here, we discuss how genomics can be integrated into RT prescription dose, the clinical potential for this approach and how genomic-optimization of RT dose could lead to new understanding of the clinical benefit of RT.

Dynamic Characteristics and Predictive Capability of Tumor Voxel Dose-Response Assessed Using 18F-FDG PET/CT Imaging Feedback

Purpose

Tumor voxel dose–response matrix (DRM) can be quantified using feedback from serial FDG-PET/CT imaging acquired during radiotherapy. This study investigated the dynamic characteristics and the predictive capability of DRM.

Methods

FDG-PET/CT images were acquired before and weekly during standard chemoradiotherapy with the treatment dose 2 Gy × 35 from 31 head and neck cancer patients. For each patient, deformable image registration was performed between the pretreatment/baseline PET/CT image and each weekly PET/CT image. Tumor voxel DRM was derived using linear regression on the logarithm of the weekly standard uptake value (SUV) ratios for each tumor voxel, such as SUV measured at a dose level normalized to the baseline SUV₀. The dynamic characteristics were evaluated by comparing the DRM_i estimated using a single feedback image acquired at the ith treatment week (i = 1, 2, 3, or 4) to the DRM estimated using the last feedback image for each patient. The predictive capability of the DRM estimated using 1 or 2 feedback images was evaluated using the receiver operating characteristic test with respect to the treatment outcome of tumor local–regional control or failure.

Results

The mean ± SD of tumor voxel SUV measured at the pretreatment and the 1st, 2nd, 3rd, 4th, and last treatment weeks was 6.76 ± 3.69, 5.72 ± 3.43, 3.85 ± 2.22, 3.27 ± 2.25, 2.5 ± 1.79, and 2.23 ± 1.27, respectively. The deviations between the DRM_i estimated using the single feedback image obtained at the ith week and the last feedback image were 0.86 ± 4.87, −0.06 ± 0.3, −0.09 ± 0.17, and −0.09 ± 0.12 for DRM₁, DRM₂, DRM₃, and DRM₄, respectively. The predictive capability of DRM₃ and DRM₄ was significant (p < 0.001). The area under the curve (AUC) was increased with the increase in treatment dose level. The DRMs constructed using the single feedback image achieved an AUC of 0.86~1. The AUC was slightly improved to 0.94~1 for the DRMs estimated using 2 feedback images.

Conclusion

Tumor voxel metabolic activity measured using FDG-PET/CT fluctuated noticeably during the first 2 treatment weeks and obtained a stabilized reduction rate thereafter. Tumor voxel DRM constructed using a single FDG-PET/CT feedback image after the 2nd treatment week (>20 Gy) has a good predictive capability. The predictive capability improved continuously using a later feedback image and marginally improved when two feedback images were applied.

Inter/intra-tumoral dose response variations assessed using FDG-PET/CT feedback images: Impact on tumor control and treatment dose prescription

PURPOSE

To quantify inter/intra-tumoral variations of baseline metabolic activity and dose response. To evaluate their impact on tumor control and treatment dose prescription strategies.

METHODS AND MATERIALS

Tumor voxel baseline metabolic activity, SUV₀, and dose response matrix, DRM, quantified using the pre-treatment and weekly FDG-PET/CT imaging feedback for each of 34 HNSCC patients (25 HPV+ and 9 HVP-) were evaluated. Inter/intra-tumoral variations of tumor voxel (SUV₀, DRM) for each of the HPV- and HPV+ tumor groups were quantified and used to evaluate the variations of individual tumor control probabilities and the efficiency of uniform vs non-uniform treatment dose prescription strategies.

