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Published online before print May 31, 2007
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From the Departments of Dermatology,* Gynaecology, and Radiotherapy, Martin Luther University Halle-Wittenberg, Halle, Germany; the Department of Radiotherapy, University of Lübeck, Lübeck, Germany; the Institute of Pathology,¶ Medical University of Vienna, Vienna, Austria; and the Department of Dermatology,|| University of Heidelberg, Heidelberg, Germany
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Abstract |
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The effect of an intermediate radio dose is another important aspect that has not yet been sufficiently investigated. Although the studies cited above mainly focused on tumoral target doses, investigation of the effect of a "tumor-environmental dose" (about two-thirds of the target volume dose) seems to be most promising, because the tumor environment is the major site of development of local recurrences or skin metastases.6
The detection of molecules that are relatively specifically expressed by lymphatic endothelial cells, like podoplanin, lymphatic vessel endothelial hyaluronate receptor 1 (LYVE-1), vascular endothelial growth factor receptor (VEGFR)-3, prospero-related homeobox gene PROX-1, desmoplakin-1 (2.17), and ß-chemokine receptor D6, has facilitated new insights into the molecular mechanisms that control lymphatic vessel development.7-12 Lymphangiogenesis is stimulated during embryogenesis, after trauma, in ischemic tissue, chronic wounds, acute lymphedema, or by malignant tumors.13-16 Two members of the VEGF family, VEGF-C and VEGF-D, are important for the induction of lymphangiogenesis.7,17-21 These factors are ligands of the lymphatic endothelial VEGF receptor-3 that is expressed on the surface of lymphatic endothelial cells. VEGF-C is produced by macrophages, dendritic cells, endothelial cells, platelets, basophilic granulocytes, lymphocytes, and tumor cells.15,22 Like other VEGFs, VEGF-C controls not only angiogenesis of blood and lymphatic vessels but also microvessel permeability and migration of endothelial cells.7,11,17,23
In this study, we investigated the changes of the lymphatic and blood microvasculature during the first years after postoperative radiotherapy for breast cancer in a standardized group of patients by comparing intraindividual control samples of unirradiated skin taken from exactly symmetrically contralateral areas. In addition, we used for comparison a group of melanoma patients who received no radiotherapy but were treated by operation (lymph node dissection).
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Materials and Methods |
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Patients
We studied 80 samples of 40 consecutive female patients of the Breast Cancer Center of Martin Luther University Halle-Wittenberg after completed standard radiation therapy because of breast cancer (Table 1)
. Patients were treated by operation, chemotherapy, and/or hormone therapy according to the standards of care (St. Gallen protocol).24
At the time of biopsy, patients were free of local recurrence or metastatic disease. Local exclusion criteria regarding the sampling regions were any clinical signs of radiation dermatitis or other inflammatory disease, palpable or sonographically detectable axillary lymph node swelling, and lymphedema of the biopsy area and its direct neighborhood (axilla and pectoral region). In contrast, clinically detectable lymphedema restricted to the arm of the irradiated site was allowed and regarded in the analysis (10 of 40 patients). General exclusion criteria were diabetes mellitus, autoimmune disease, human immunodeficiency virus infection, and reduced general condition (Karnofsky index <80%). Co-morbidity included hypertension (n = 8), arrhythmia (n = 3), bronchial asthma (n = 2), previously treated inactive pulmonary tuberculosis (n = 2), rheumatic polyarthritis (n = 1), angina pectoris (n = 1), psoriasis (n = 1), or chronic hepatitis C (n = 1). Besides specific anticancer therapy (Table 1)
, patients were treated with L-thyroxin (n = 4), acetylsalicylic acid (n = 3), ß-blockers (n = 2), angiotensin-converting enzyme inhibitors (n = 2), or calcium antagonists (n = 2).
