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

Repopulation of tumor cells during fractionated radiotherapy and detection methods (Review)

Repopulation of tumor cells during radiotherapy is believed to be a significant cause for treatment failure. The phenomenon of tumor repopulation during fractionated radiotherapy was found from clinical observations that identified that the local control rate decreased with a prolonged treatment time. A series of animal experiments with varied overall treatment time and fractionated doses were performed to demonstrate tumor cell repopulation during radiotherapy in various mouse xenograft models. However, conventional detection methods are challenging, as it is difficult to separate viable cells from those destined for apoptosis during fractionated radiotherapy. In essence, the mechanism of tumor repopulation involves the continuing proliferation of clonogenic tumor cells. In vivo imaging, tracking and targeting of the repopulation of these cells has been of clinical interest so as to administer a higher dose to the tumor repopulation regions. Currently, functional imaging methods, including 3'-deoxy-3'-¹⁸F-fluorothymidine positron emission tomography (¹⁸F-FLT PET), are showing promise in assessing the proliferation activity of tumors in vivo. This review mainly focuses on the phenomenon of tumor repopulation during radiotherapy and its conventional and novel detection methods, particularly on the feasibility of ¹⁸F-FLT PET for the detection of tumor-cell repopulation.

A role for homologous recombination and abnormal cell-cycle progression in radioresistance of glioma-initiating cells

Glioblastoma multiforme (GBM) is the most common form of brain tumor with a poor prognosis and resistance to radiotherapy. Recent evidence suggests that glioma-initiating cells play a central role in radioresistance through DNA damage checkpoint activation and enhanced DNA repair. To investigate this in more detail, we compared the DNA damage response in nontumor forming neural progenitor cells (NPC) and glioma-initiating cells isolated from GBM patient specimens. As observed for GBM tumors, initial characterization showed that glioma-initiating cells have long-term self-renewal capacity. They express markers identical to NPCs and have the ability to form tumors in an animal model. In addition, these cells are radioresistant to varying degrees, which could not be explained by enhanced nonhomologous end joining (NHEJ). Indeed, NHEJ in glioma-initiating cells was equivalent, or in some cases reduced, as compared with NPCs. However, there was evidence for more efficient homologous recombination repair in glioma-initiating cells. We did not observe a prolonged cell cycle nor enhanced basal activation of checkpoint proteins as reported previously. Rather, cell-cycle defects in the G(1)-S and S-phase checkpoints were observed by determining entry into S-phase and radioresistant DNA synthesis following irradiation. These data suggest that homologous recombination and cell-cycle checkpoint abnormalities may contribute to the radioresistance of glioma-initiating cells and that both processes may be suitable targets for therapy.

Are cancer stem cells radioresistant?

Based on findings that cancer cell clonogens exhibit stem cell features, it has been suggested that cancer stem-like cells are relatively radioresistant owing to different intrinsic and extrinsic factors, including quiescence, activated radiation response mechanisms (e.g., enhanced DNA repair, upregulated cell cycle control mechanisms and increased free-radical scavengers) and a surrounding microenvironment that enhances cell survival mechanisms (e.g., hypoxia and interaction with stromal elements). However, these radiosensitivity features are probably dynamic in nature and come into play at different times during the course of chemo/radiotherapy. Therefore, different molecularly targeted radiosensitization strategies may be needed at different stages of therapy. This article describes potential sensitization approaches based on the dynamics and changing properties of cancer stem-like cells during therapy.

Quiescent, slow-cycling stem cell populations in cancer: a review of the evidence and discussion of significance

Long-lived cancer stem cells (CSCs) with indefinite proliferative potential have been identified in multiple epithelial cancer types. These cells are likely derived from transformed adult stem cells and are thought to share many characteristics with their parental population, including a quiescent slow-cycling phenotype. Various label-retaining techniques have been used to identify normal slow cycling adult stem cell populations and offer a unique methodology to functionally identify and isolate cancer stem cells. The quiescent nature of CSCs represents an inherent mechanism that at least partially explains chemotherapy resistance and recurrence in posttherapy cancer patients. Isolating and understanding the cell cycle regulatory mechanisms of quiescent cancer cells will be a key component to creation of future therapies that better target CSCs and totally eradicate tumors. Here we review the evidence for quiescent CSC populations and explore potential cell cycle regulators that may serve as future targets for elimination of these cells.