RESULTS

Tumor voxel dose response variation of all tumor voxels assessed using FDG-PET/CT imaging feedback had the mean(CV) = 0.47(47%), which was consistent with those of previously published in vitro tumor clonogenic assay. The HPV- tumors had the mean(CV) dose response, 0.53(49%), significantly larger than those of the HPV+ tumors, 0.45(43%). However, their baseline SUVs were opposite, 6.5(56%) vs 7.7(65%). Comparing to the inter-tumoral variations, both HPV-/+ tumor groups showed larger intra-tumoral variations, (53%, 58%) vs (20%, 31%) for the baseline SUV and (38%, 37%) vs (31%, 21%) for the dose response. Due to the large dose response variations, treatment dose to control the tumor voxels has very broad range with CV of TCD₅₀ = 97% for the HPV- and 67% for the HPV+ tumor group respectively. As a consequence, heterogeneous prescription dose could potentially reduce the treatment integral dose for 92% of the HPV+ tumors and 78% of the HPV- tumors.

CONCLUSIONS

The study demonstrates that tumor dose response assessed using FDG-PET/CT feedback images had a similar distribution to those assessed conventionally using in vitro tumor clonogenic assay. Inter-tumoral dose response variation seems larger for HPV- tumors, but intra-tumoral dose response variations are similar for both HPV groups. These variations cause very large variation on the individual tumor control probability and limit the efficacy of dose escalation and de-escalation in conventional clinical practice. On the other hand, heterogeneous dose prescription guided by metabolic imaging feedback has a potential advantage in radiotherapy.

Tumor Voxel Dose-Response Matrix and Dose Prescription Function Derived Using 18F-FDG PET/CT Images for Adaptive Dose Painting by Number

PURPOSE

To construct a tumor voxel dose response matrix (DRM) and dose prescription function (DPF) for adaptive dose painting by number (DPbN) based on treatment feedback of fluoro-2-deoxyglucose (FGD) positron emission tomography (PET)/computed tomography (CT) imaging.

METHODS AND MATERIALS

FDG-PET/CT images obtained before and after chemoradiation therapy and at weekly chemoradiation therapy sessions for each of 18 patients with head and neck cancer, as well as the treatment outcomes, were used in the modeling. All weekly and posttreatment PET/CT images were registered voxel-to-voxel to the corresponding pretreatment baseline PET/CT image. Tumor voxel DRM was created using serial FDG-PET imaging of each patient with respect to the baseline standardized uptake value (SUV₀). A tumor voxel control probability (TVCP) lookup table was created using the maximum likelihood estimation on the tumor voxel (SUV₀, DRM) domain of all tumors. Tumor voxel DPF was created from the TVCP lookup table and used as the objective function for DPbN-based inverse planning optimization.

RESULTS

Large intertumoral and intratumoral variations on both tumor voxels (SUV₀, DRM) were identified. Tumor voxel dose resistance did not show correlation with its baseline SUV₀ value and was the major cause of the tumor local failures. Tumor voxel DPF as the function of tumor voxel (SUV₀, DRM) values also showed a very large intertumoral and intratumoral heterogeneity. Most human papillomavirus-negative tumors require a treatment dose >100 Gy to certain local tumor regions. These treatment doses, which are most unlikely to be implementable in conventional radiation therapy, can be achieved using adaptive DPbN treatment. Clinical feasibility was evaluated by comparing the adaptive DPbN treatment plan with the conventional intensity modulated radiation therapy plan.

CONCLUSIONS

Tumor voxel (SUV₀, DRM) provides an intratumoral prognostic map to target tumor locoregional-resistant regions. The corresponding TVCP or DPF provides a quantitative objective to optimize the intratumoral dose distribution for the individuals. The adaptive DPbN with FDG-PET/CT imaging feedback is feasible to implement in clinics.