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Before sampling, patients had received standard radiotherapy with 6-MV photons via tangential fields (Table 1)
. Some patients with large breasts and larger dose inhomogeneities received part of the radiation dose with 10-MV photons, mainly about 40% of the total dose (20 of 50 Gy). Radiotherapy was delivered in conventional fractionation with five fractions per week in all patients. No bolus was used. Three of 40 patients with four or more positive nodes received additional radiotherapy to the supraclavicular fossa; treatment was delivered via a single anterior portal with 6-MV photons, and the radiation field did not cover the skin biopsy side (see below). These patients with additional supraclavicular radiotherapy belonged to the patient groups "2 to 8 weeks" (n = 1), "11 to 14 months" (n = 1), or "17 to 25 months after radiotherapy" (n = 1). In 38 of 40 patients, the total dose calculated at the International Commission on Radiation Units and Measurements reference point within the target volume was 50.4 Gy (in 28 single doses of 1.80 Gy). One patient of 40 received 54 Gy (30 x 1.8 Gy), and one patient of 40 received 44.8 Gy (28 x 1.6 Gy). Both of these dose-deviant patients belonged to the same patient subgroup "2 to 8 weeks after radiotherapy" (see below). The estimated skin dose was 60 to 70% of the reference dose (see Discussion).
Skin Samples
Punch biopsies (6 mm) were taken 2 to 8 weeks (median, 4 weeks; n = 13 patients), 11 to 14 months (median, 12 months; n = 11), 17 to 25 months (median, 24; n = 8), or 37 to 157 months (median, 43.5; n = 8) after radiotherapy from the skin of the superior anterior axillary line that was directly included in the target region of all fractions of the radiotherapy (2 to 3 cm proximal from lymph node dissection scar) and from the corresponding unirradiated skin area of the opposite anterior axillary line of each patient for control. Biopsies were divided into two parts. One was snap-frozen in liquid nitrogen and stored at –70°C, and the other was fixed overnight in 4% formalin and paraffin-embedded.
Skin Samples from Melanoma Control Patients
Twenty-two samples from 11 melanoma patients who had not undergone radiation but underwent similar procedure of operation (lymph node dissection) were used for comparison. Eleven biopsies were taken 2 to 3 cm proximal to a lymph node dissection scar either from the axilla (9 of 11 cases) or from inguinal region (2 of 11), and 11 additional biopsies were taken from the exactly contralateral skin area of the same 11 patients: age, 61.8 ± 12.8 years; sex, 10 female, one male; time between operation and biopsy median, 14.4 ± 2.8 months (minimum, 9; maximum, 19 months); stage of disease (American Joint Committee on Cancer 200225 ), 1b/2a/3b/3c, n = 1/1/8/1; type of operation, radical lymph node dissection (9 of 11) or sentinel node biopsy (2 of 11); and number of dissected lymph nodes, 13.9 ± 7.2. At the time of biopsy, patients were free of local recurrence or metastatic disease. Biopsies were fixed overnight in 4% formalin and paraffin-embedded.
Antibodies
Polyclonal rabbit anti-human podoplanin IgG was raised against the recombinant human homolog of the rat 43-kd glycoprotein podoplanin. Rabbit sera were affinity-purified using nitrocellulose strips containing recombinant protein, as described previously.26
Other primary antibodies were as follows: monoclonal mouse anti-human CD34, class II clone QBEnd10, isotype IgG1, 
(Dako, Glostrup, Denmark); monoclonal mouse anti-human CD68, clone KP1, isotype IgG1, (Dako); monoclonal mouse anti-human CD68, clone PG-M1, fluorescein isothiocyanate conjugate (Dako); rabbit anti-human LYVE-1 (AngioBio, Del Mar, CA); rabbit anti-human VEGF-C (Zymed Inc., South San Francisco, CA); and goat anti-human VEGF-D (R&D Systems, Minneapolis, MN). Fluorochrome-labeled secondary monoclonal antibodies were as follows: goat anti-rabbit IgG, F(ab')2-tetramethylrhodamine isothiocyanate (Santa Cruz Biotechnology Inc., Santa Cruz, CA) and donkey anti-goat IgG, F(ab')2-tetramethylrhodamine B isothiocyanate (Santa Cruz).
Histological Preparation of Paraffin Sections
Paraffin sections (5 µm) of each sample were stained with hematoxylin and eosin (H&E). For immunohistology, consecutive sections were deparaffinized in a graded ethanol series. Endogenous peroxidase was blocked (0.3% H2O2 in methanol). Then, the sections were microwaved (10 mmol/L citrate buffer, pH 6, four times for 5 minutes, and 600 W) for antigen demasking. The sections were incubated with anti-podoplanin (1:250), anti-CD34 (1:40), anti-CD68/KP1 (1:50), or anti-VEGF-C (1:100) for 30 minutes at room temperature. In VEGF-C or CD68 staining, undiluted rabbit or mouse primary antibody isotype controls (08-6199 or 08-6599; Zymed) were used for negative control. Further steps were performed with the Elite-ABC kit (Vector Laboratories, Burlingame, CA) according to the manufacturer’s instructions. Finally, the sections were counterstained with 25% hemalum (3 minutes; Merck, Darmstadt, Germany), rinsed with tap water, and mounted (gelatin mounting medium; Dako).