Cancer stem cells and tumor response to therapy: current problems and future prospects

The presence of a subpopulation of cells within tumors, so-called cancer stemlike cells, that is uniquely capable of reestablishing the tumor during and after definitive radio(chemo)therapy and must be effectively controlled for a long-term cure is being increasingly appreciated. The existence and physiology of a rare cancer cell population, termed cancer cell clonogens, with similar properties has been extensively described in the radiobiology literature for several decades based on studies using tumor cells transplanted into syngeneic or immunodeficient animals. The earlier studies have identified important features that govern tumor establishment; tumor growth and homeostasis; and therapeutic resistance, including clonogen number, tumor type, vascular status, hypoxia, repopulation dynamics during treatment, and immunologic and microenvironmental status. These discoveries led to therapeutic strategies, some of which have shown efficacy and have become current standard clinical practice (eg, concomitant boost and concurrent radio chemotherapy). Although the identity of cancer stemlike cells and cancer cell clonogens has not been definitively shown, recent characterization of molecular signaling pathways controlling stem cells and their microenvironmental niche combined with the earlier findings on clonogen physiology may now lead to the development of molecularly targeted strategies to overcome therapeutic resistance of this rare subpopulation of tumor cells. Along these lines, we describe 3 unique treatment settings (ie, before, during, and after definitive radio[chemo]therapy) in which molecularly targeted approaches might specifically counteract cancer stemlike cell resistance mechanisms and enhance the curative efficiency of radio(chemo)therapy.

Differentiation status of human squamous cell carcinoma xenografts does not appear to correlate with the repopulation capacity of clonogenic tumour cells during fractionated irradiation

PURPOSE

To investigate the magnitude and kinetics of repopulation in a moderately well differentiated UT-SCC-14 human squamous cell carcinoma [hSCC] in nude mice. This question is of interest because clinical data indicate a higher repopulation capacity in those SCC that have preserved characteristics of differentiation, which appears to be in contrast to results on FaDu and GL hSCC previously reported from this laboratory.

METHODS AND MATERIALS

UT-SCC-14 tumours were transplanted subcutaneously into the right hind leg of NMRI nu/nu mice. Fractionated radiation treatments were delivered, either under clamped hypoxia at 5.4 Gy/fraction or under ambient conditions (consistent with an OER of 2.7). Tumours were irradiated every day, every 2nd day, or every 3rd day with 6, 12 or 18 fractions. 1, 2 or 3 days after the last fraction, graded top-up-doses under clamped conditions were given for the purpose of estimating the 50% tumour control dose (TCD50). A total of 22 TCD50 assays were performed and analysed using maximum likelihood techniques.

RESULTS

The data demonstrate a slow but significant repopulation of clonogenic cells during fractionated irradiation of UT-SCC-14 hSCC. The results under hypoxic conditions are consistent with a constant repopulation rate, with a clonogenic doubling time (Tclon) of 15.6 days (95% CI: 9.7, 21.4). This contrasts with ambient conditions where Tclon was 68.5 days (95% CI: 124, 161). Both Tclon values are longer than the 6-day volume doubling time of untreated tumours.

CONCLUSIONS

Less pronounced repopulation for irradiation under ambient compared to clamped hypoxic conditions might be explained by preferential survival of hypoxic and therefore non-proliferating clonogenic cells. Taken together with previous studies on poorly differentiated FaDu and moderately well differentiated GL hSCC, the results are consistent with considerable variability in the magnitude and kinetics of repopulation in different experimental squamous cell carcinomas, and with a relationship between reoxygenation and repopulation during fractionated irradiation. The differentiation status of hSCC growing in nude mice does not to appear to correlate with the proliferative capacity of clonogenic tumour cells during treatment. The results do not support the hypothesis gained from clinical data of higher repopulation in well-differentiated tumours.