Feasibility of Primary Tumor Culture Models and Preclinical Prediction Assays for Head and Neck Cancer: A Narrative Review

Primary human tumor culture models allow for individualized drug sensitivity testing and are therefore a promising technique to achieve personalized treatment for cancer patients. This would especially be of interest for patients with advanced stage head and neck cancer. They are extensively treated with surgery, usually in combination with high-dose cisplatin chemoradiation. However, adding cisplatin to radiotherapy is associated with an increase in severe acute toxicity, while conferring only a minor overall survival benefit. Hence, there is a strong need for a preclinical model to identify patients that will respond to the intended treatment regimen and to test novel drugs. One of such models is the technique of culturing primary human tumor tissue. This review discusses the feasibility and success rate of existing primary head and neck tumor culturing techniques and their corresponding chemo- and radiosensitivity assays. A comprehensive literature search was performed and success factors for culturing in vitro are debated, together with the actual value of these models as preclinical prediction assay for individual patients. With this review, we aim to fill a gap in the understanding of primary culture models from head and neck tumors, with potential importance for other tumor types as well.

Abrogation of radioresistance in glioblastoma stem-like cells by inhibition of ATM kinase

Resistance to radiotherapy in glioblastoma (GBM) is an important clinical problem and several authors have attributed this to a subpopulation of GBM cancer stem cells (CSCs) which may be responsible for tumour recurrence following treatment. It is hypothesised that GBM CSCs exhibit upregulated DNA damage responses and are resistant to radiation but the current literature is conflicting. We investigated radioresistance of primary GBM cells grown in stem cell conditions (CSC) compared to paired differentiated tumour cell populations and explored the radiosensitising effects of the ATM inhibitor KU-55933. We report that GBM CSCs are radioresistant compared to paired differentiated tumour cells as measured by clonogenic assay. GBM CSC's display upregulated phosphorylated DNA damage response proteins and enhanced activation of the G2/M checkpoint following irradiation and repair DNA double strand breaks (DSBs) more efficiently than their differentiated tumour cell counterparts following radiation. Inhibition of ATM kinase by KU-55933 produced potent radiosensitisation of GBM CSCs (sensitiser enhancement ratios 2.6-3.5) and effectively abrogated the enhanced DSB repair proficiency observed in GBM CSCs at 24 h post irradiation. G2/M checkpoint activation was reduced but not abolished by KU-55933 in GBM CSCs. ATM kinase inhibition overcomes radioresistance of GBM CSCs and, in combination with conventional therapy, has potential to improve outcomes for patients with GBM.

Predictive value of modelled tumour control probability based on individual measurements of in vitro radiosensitivity and potential doubling time

OBJECTIVE

The aim of this study was to compare patient-specific radiobiological parameters with population averages in predicting the clinical outcome after radiotherapy (RT) using a tumour control probability (TCP) model based on the biological effective dose (BED).

METHODS

A previously published study of 46 head and neck carcinomas with individually identified radiobiological parameters, radiosensitivity and potential doubling time (Tpot), and known tumour size was investigated. These patients had all been treated with external beam RT, and the majority had also received brachytherapy. The TCP for each individual based on the BED using patient-specific radiobiological parameters was compared with the TCP based on the BED using average radiobiological parameters (α=0.3 Gy(-1), Tpot=3 days).

RESULTS

43 patients remained in the final analysis. There was only a weak trend for increasing local tumour control with increasing BED in both groups. However, when the TCP was calculated, the use of patient-specific parameters was better for identifying local control correctly. The sensitivity and specificity for tumour-specific parameters were 63% and 80%, respectively. The corresponding values for population-based averages were 0% and 91%, respectively. The positive predictive value was 92% when tumour-specific parameters were used compared with 0% for population-based averages. A receiver operating characteristic curve confirmed the superiority of patient-specific parameters over population averages in predicting local control.

CONCLUSION

Individual radiobiological parameters are better than population-derived averages when used in a mathematical model to predict TCP after curative RT in head and neck carcinomas.

ADVANCES IN KNOWLEDGE

TCP based on individual radiobiological parameters is better than TCP based on population-based averages for identifying local control correctly.