Immunofluorescence Investigations
Cryostat sections (5 µm) were triple-stained with fluorescein isothiocyanate conjugate of anti-CD68/PG-M1 (1:50; 30 minutes; room temperature), anti-VEGF-C (1:100; 30 minutes; room temperature), or anti-VEGF-D (1:80; 30 minutes; room temperature) and DNA fluorochrome (Hoechst 33258; Sigma). Tetramethylrhodamine isothiocyanate-fluorochrome-labeled secondary monoclonal antibodies [goat anti-rabbit IgG, F(ab')2; donkey anti-goat IgG, F(ab')2] were both applied at 1:100 (30 minutes). For demonstration of LYVE-1+ lymphatic microvessels, 8-µm cryostat sections were double-stained with anti-LYVE-1 (1:100; 30 minutes; room temperature) and DNA fluorochrome. Tetramethylrhodamine isothiocyanate-fluorochrome-labeled goat anti-rabbit IgG, F(ab')2 was applied 1:100 (30 minutes).
Negative Controls
In VEGF-C, VEGF-D, or LYVE-1 staining, 0.5 µg/ml primary antibody isotype controls of rabbit (Zymed) or goat (Abcam, Cambridge, UK) were used for negative control. In VEGF-C, negative controls were additionally preblocked with human recombinant VEGF-C (5 µg/ml; BioVision, Mountain View, CA).
Analysis
Sections were studied for general description of changes in comparison with the controls. Immunohistologically treated sections were evaluated in a "blinded" setting. At least three different sections of each sample were evaluated in the upper horizontal dermal plexus. Densities were calculated relative to skin surface (n/mm). The lymphatic vessels were grouped by diameter: <10 µm, 10 to 17 µm, 18 to 24 µm, and 
25 µm (only in podoplanin investigations), as were the blood vessels: <12 µm, 12 to 15 µm, and 16 µm. For this, the shortest transversal axis of each vessel section was assessed under a measuring field ocular. The numbers of the visible microvessel sections were counted using the diameter groups. Only clearly identifiable microvessels were taken into account (Figure 1, j and k)
. The interobserver coefficient of variation of this method was calculated in sixfold counting of the podoplanin-stained sections of 10 different cases with a total of 57 sections at 0.076 ± 0.045.
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); t-test for paired samples (t-test); Wilcoxon test, exact, two-sided significance (Wilcoxon test); Mann-Whitney U-test, exact, two-sided significance (U-test); and logistic regression. Categorical, ordinal covariates were median-dichotomized in logistic regression. In LYVE-1 immunofluorescence investigations, the number of positive cells or microvessel segments was calculated from five sections of each analyzed sample. |
Results |
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In the H&E-stained sections of irradiated skin of the breast cancer patients, a sparse perivascular-oriented mononuclear infiltrate and qualitative degeneration of microvessels, hair follicles, or sebaceous glands were evident in the samples harvested 2 to 8 weeks after the end of radiation (Figure 1, a–d)
. Venules presented diameter enlargement. The basal epidermis showed single-cell necroses. Twelve months after radiation, a proportion of 10 to 30% disfigured microvessels was visible (Figure 1e)
. These changes were persistently found in other samples from later stages that included a time up to 157 months after radiotherapy (Figure 1f)
. In addition, basophilic degeneration of the subepidermal connective tissue could be found in some cases >3 years after radiation.