On mammary stem cells

Mammary gland stem cells are a quiescent and self-renewing population within the mammary gland that are capable of giving rise to the differentiated ductal, alveolar and myoepithelial cells. To identify mammary gland stem cells, several investigators have employed a variety of methods including: non-adherent mammosphere cultures; 5-bromo-2-deoxy-uridine (BrdU) label-retention studies; cell-surface markers, such as Sca1 and CD49f; and Hoechst dye efflux. These methods have helped identify and further characterize signal transduction pathways such as the Notch, Wnt and Hedgehog pathways that may be important for the self-renewal and fate determination of mammary gland stem cells. Stem cells within the mammary gland have been proposed to underpin many types of breast cancer. A better understanding of the signal transduction pathways and the molecules that are responsible for the self-renewal and survival of these cells will be essential in the design of more effective therapies aimed at the eradication of both cancer-initiating cells and breast cancer stem cells.

Nucleosomes in pancreatic cancer patients during radiochemotherapy

Nucleosomes appear spontaneously in elevated concentrations in the serum of patients with malignant diseases as well as during chemo- and radiotherapy. We analyzed whether their kinetics show typical characteristics during radiochemotherapy and enable an early estimation of therapy efficacy. We used the Cell Death Detection Elisa plus (Roche Diagnostics) and investigated the course of nucleosomes in the serum of 32 patients with a local stage of pancreatic cancer who were treated with radiochemotherapy for several weeks. Ten of them received postsurgical therapy, 21 received primary therapy and 1 received therapy for local relapse. Blood was taken before the beginning of therapy, daily during the first week, once weekly during the following weeks and at the end of radiochemotherapy. The response to therapy was defined according to the kinetics of CA 19-9: a decrease of CA 19-9 > or =50% after radiochemotherapy was considered as 'remission'; an increase of > or =100% (which was confirmed by two following values) was defined as 'progression'. Patients with 'stable disease' ranged intermediately. Most of the examined patients showed a decrease of the concentration of nucleosomes within 6 h after the first dose of radiation. Afterwards, nucleosome levels increased rapidly, reaching their maximum during the following days. Patients receiving postsurgery, primary or relapse therapies did not show significant differences in nucleosome values during the time of treatment. Single nucleosome values, measured at 6, 24 and 48 h after the application of therapy, could not discriminate significantly between patients with no progression and those with progression of disease. However, the area under the curve of the first 3 days, which integrated all variables of the initial therapeutic phase, showed a significant correlation with the progression-free interval (p=0.008). Our results indicate that the area under the curve of nucleosomes during the initial phase of radiochemotherapy could be valuable for the early prediction of the progression-free interval.

Repopulation of moderately well-differentiated and keratinizing GL human squamous cell carcinomas growing in nude mice

PURPOSE

It has been suggested that, reminiscent of the regulated proliferative response of normal squamous epithelium, squamous cell carcinomas that have preserved characteristics of differentiation have a greater repopulation capacity during fractionated irradiation than undifferentiated tumors. The aim of the present study was to investigate repopulation in moderately well-differentiated and keratinizing GL human squamous cell carcinomas in nude mice.

METHODS AND MATERIALS

GL human squamous cell carcinomas were transplanted s.c. into the right hind leg of NMRI nu/nu mice. Irradiation was performed with 5.4 Gy fractions under clamp hypoxia or with 2 Gy fractions under ambient conditions. Six, 12, or 18 fractions were given daily, every second day, or every third day. Graded top-up doses were applied under clamp hypoxia to determine the tumor control dose 50% (TCD(50)). A total of 20 TCD(50) assays were performed and analyzed using maximum-likelihood techniques.

RESULTS

With an increasing number of daily 5.4 Gy fractions under clamp hypoxia, the top-up TCD(50) values decreased significantly from 50.9 Gy (95% CI: 47, 54) after single doses to 0 Gy after 18 fractions. For the same number of fractions, the top-up TCD(50) increased with increasing overall treatment time. The results are consistent with a constant repopulation rate with a clonogenic doubling time (T(clon)) of 12.7 days (8.6, 16.8). Under ambient blood flow, the top-up TCD(50)s for daily 2 Gy fractions decreased significantly, but were less pronounced than for 5.4 Gy fractions under clamp hypoxia. For a given number of fractions under ambient conditions, the top-up TCD(50)s did not increase significantly with overall treatment time, except for irradiation with 12 fractions in 36 days compared to 12 and 24 days. The T(clon) value from these data was 24.0 days (11.6, 36.4).