Clinically relevant biomarkers in targeted radiotherapy

Three classic parameters have been recognized as predictors or biomarkers of radiation response: intrinsic radiosensitivity, degree of hypoxia and repopulation capacity of clonogenic cells during a course of fractionated radiation therapy. Although good functional assays exist to measure these tumor parameters, and their use has led to the understanding of factors affecting outcome after radiotherapy, their application in clinical practice is hampered by technical difficulties, the length of time needed to obtain results and the lack of prospective randomized clinical trials. Recently, with the progress in molecular biology, genome-wide screening methods have been used to look for genetic signatures that can distinguish between good and bad outcome after radiotherapy. One of the most promising candidates is the epidermal growth factor receptor which is overexpressed or mutated in a variety of malignancies, such lung and head and neck cancer. Inhibition of this receptor has led to radio-sensitization with the prolongation of median survival in several cancers. Since there is significant variability in the response of patients with the same disease to radiotherapy, it would be very valuable to be able to predict which patients would benefit from a molecularly targeted therapy administered with concomitant radiation in order to increase the response rate (and cure rate) of those patients with radioresistant tumors. Optimally, this assay should be able to provide results in an efficient and reproducible manner and detect tumor genetic mutations that would provide specificity to the intervention. One approach currently in clinical practice to overcome intrinsic radioresistance and repopulation is stereotactic body radiotherapy coupled with image-guided radiation, a highly precise and powerful form of radiation, allowing radiation oncologist to treat tumors with more aggressive biological doses of radiation without causing serious normal tissues injury.

A mathematical model of the treatment and survival of patients with high-grade brain tumours

More years of life per patient are lost as the result of primary brain tumours than any other form of cancer. The most aggressive of these is known as glioblastoma (GBM). The median survival time of patients with GBM is under 10 months and the outlook has hardly improved over the past 20 years. Generally, these tumours are remarkably resistant to radiotherapy and yet about 2-3% of all GBMs appear to be cured. The objectives of this study were to formulate a mathematical and phenomenological model of tumour growth in a population of patients with GBM to predict survival, and to use the model to extract biological information from clinical data. The model describes the growth of the tumour and the resulting damage to the normal brain using simple concepts borrowed from chemical reaction engineering. Death is assumed to result when the amount of surviving normal brain falls to a critical level. Radiotherapy is assumed to destroy tumour but not healthy brain. Simple rules are included to represent approximately the clinician's decisions about what type of treatment to offer each patient. A population of patients is constructed by assuming that key parameters can be sampled from statistical distributions. Following Monte Carlo simulation, the model can be fitted to data from clinical trials. The model reproduces clinical data extremely accurately. This suggests that the long-term survivors are not a separate sub-population but are the 'lucky tail' of a unimodal distribution. The estimated values of radiation sensitivity (represented as SF2, the survival fraction after 2Gy) suggest the presence of severe hypoxia, which renders cells less sensitive to radiation. The model can predict the probable age distribution of tumours at presentation. The model shows the complicated effects of waiting times for treatment on the survival outcomes, and is used to predict the effects of escalation of radiotherapy dose. The model may aid the design of clinical trials using radiotherapy for patients with GBM, especially in helping to estimate the size of trial required. It is also designed in a generic form, and might be applicable to other tumour types.

Mathematical Modelling of Survival of Glioblastoma Patients Suggests a Role for Radiotherapy Dose Escalation and Predicts Poorer Outcome After Delay to Start Treatment

AIMS

The outcome of patients with glioblastoma (GBM) remains extremely poor. We have developed a mathematical model, using pathological and radiation biology concepts, to assess the detrimental effect of delay to start radiotherapy, the possible benefit from dose escalation, and to extract biological data from clinical data.