Lymphatic and Blood Microvessel Densities
Absolute Density Values
The mean density of total podoplanin+ lymphatic vessels in the samples of the treated skin of the breast cancer patients proved 18% higher than that of the individual control skin. This difference was significant (Table 2)
. Detailed analysis showed that this was mainly a consequence of an increase of the smallest lymphatics (with a diameter <10 µm) in the irradiated skin. In all patient subgroups together, the mean density of small lymphatics <10 µm was 44% higher in the treated side versus control side (Table 2)
. The source of this effect could be localized by time-subgroup analysis: We found more than duplication (107%) of the mean density of small lymphatics <10 µm in the subgroup of patients who had undergone radiotherapy 11 to 14 months earlier (Table 2
; Figure 1, g–k
). This effect was not yet visible in samples gathered in the first weeks after therapy and tended to persist beyond 14 months after therapy (Table 2)
. Neither the larger lymphatic microvessel calibers (10 µm) nor the total CD34+ blood microvessels showed significant radiogenic effect regarding the vessel densities (Table 2)
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). Intraindividual Correlation to the Controls
We found intraindividual correlation between the microvessel densities of the treated skin of the breast cancer patients and those of unirradiated individual control skin (total lymphatics, r = 0.415, P = 0.008; small lymphatics <10 µm diameter, r = 0.357, P = 0.024; small blood microvessels <12 µm, r = 0.534, P < 0.001; medium blood microvessels 12 to 15 µm, r = 0.668, P < 0.001; n = 40). A similar tendency of intraindividual correlations between the therapeutic and control sides of the lymphatic microvessel densities was found even in the melanoma control patients (total lymphatics, r = 0.549, P = 0.081; small lymphatics <10 µm diameter, r = 0.653, P = 0.029; lymphatics 10 to 17 µm, r = 0.559, P = 0.074; lymphatics >25 µm, r = 0.522, P = 0.100; n = 11). There was no significant intraindividual correlation in all other vessel diameters (Supplemental Table 1 at http://ajp.amjpathol.org). Interindividual differences between the normal control skin of different patients were considerable in all investigated vessel diameter groups (Table 2)
. Moreover, there were stochastic differences in the normal values of the control skin between the "time subgroups." Thus, individual microvessel density in irradiated skin seemed to be a result of the normal individual state and the therapeutic effects. To overcome this problem, we used the individual ratio of treated skin to untreated control [treated-to-control ratio (T/C ratio)] in addition to the absolute density.
T/C Ratios of Microvessel Densities
The T/C ratios of the densities of total podoplanin+ lymphatics or CD34+ blood microvessels of the different time groups are presented in Figure 2, a and b
. Separate analysis of each vessel diameter subgroup revealed a clear effect of the time between radiotherapy and biopsy on the T/C ratios. Although the densities of the smallest podoplanin+ lymphatics (<10 µm) in the biopsies taken shortly after radiation (2 to 8 weeks) showed a T/C ratio of 0.91 ± 0.74 (mean ± SD), it was 2.04 ± 1.09 in the biopsies taken 11 to 14 months after radiation.
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. Density of LYVE-1+ Lymphatic Microvessels
To audit the results of podoplanin staining, the density of LYVE-1+ lymphatic microvessels was investigated in the subgroup of patients who had undergone radiotherapy 11 to 14 months before biopsy (Supplemental Figure 1, A and B, at http://ajp.amjpathol.org). In total lymphatics, we found 3.24 ± 0.94/mm in the irradiated skin and 1.98 ± 0.59/mm in the unirradiated control skin (or skin of unirradiated controls). This difference was significant (P = 0.017, n = 10, Wilcoxon test).
These values showed individual correlation to the total lymphatic density results of podoplanin investigations (r = 0.698, P = 0.003, n = 20, Pearson correlation). Moreover, the mean values of the podoplanin results (given in Table 2
, first line) did not differ significantly from that of the results of LYVE-1 investigations given above (P = 0.14, n = 10 or P = 0.11, n = 10 for the irradiated or unirradiated biopsies, respectively, all Wilcoxon test).
CD68 and VEGF-C Expression
Increased densities of CD68+ and VEGF-C+ cells in the samples from the first weeks after radiotherapy (2 to 8 weeks) were some of the most striking effects found in this work (Table 2
; Figures 1, l–p, and 3, a and b
; Supplemental Figure 1C at http://ajp.amjpathol.org). In the samples obtained 2 to 8 weeks after radiotherapy, the densities of CD68+ cells were elevated in the irradiated skin (94.7 ± 31.4/mm) in comparison with the controls (62.8 ± 22.3/mm). Likewise, the densities of the VEGF-C+ cells were elevated in this time group in the irradiated sites (21.1 ± 7.7/mm) in comparison with the controls (12.4 ± 6.3/mm). Although the number of CD68+ cells remained elevated in the samples taken thereafter, the frequency of VEGF-C+ cells showed "normalization" to the control values in the biopsies taken later (Figure 3, a and b)
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We found high intraindividual correlation between CD68 and VEGF-C expression in all samples (r = 0.510; P < 0.001, n = 44, paraffin-embedded material). We could demonstrate by triple immunofluorescence that VEGF-C was highly coexpressed with CD68 (cryosections). Evaluation of 100 randomly selected VEGF-C+ cells (defined as VEGF-C+ cells with DNA fluorochrome staining nucleus) of eight sections revealed 82% coexpression of VEGF-C with CD68 (Supplemental Figure 1C at http://ajp.amjpathol.org).