CONCLUSION

Our data demonstrate a slow but significant rate of repopulation of clonogenic tumor cells during fractionated irradiation of GL human squamous cell carcinomas under clamp hypoxia without indication of a change of the repopulation rate during treatment. Less pronounced repopulation was observed for irradiation under ambient conditions, which might be explained by preferential survival of hypoxic and therefore nonproliferating cells. Taken together with our previous studies on poorly differentiated FaDu tumors (Petersen et al., IJROBP 2001;51:483-493), the results support important heterogeneity of kinetics and mechanisms of repopulation, in particular of the influence of the oxygenation status of surviving clonogenic cells on the repopulation rate during fractionated irradiation, in different experimental squamous cell carcinoma.

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.

Poly(L-glutamic acid)-paclitaxel conjugate is a potent enhancer of tumor radiocurability

PURPOSE

Conjugating drugs with polymeric carriers is one way to improve selective delivery to tumors. Poly (L-glutamic acid)-paclitaxel (PG-TXL) is one such conjugate. Compared with paclitaxel, its uptake, tumor retention, and antitumor efficacy are increased. Initial studies showed that PG-TXL given 24 h before or after radiotherapy enhanced tumor growth delay significantly more than paclitaxel. To determine if PG-TXL-induced enhancement is obtained in a more clinically relevant setting, we investigated PG-TXL effects on tumor cure.

METHODS AND MATERIALS

Mice bearing 7-mm-diameter ovarian carcinomas were treated with PG-TXL at an equivalent paclitaxel dose of 80 mg/kg, single dose or 5 daily fractions of radiation or both PG-TXL and radiation. Treatment endpoint was TCD(50) (radiation dose yielding tumor control in 50% of mice). Acute radioresponse of jejunum, skin, and hair was determined for all treatments.

RESULTS

PG-TXL dramatically improved tumor radioresponse, reducing TCD(50) of single-dose irradiation from 53.9 (52.2-55.5) Gy to 7.5 (4.5-10.7) Gy, an enhancement factor (EF) of 7.2. The drug improved the efficacy of fractionated irradiation even more, reducing the TCD(50) of 66.6 (62.8-90.4) Gy total fractionated dose to only 7.9 (4.3-11.5) Gy, for an EF of 8.4. PG-TXL did not affect normal tissue radioresponse resulting from either single or fractionated irradiation.

CONCLUSION

PG-TXL dramatically potentiated tumor radiocurability after single-dose or fractionated irradiation without affecting acute normal tissue injury. To our knowledge, PG-TXL increased the therapeutic ratio of radiotherapy more than that previously reported for other taxanes, thus, PG-TXL has a high potential to improve clinical radiotherapy.

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.

Repopulation of FaDu human squamous cell carcinoma during fractionated radiotherapy correlates with reoxygenation

PURPOSE

FaDu human squamous cell carcinoma (FaDu-hSCC) showed a clear-cut time factor during fractionated radiotherapy (RT) under ambient blood flow. It remained unclear whether this is caused solely by proliferation or if radioresistance resulting from increasing hypoxia contributed to this phenomenon. To address this question, repopulation of clonogenic FaDu cells during fractionated RT under clamp hypoxia was determined by local tumor control assays, and compared to the results after irradiation with the same regimen under ambient blood flow.

METHODS AND MATERIALS

FaDu-hSCC was transplanted into the right hind leg of NMRI nu/nu mice. In the first set of experiments, irradiation was performed under clamp hypoxia. After increasing numbers of 3 Gy fractions (time intervals 24 h or 48 h), graded top-up doses were given to determine the TCD(50) (dose required to control 50% of the tumors). In the second set of experiments, all 3 Gy fractions were applied under ambient conditions, but as in the previous experiments the graded top-up doses were given under clamp hypoxia. A total of 26 TCD(50) assays were performed and analyzed using maximum likelihood techniques.

RESULTS

With increasing numbers of daily fractions, the top-up TCD(50) under clamp hypoxia decreased from 39.4 Gy [95% CI 36, 42] after single dose to 19.8 Gy [15, 24] after 18 fractions in 18 days and to 37.8 Gy [31, 44] after 18 fractions in 36 days. The results were consistent with biphasic repopulation, with a switch to rapid repopulation after about 22 days [13, 30]. The clonogen doubling time (T(clon)) decreased from 9.8 days [0, 21] in the beginning of RT to 3.4 days after 22 days. Under ambient blood flow the top-up TCD(50) decreased from 37.6 Gy [34, 40] after single dose irradiation to 0 Gy [0, 1] after 18 fractions in 18 days and 22.4 Gy [18, 27] after 18 fractions in 36 days. Similar to results from irradiations under clamp hypoxia, the ambient data were consistent with a biphasic course of clonogen inactivation. Comparison of both data sets revealed significant reoxygenation after 12 fractions.