MATERIALS AND METHODS

Survival data were available for 154 adult patients with GBM treated in our centre with curative intent to a dose of 60 Gy in 30 fractions between 1996 and 2002. Survival data for 129 patients from the 60 Gy arm of the MRC BR02 randomised trial of radiotherapy dose were obtained for comparison. The model generates the equivalent of individual patients with a brain tumour, and produces an explicit outcome, either death or survival. The tumour, assumed to be growing exponentially, causes normal cell damage in the brain, and death occurs when the number of normal brain cells falls below a critical level. The outcome for an individual patient is determined by values of the variables assigned by the model. Parameters for the single patient include tumour doubling time, surviving fraction of tumour cells after each fraction of radiotherapy, and a waiting time from presentation to the start of radiotherapy. A surrogate for performance status is implemented, using a rule that rejects patients whose tumours are too advanced at presentation to be suitable for radical radiotherapy. Values for the parameters that determine individual patient outcome are randomly assigned from a set of probability distributions, using Monte Carlo simulation. The simulation constructs survival results for a population, typically 2000 individuals. The descriptors of the probability distributions that are used to determine the parameters that define the patient characteristics are adjusted to optimise the fit of the modelled population to real clinical data, using a combination of folding polygon and simulated annealing techniques.

RESULTS

The model fits the clinical data well. The results suggest that the surviving fraction of tumour cells after a radiation dose of 2 Gy (SF2) does influence patient outcome. The mean in vivo SF2 for the Addenbrooke's data is 0.80, implying that hypoxia is a serious problem in radiotherapy for GBM. The Addenbrooke's data suggest a mean tumour doubling time of 24 days, so that a delay to start radiotherapy would be expected to have an adverse effect. Considering patients by treatment intent, median survival plummets as delay increases, and almost no patients survive long term after a 70-day delay. Radiotherapy dose escalation has an important predicted effect on survival. Assuming that the treatment could be delivered safely, a dose of 74 Gy, given at 2 Gy/fraction, would extend the survival of all patients. The proportion of long-term survivors would increase, from 2.4% with 60 Gy, to 6.4% with 74 Gy. The model can be used to derive gamma50, which has a value of 0.42, lower than the typical value of 1-2.

CONCLUSION

Using the model, we have extracted biological information from clinical data. The model could be used to assess the potential benefit, or lack of benefit, from a proposed radiotherapy trial, and to estimate the necessary size. It shows that a single modality is unlikely to achieve a major improvement in long-term survival, although radiotherapy dose escalation should have a role, provided it can be given safely. The model could be extended to include chemotherapy, bio-reductive drugs, or gene therapy.

Reduced DNA-dependent protein kinase activity in two cell lines derived from adult cancer patients with late radionecrosis

Epstein-Barr virus-immortalized lymphoblastoid cell lines were derived from five patients with late radionecrosis. Two of these cell lines exhibited postradiation viability levels intermediate between normal cell lines and that from an individual with ataxia telangiectasia. Compared with controls, these two cell lines exhibited impaired ability to rejoin DNA double-strand breaks on pulsed-field gel electrophoresis and 6-10-fold reduced DNA-dependent protein kinase (DNA-PK) activity in vitro in cell-free extracts. Immunoblotting showed normal levels of Ku70, Ku80 and XRCC4 and the presence of DNA-PKcs in both cell lines. These findings suggest that DNA-PK might be an important factor affecting the predisposition of radiotherapy patients to late radionecrosis.

Tumor radiosensitivity (SF2) is a prognostic factor for local control in head and neck cancers

PURPOSE

To evaluate prospectively the prognostic value of SF2 for local control and survival in patients undergoing radiation therapy for head and neck cancers.

METHODS AND MATERIALS

Following informed consent tumor specimens were obtained from 156 patients with primary carcinomas of the head and neck region. The specimens were assessed for the ability to grow in vitro (colony forming efficiency, CFE) and inherent radiosensitivity measured as the surviving fraction at 2 Gy (SF2) using a soft-agar clonogenic assay. Patients were treated mainly with neoadjuvant chemotherapy plus radiation therapy usually as a combination of accelerated external beam and interstitial radiotherapy. The probabilities of local control and survival were analyzed by univariate, bivariate and Cox multivariate analyses.