Expression of VEGF-D
In the cryosections, we could demonstrate that VEGF-D expression was low (Figure 3c
; Supplemental Figure 1D at http://ajp.amjpathol.org). Furthermore, we found no significant difference between the irradiated skin and the individual control samples (densities, 3.9 ± 2.9 and 3.6 ± 2.5/mm VEGF-D+ cells, respectively; P = 0.351, Wilcoxon test, n = 20).
Correlation with Other Patient Features
In the irradiated skin, we found a negative correlation of chronological age with the T/C ratio and the absolute densities of small lymphatics <10 µm (r = –0.384, P = 0.017, n = 38; or r = –0.343, P = 0.03, n = 40, respectively).
Clinically detectable lymphedema occurred in 10 of 40 cases (four cases in the patient group 2 to 8 weeks after radiotherapy, and six cases of persistent lymphedema in the patients 11 months after radiotherapy). In the six samples from previously irradiated skin of patients with persistent lymphedema, the mean density of small lymphatics (<10 µm) was 0.43 ± 0.40/mm (T/C ratio, 1.05 ± 0.63), whereas in the 21 cases without lymphedema, this value was 1.23 ± 1.02/mm (T/C ratio, 2.16 ± 1.76) (P = 0.033 for absolute values and P = 0.209 for T/C ratios, U-test). There was no significant difference in the other diameter groups of the lymphatic microvessels (Supplemental Table 2 at http://ajp.amjpathol.org). No correlation of histological tumor features (T or N status, number of positive nodes, primary grading, estrogen, or progesterone receptor status), drugs, or co-morbidity was detectable with microvessel densities, macrophage, or VEGF-C+ macrophage densities.
Influence of Therapy
In the 26 samples harvested 2 to 8 weeks after radiotherapy, we found correlations between previous radiotherapy and total CD68+ macrophage density ( = 0.49, P = 0.012) as well as between radiotherapy and the density of VEGF-C+ macrophages ( = 0.55, P = 0.006). Axillary dissection, too, correlated with total macrophage density in these samples ( = 0.45, P = 0.020) but not with the number of VEGF-C+ macrophages. Nevertheless, logistic regression revealed radiotherapy as the only significant factor that predicted total macrophage density (odds ratio, 7.50; 95% confidence interval, 1.31 to 43.03; P = 0.024). The number of dissected lymph nodes, mastectomy (versus breast-conserving therapy), or systemic therapies (chemotherapy and hormone therapy) showed correlation with neither the total macrophage density nor the density of VEGF-C+ macrophages.
In the 54 biopsies that were taken 11 or more months after radiotherapy, the density of small lymphatics (<10 µm) correlated with previous radiotherapy ( = 0.29, P = 0.031). All other therapeutic factors (axillary dissection, number of dissected lymph nodes, mastectomy versus breast-conserving therapy, or systemic therapies) showed no correlation with the density of small lymphatics. Of all therapeutic procedures, logistic regression demonstrated that radiation was the only significant predictor of the density of small lymphatics (<10 µm) in these samples (odds ratio, 4.04; 95% confidence interval, 1.30 to 12.58; P = 0.016).
To delimit the influence of surgery from radio effects, a group of unirradiated patients was investigated for the presence of podoplanin+ lymphatic vessels in the upper dermis using the same conditions. Results were compared with those of the irradiated patients. In this comparison, 22 skin samples from 11 melanoma patients who had undergone lymph node dissection (but no radiation) 9 to 19 months previously were studied. We found no significant difference between the absolute lymphatic vessel counts of samples from operated and contralateral control areas (Table 3)
. T/C ratios showed no significant distinction from normal (Table 3)
. Furthermore, both the operated and the control skin of the melanoma patient control group showed high agreement with the normal control values of the breast cancer patients (Tables 2 and 3)
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Discussion |
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A major result of our study is that, in contrast to studies with higher doses,2,28-30 neither blood vessels nor lymphatics are reduced in the early after-radiation phase in the tumor environment that received an intermediate radiation dose. This, together with our finding of VEGF-C increase and the known regulatory effects of VEGFs on tumor cells,31-34 might be of importance for the potency of local recurrence and metastasis development in case of remaining tumor cells in the tumor environment.