CONCLUSIONS

Our data are most consistent with a biphasic course of clonogen repopulation during fractionated RT of FaDu-hSCC under clamp hypoxia with a switch in T(clon) after about 22 days of treatment ("dog-leg"). A similar biphasic course of cell repopulation was observed under ambient conditions. The temporal coincidence between repopulation and reoxygenation suggests that the latter might be the stimulus for proliferation in FaDu tumors.

No evidence for a different magnitude of the time factor for continuously fractionated irradiation and protocols including gaps in two human squamous cell carcinoma in nude mice

BACKGROUND AND PURPOSE

To study whether the magnitude of the time factor is different for continuously fractionated irradiation and for fractionation protocols including gaps.

MATERIALS AND METHODS

Two human head and neck squamous cell carcinomas (SCCs), FaDu and GL, were transplanted subcutaneously into the right hindleg of NMRI (nu/nu) mice and irradiated with 30 fractions under ambient conditions within 2, 6 and 10 weeks. Irradiations within 6 and 10 weeks were given either as a continuous course or with a mid-course gap of 3 weeks. The end-point of the experiments was local tumor control at day 120 (FaDu) or day 180 (GL) after the end of treatment.

RESULTS

In FaDu tumors, two experimental cohorts (A, B) yielded significantly different results and were analyzed separately. In cohort A, the tumor control dose 50% (TCD50) increased from 37 to 89 Gy when the treatment time of continuous fractionated irradiation was extended from 2 to 10 weeks. The recovered dose/day (D(r)) was 0.98 Gy (95% confidence interval, 0.72; 1.27). In cohort B, the TCD50 increased from 35 to 63 Gy, and the D(r) was 0.51 Gy (0.24; 0.75). In GL tumors, the TCD50 for continuously fractionated irradiation increased from 41 to 48 Gy. This increase was not significant, and the D(r) was 0.15 Gy (0; 0.30). None of the TCD50 and D(r) values obtained in both tumor models for continuous irradiation vs. irradiation with a gap were significantly different.

CONCLUSIONS

Prolongation of the overall treatment time of fractionated irradiation resulted in a pronounced decrease of local control in human FaDu SCC and little decrease of local control in human GL SCC. No evidence was found that the magnitude of the time factor in these tumors is different for continuous fractionation or fractionation protocols including gaps.

Nucleosomes in serum of patients with benign and malignant diseases

High quantities of mono- and oligonucleosomes circulate in the blood of patients with malignant tumors. For their direct quantification in serum, we modified the Cell Death Detection(plus)-ELISA for its application in liquid materials. We examined sera samples from 590 persons, including 418 patients with malignant tumors, 109 patients with benign diseases and 63 healthy persons. We also observed the kinetics of the concentration of nucleosomes in serum samples from 20 patients undergoing chemotherapy and from 16 patients undergoing radiotherapy. Sera of patients with malignant tumors contained considerably higher concentrations of nucleosomes (mean = 350 arbitrary units [AU], median = 190 AU) compared with those of healthy persons (mean = 36 AU, median = 24 AU; p = 0.0001) and patients with benign diseases (mean = 264 AU, median = 146 AU; p = 0.072). Concerning the follow-up investigations, the concentration of nucleosomes in serum increased 24-72 hr after the first application of chemotherapy and 6-24 hr after the start of radiotherapy. A subsequent decrease was often correlated with regression of the tumor. In patients undergoing chemotherapy, an increase in the baseline values of circulating nucleosomes >50%, which were determined before each new therapeutic cycle, was correlated with progression of disease; all patients with disease regression showed a decrease >50% of the baseline values. In patients undergoing radiotherapy, an early decrease of the nucleosomal concentration (< or = 1 day after the initial peak during therapy) to low minimum levels (< or = 100 AU) correlated with good clinical outcome; a late decrease (>1 day) to higher minimum levels (>100 AU) was associated with a worse clinical outcome. Thus, the concentration of nucleosomes in serum might be a useful tool for monitoring the biochemical response during antitumor therapy, especially for the early estimation of therapeutic efficacy.