RESULTS

Successful growth was achieved in 110/156 specimens and SF2 values were obtained from 99/156. Eighty four out of these patients underwent radical treatment. The median SF2 value for the 84 tumors was 0.40. At a mean follow-up time of 25 months (range 7-65) the median SF2 value of tumors from 14 patients who developed local recurrence was 0.53, which was significantly higher than the median of 0.38 for tumors from 70 patients without local recurrence (p = 0.015). Tumor SF2 was a significant prognostic factor for local control (p = 0.036), but not for overall survival (p = 0.20). Tumor SF2 was an independent prognostic factor for local control within bivariate and Cox multivariate analyses.

CONCLUSIONS

This study has shown that tumor radiosensitivity measured as SF2 is a significant prognostic factor for local control in head and neck cancers.

Time factors in larynx tumor radiotherapy: lag times and intertumor heterogeneity in clinical datasets from four centers

PURPOSE

To use the time-dependent linear-quadratic model, both in the standard form and in a form modified to incorporate intertumor heterogeneity, in a reanalysis of 4 datasets for larynx tumor control, to provide more representative and direct estimates of the lag period, the time factor (lambda/alpha), and the clonogen population inactivation dose ([lnk]/alpha).

METHODS AND MATERIALS

The data comprised 2,225 patients treated in Edinburgh (UK), Glasgow (UK), Manchester (UK), or Toronto (Canada), with tumor control assessed after at least 2 years. Heterogeneity in each series was taken into account using the coefficient of variation (CV) of the clonogen radiosensitivity (alpha). Maximum likelihood techniques were used to provide best estimates of the parameters, and also direct estimation of the more stable parameter ratios of interest.

RESULTS

The use of different heterogeneity factors for the different series allowed common dose/time parameters to be fitted across all four series in a way not possible using the standard model, enabling the inherent effect of heterogeneity in flattening dose-response curves and in reducing time factors to be separated from the underlying more-representative values. Radiosensitivity CVs were calculated to be 30% (Edinburgh), 36% (Glasgow), 40% (Manchester), and 71% (Toronto). The lag phase was 32 days (95% CL 20-38 days) which was longer than the value of 23 days (11-36 days) deduced using the standard model without the heterogeneity parameter. The time factor was 1.2 (0.8-2.2) Gy/day, again greater than the value of 0.80 (0.54-1.41) Gy/day derived using the standard model. Similar larger time factors and longer lag periods could be reproduced using the standard model either by using a parameterization based on parameter ratios, or by omitting the discordant Toronto data and refitting just the data from the three UK centers.

CONCLUSION

It was concluded that the heterogeneity model provides a better representation of the time factor for tumor control when data are analyzed comprising different stages of disease treated at different centers. The model allows different amounts of heterogeneity in different series, which tend to flatten dose-responses curves and reduce time factors, to be taken in to account. Also, direct maximum likelihood estimates can be made of the lag period, the time factor (lambda/alpha), and the fractionation sensitivity (beta/alpha), as well as the clonogen population inactivation dose (lnk)/alpha. Values of these parameter ratios are more robust and stable than the individual parameter values. The results of the present analysis using a total of 2,225 patients from four centers indicate that the average lag period may be somewhat longer and the average time factor somewhat greater (and the 95% confidence limits of the time factor exclude previous estimates), than the values deduced previously using simpler models and more diverse multi-center datasets.

The immunohistochemical expression of DNA-PKCS and Ku (p70/p80) in head and neck cancers: relationships with radiosensitivity

PURPOSE

The DNA-PK complex is one of the major pathways by which mammalian cells respond to DNA double-strand breaks induced by ionizing radiation. This study evaluated the relationship between the immunohistochemical expression of the individual components of DNA-PK and cellular radiosensitivity in head and neck cancers.

METHODS AND MATERIALS

Biopsies from patients with previously untreated squamous cell carcinomas of the head and neck were assessed for inherent tumor radiosensitivity measured as the surviving fraction at 2 Gy (SF2) using a soft agar clonogenic assay. Paraffin-embedded tumor material from 64 successfully grown specimens was immunohistochemically stained for expression of DNA-PKcs and Ku (p70/p80). The same tumor material was previously analyzed for the immunohistochemical expression of p53.