Summarizing our results on lymphatic vessels after radiotherapy, we found significantly higher densities of lymphatic microvessels in biopsies taken 1 or more years after radiotherapy than in the control skin and in the skin shortly after radiotherapy. The increase was mainly produced by a duplication of the density of the small lymphatic capillaries within the 1st year after radiation. This constellation could be regarded as a strong sign of interim lymphangiogenesis. It should be pointed out in this context that the analyzed subgroups were relatively small.
One of the most interesting aims of this study was to provide evidence that lymphogenic effects do not result from surgery. In all patients, the biopsy sides were clearly spatially separated from the sides of previous surgical activities (>2 cm) while they were covered completely by radiation. Thus, this study was primarily designed to investigate the influence of radiation but not that of wound healing. In accordance, logistic regression demonstrated that the increase in lymphatic densities in the breast cancer patients was linked to previous radiotherapy but not to operative therapy or other covariates. Furthermore, we compared our results with a group of unirradiated patients who had undergone lymph node dissection for melanoma. In these unirradiated controls, we found no similar increase of lymphatic densities in the 1st year after therapy (neither significance nor tendency). This allows the conclusion that the increase in the density of lymphatics during the 1st therapeutic year is primarily radiogenic.
It should be noted that this effect was observed after a total dose of less than 40 Gy administered with single doses of about 1.5 Gy or less because the radiation dose in the superficial skin in the treatment of breast cancer (with the radiation technique mentioned) is about 30% lower than the reference dose in the target volume. According to standard radiobiological models such as the linear-quadratic model, this dose (assumed skin dose of 35 Gy in single doses of 1.4 Gy) would be considered equivalent to a total dose of 30 Gy in single doses of 2 Gy (assuming an 
-to-ß ratio of 3 Gy for late effects). This dose is far lower than the tolerance dose currently assumed for blood vessels. Thus, significant and persistent changes with regard to lymphangiogenesis might be provoked by radiation doses that are much lower than previously assumed. This hypothesis is supported by recent data suggesting that microenvironmental changes and especially angiogenic cytokines may play a critical role in the radiation response of normal and tumor tissues.35
In our study, the chronological age of the patients seemed to suppress the individual lymphangiogenic potential. Furthermore, cases with coexisting chronic lymphedema showed a reduction of radiogenic effect in the small lymphatics. Lymphedema occurs in up to 40% of breast cancer patients after axillary lymph node dissection in combination with radiotherapy.36 Two peaks of incidence of lymphedema can be distinguished: reversible acute lymphedema (in the first 3 months after radiotherapy) and chronic lymphedema (starting frequently after 1 or more years). Our data give first indication of a possible role of reduced individual ability to develop small lymphatics in the development of chronic lymphedema.