Angiogenesis vs. response after combined chemoradiotherapy of squamous cell head and neck cancer

Oxygen/drug supply to cancer cells is an important factor defining response to radiotherapy and chemotherapy. Although tumor angiogenesis is considered an important prognostic marker, its role in the outcome of chemotherapy and radiotherapy is unknown. In the present study we examined the possible correlation of the degree of angiogenesis with response to cytotoxic therapy in locally advanced squamous cell head and neck cancer (HNC). Vascular grade (VG) was assessed immunohistochemically using the JC70 monoclonal antibody (MAb) in tumor specimens from 76 patients treated with platinum/5-fluorouracil (with or without methotrexate) induction chemotherapy (ICT) (n = 37) or concurrent chemoradiotherapy (CCRT) with cisplatin or carboplatin (n = 39). Seventeen of 76 analyzed patients had an overall microvessel score of < 11 (VGI), 25/76 of 11-30 (VG2), 16/76 of 31-50 (VG3) and 18/76 of > 50 (VG4). Complete response rate after ICT or after CCRT was higher in cases with an intermediate vascularization (VG2,3). Both local relapse-free and overall survival were significantly better in the VG2 group. Patients treated with CCRT had a better survival compared to those treated with ICT. This was mainly observed in VG1 tumors. Multivariate analysis showed that VG and treatment modality were independent prognostic factors for local relapse and survival. Intratumoral angiogenesis correlation with the cytotoxic therapy outcome is likely to follow a bell-shaped relation, the response being better in cases with an intermediate VG. This may be the consequence of 2 vasculature-dependent factors, i.e., the drug/oxygen availability and the ability of cancer cells to undergo rapid repopulation in optimally oxygenated conditions. Our pilot study stresses the importance of individualization of therapy according to VG.

Impact of overall treatment time on local control of slow growing human GL squamous cell carcinoma in nude mice treated by fractionated irradiation

BACKGROUND AND PURPOSE

The impact of overall treatment time of fractionated irradiation on local control of slow growing human GL squamous cell carcinoma (SCC) was determined.

MATERIALS AND METHODS

Moderately well differentiated and keratinizing human GL SCC with a volume doubling time of 8 days were transplanted subcutaneously into the right hindleg of NMRI (nu/nu) mice and irradiated with 30 fractions under ambient conditions over 2, 3, 4.5, 6 and 10 weeks. Endpoint of the experiments was local tumor control at day 180 after end of treatment.

RESULTS

The tumor control dose 50% (TCD50) increased from 40 to 57 Gy when the treatment time was extended from 2 to 10 weeks. The data can be well described by a linear increase in TCD50 with time. The recovered dose per day (D(r)) was 0.28 Gy (95% confidence interval 0.06; 0.48). The fit to the data was not significantly improved by assuming a biphasic (dog-leg) time course with constant TCD50 values in the initial part of treatment followed by a more rapid increase of TCD50 thereafter.

CONCLUSIONS

D(r) in GL SCC was significantly less than the value of 1.0 Gy (0.7; 1.3) previously reported for poorly differentiated, non-keratinizing and fast growing human FaDu SCC (Baumann M, Liertz C, Baisch H, Wiegel T, Lorenzen J, Arps H. Impact of overall treatment time of fractionated irradiation on local control of human FaDu squamous cell carcinoma in nude mice. Radiother. Oncol. 1994:32:137-143), indicating important heterogeneity of the time factor between different tumors of the same histological type.

Effect of filgrastim (G-CSF) during chemotherapy and abdomino-pelvic radiation therapy in patients with ovarian carcinoma

PURPOSE

To evaluate the safety and effectiveness of filgrastim (granulocyte colony-stimulating factor, G-CSF) in reducing neutropenia and treatment interruptions during whole abdominal radiotherapy for ovarian cancer.

METHODS AND MATERIALS

Sixteen patients with ovarian cancer treated with 2 to 6 courses of cisplatin-containing chemotherapy and abdomino-pelvic radiation therapy received filgrastim for neutrophil counts <2 x 10(9)/L. Endpoints for analysis included the ability to maintain the neutrophil count in the target range, number of treatment interruptions due to neutropenia, and toxicity attributed to filgrastim.