RESULTS

A significant correlation was found between the degree of expression of DNA-PKcs and Ku (p70/p80) (r = 0.55, p<0.001). There were no overall significant differences in the levels of expression of DNA-PKcs and Ku (p70/p80) in tumors from patients of either sex, different sites, histologies, and stages. No relationship was found between SF2 and the expression of either DNA-PKcs (r = 0.22, p = 0.081) or Ku (p70/p80) (r = 0.064, p = 0.62). Comparison with previous immunohistochemical characterization showed no significant correlations between the expression levels of p53 and either DNA-PKcs (r = 0.093, p = 0.46) or Ku (p70/p80) (r = -0.17, p = 0.17).

CONCLUSIONS

This study suggests that determining the immunohistochemical expression of DNA-PK in head and neck cancers from multiple sites does not have a role as a predictive assay of tumor in vitro radiosensitivity.

The lack of correlation between proliferation (Ki-67, PCNA, LI, Tpot), p53 expression and radiosensitivity for head and neck cancers

A study was made of the relationship between measurements of radiosensitivity versus proliferation and p53 status in head and neck cancers. Inherent tumour radiosensitivity was assessed as surviving fraction at 2 Gy (SF2) using a clonogenic soft agar assay (n = 77). The results were compared to data on proliferation obtained by both flow cytometry (labelling index (LI), the potential doubling time (Tpot) n = 55) and immunohistochemistry (Ki-67 and PCNA; n = 68), together with immunohistochemical p53 expression (n = 68). There were no overall significant differences in the median values of the various parameters analysed for the different sites within the head and neck region, disease stages, grades of tumour differentiation or nodal states. A subgroup analysis showed that oropharyngeal (n = 22) versus oral cavity (n = 35) tumours were more radiosensitive (P = 0.056) and had a higher Ki-67 index (P = 0.001). Node-positive tumours had higher LI (P = 0.021) and a trend towards lower Tpot (P = 0.067) values than node-negative ones. No correlations were seen between SF2 and any of the parameters studied. The long-standing dogma of an increased radiosensitivity of rapidly proliferating cells in contrast to slowly proliferating cells was not confirmed. The study shows that parallel measurements of different biological markers can be obtained for a large number of patients with head and neck cancers. The independence of the various parameters studied suggests that there may be potential for their combined use as prognostic factors for the outcome of radiotherapy.

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.

Discrimination of human tumor radioresponsiveness using low-dose rate irradiation

PURPOSE

Evaluation of the theoretical and practical value of using low-dose rate (LDR) irradiation to increase the resolution of radiosensitivity testing of primary human tumors using clonogenic assays.

METHODS AND MATERIALS

Fourteen human tumor cell lines were assessed for surviving fraction at 2-8 Gy (SF2-SF8) using low-dose rate irradiation and a clonogenic assay. Further data were collected from the literature for 64 low-dose rate irradiation survival curves from human tumor cell lines. The data were grouped into five different radioresponsiveness categories (A-E). An analysis was made of the ability of the graded survival levels to discriminate between the different radioresponse groups and compared with previous analyses for high-dose rate SF2. Fifteen human cervical carcinoma specimens were analysed for SF2 and SF3.5 following high- and low-dose rate irradiation.

RESULTS

Low-dose rate irradiation increased the spread of tumor cell line radiosensitivity data and the ability to discriminate between radioresponse groups was greater at low than at high-dose rates. Using low-dose rate irradiation on primary tumor specimens and a soft agar clonogenic assay decreased the success rate in obtaining data. The latter dropped from 70% for high-dose rate SF2 to 51% for low-dose rate SF3.5.

CONCLUSIONS

The work on cell lines illustrates that low-dose rate irradiation does improve the ability of clonogenic radiosensitivity measurements to discriminate between tumors of different radioresponsiveness groups. However, using low-dose rate irradiation on primary human tumors with a soft agar clonogenic assay was not practical because of reducing the success rate for obtaining data for radiosensitivity measurements.