Looking for the potential mechanism of induction of lymphangiogenesis, we found increased numbers of CD68+/VEGF-C+ cells in the first weeks after radiation, suggesting a regulation of lymphangiogenesis by VEGF-C-expressing macrophages. Interestingly, a slightly elevated number of CD68+ macrophages persisted during the first years after radiotherapy, whereas VEGF-C expression was reduced to the control values. Approximately 20% of the CD68+ cells were VEGF-C+ in the early samples (taken 2 to 8 weeks after radiation) in our study. A similar frequency of VEGF-C expression was observed previously in tumor-associated macrophages of the peritumoral tissue.15 Moreover, we have previously shown that the density of macrophages correlates with lymphatic capillary proliferation.15 Macrophages seem to support lymphangiogenesis in two different ways, either by trans-differentiating and direct incorporation into the endothelial layer or by stimulating division of pre-existent local lymphatic endothelial cells.37 VEGF-C itself stimulates recruitment of macrophages and increases inflammatory response in malignant tumors.13 In this, VEGF-C seems to be chemotactic for macrophages via VEGFR-3.13 The role of VEGF-C stimulation on lymphangiogenesis could be demonstrated recently by blockade of VEGFR-3.38 Other studies have shown that lymphangiogenesis is driven by co-stimulation with VEGF-C.39,40 In contrast to VEGF-C, we found no major stimulation of VEGF-D expression. VEGF-D (beside VEGF-C) is another ligand of the tyrosine kinase VEGFR-3 receptor. It has previously been shown that VEGF-D is important in neonatal and tumor lymphangiogenesis but seems to play a subordinate role in comparison with VEGF-C.15,18,41-43
Metastasis of breast cancer occurs primarily through the lymphatic system. It has previously been shown that induction of tumor lymphangiogenesis by tumor cell-derived VEGF-C promotes breast cancer metastasis.10 Thus, it can be hypothesized that both lymphangiogenesis and the direct tumor cell-stimulating potential of VEGF-C might contribute to the development of local recurrences in case of surviving tumor cells in the after-irradiation tumor environment. It should be pointed out that our results do not contradict the indisputable curative antitumoral effect of the radiation in the target fields. Radiogenic lymphangiogenesis can be regarded as a factor for local recurrences only in case of tumor cells outside of surviving tumor cells in the environment of the irradiated tumor. Furthermore, radiation therapy clearly increases the risk of lymphedema. Thus, the increase in small lymphatic vessels with radiation does not seem to increase the functional lymphatic drainage in and around an irradiated tumor bed.
It should be further noted that some aspects cannot be completely clarified with our material and the setting of our study (sampling followed routine follow-up schedule). The first aspect is that the number of VEGF-C-expressing macrophages was highest at 0.5 to 2 months after therapy, whereas high lymphatic vessel numbers were observed in samples of approximately 1 year after therapy. Thus, the time window of the increase of lymphatic density can be identified between >2 and <11 months after therapy by our material. From the early increase of VEGF-C expression, it is probable that the increase of lymphatic density might take place in the earlier part of the identified time window—a period that needs further investigation. Second, it is reasonable to assume that some factors other than VEGF-C may be contributing as well, possibly at the time point later than 2 months after therapy. Third, although observation on the role of macrophages in lymphogenesis has been made in peritumoral tissue or renal transplants,15,44 our findings indicate, but cannot completely prove, a similar process in the skin.
We used CD34 for investigation of blood microvessels. It has been shown previously that CD34 is expressed in blood endothelial cells and lymphatic endothelial cells of human malignancies.45 Nevertheless, in our setting, it can be regarded as a marker for blood vessels in the environment of non-neoplastic skin.45,46 According to the results of our study, blood microvessels seem to be less influenced by intermediate-dose radiotherapy. We found qualitative microscopic radiation effects on microvessels (disfiguring). The only quantitative alteration of blood microvessels was an early and transient increase of the number of bigger vessels (>16 µm) as had already been shown previously in porcine skin.1 It can be concluded from the vessel diameter and microstructure that this effect was generated by an acute postexpositional increase in the number of enlarged venules.27 Most probably, this is an effect of higher tissue perfusion in the postinflammatory phase of the early postradiation period.
Summarizing, we demonstrated a considerable increase in the density of small lymphatic microvessels during the 1st year after an intermediate (tumor-environmental) dose of radiotherapy in the skin. We could demonstrate that radiogenic lymphangiogenesis might be mediated by macrophage-expressed VEGF-C. Age and occurrence of lymphedema were found to be linked with impaired capacity to develop lymphatic capillaries. Blood microvasculature revealed a transient expansion of postcapillary venules during the first weeks, but neither signs of angiogenesis nor substantial depletion of microvessel density during the first years after intermediate-dose radiotherapy was observed.
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Footnotes |
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Supported by the German Society of Lymphology (to P.H.), the Ministry of Education and the Arts of the State of Saxony-Anhalt, Germany (to P.H. and E.F.), and the European Union 6th Framework Integrated Project "Lymphangiogenesis" (LSGH-2004-503573 to D.K.).
S.J. and M.J. contributed equally to this article.
Supplemental material for this article can be found on http://ajp.amjpathol.org.
Accepted for publication March 27, 2007.
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References |
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B signaling pathway. J Biol Chem 2003, 278:5750-5759
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, largest to smallest observed values that are not outliers; 
, outlier more than 1.5 box lengths from the 25 or 75 percentile; and asterisk, outlier value >2 x SD from mean.