RESULTS

Fourteen patients received a mean of 2.9 courses of filgrastim (each with a mean duration of 4.1 days), with no treatment interruptions due to neutropenia. The majority of neutrophil counts were maintained above the target range of 2 x 10(9)/L during treatment. Thrombocytopenia requiring treatment interruption was seen in six patients and necessitated platelet transfusions in one. Thrombocytopenia occurred at a mean abdominal radiation dose of 2207 cGy and in all but one patient was preceded by one or more episodes of neutropenia. In comparison with a control group of 31 patients treated without filgrastim there was no reduction in treatment interruptions. Four patients did not complete treatment because of persistent thrombocytopenia yet received a mean of 94% of the planned abdominal radiation dose and 69% of the planned pelvic dose. Filgrastim toxicity was limited to mild skeletal pains in six patients and a Grade 1 skin rash in two patients.

CONCLUSIONS

Filgrastim is safe and effective in preventing neutropenia and reducing neutropenic treatment interruptions during abdominal radiotherapy in patients with ovarian cancer. However, there was no clear benefit to the use of filgrastim as thrombocytopenia became the dose-limiting toxicity resulting in a risk of treatment interruptions and early termination of radiotherapy.

Analysis of the radiobiology of ytterbium-169 and iodine-125 permanent brachytherapy implants

Recently, Yb-169 has been considered as a potential replacement for I-125 and Pd-103 in permanent implants. In spite of the uncertainties in the parameters necessary for an accurate radiobiological modelling, the linear quadratic model can be useful in the comparative evaluation of the radiotherapeutic merit of similar implants. In order to find out if a Yb-169 permanent implant can be made biologically 'equivalent' to an I-125 implant, we studied the dependence of local control on the tumour cell radiosensitivity and on the balance between the rate of tumour cell killing and tumour cell proliferation, for rapidly and slowly proliferating tumours. The extrapolated response dose (ERD) has been calculated for tumour and late reacting normal tissue for both types of implants and the possible biological restrictions due to the normal tissue tolerance have been discussed. Our theoretical analysis is consistent with the clinical results published for I-125 permanent implants in prostate tumours and meningiomas. It predicts that Yb-169, which has only recently been used in human tumours, can provide comparable tumour control for permanent implants in slowly proliferating tumours with an initial dose rate of 13 cGy h-1. Control might be extended to rapidly proliferating tumours by increasing the initial dose rate within a range consistent with an acceptable level of normal tissue late reaction.

The effect of time delays on the dynamics of avascular tumor growth

During avascular tumor growth, the balance between cell proliferation and cell loss determines whether the colony expands or regresses. Mathematical models describing avascular tumor growth distinguish between necrosis and apoptosis as distinct cell loss mechanisms: necrosis occurs when the nutrient level is insufficient to sustain the cell population, whereas apoptosis can occur in a nutrient-rich environment and usually occurs when the cell exceeds its natural lifespan. Experiments suggest that changes in the proliferation rate can trigger changes in apoptotic cell loss and that these changes do not occur instantaneously: they are mediated by growth factors expressed by the tumor cells. In this paper, we consider two ways of modifying the standard model of avascular tumor growth by incorporating into the net proliferation rate a time-delayed factor. In the first case, the delay represents the time taken for cells to undergo mitosis. In the second case, the delay represents the time for changes in the proliferation to stimulate compensatory changes in apoptotic cell loss. Numerical and asymptotic techniques are used to show how a tumor's growth dynamics are affected by including such delay terms. In the first case, the size of the delay does not affect the limiting behavior of the tumor: it simply modifies the details of its evolution. In the second case, the delay can alter the tumor's evolution dramatically. In certain cases, if the delay exceeds a critical value, defined in terms of the system parameters, then the underlying radially symmetric steady state is unstable with respect to time-dependent perturbations. (For smaller delays, this steady state is stable). Using the delay as a measure of the speed with which a tumor adapts to changes in its structure, we infer that, for the second case, a highly responsive tumor (small delay) has a better chance of surviving than does a less-responsive tumor (large delay). We also conclude that the tumor's evolution depends crucially on the manner and speed with which it adapts to changes in its surroundings and composition.