Ketan
Gajjar
ab,
Lara D.
Heppenstall
a,
Weiyi
Pang
a,
Katherine M.
Ashton
b,
Júlio
Trevisan
a,
Imran I.
Patel
a,
Valon
Llabjani
a,
Helen F.
Stringfellow
b,
Pierre L.
Martin-Hirsch
ab,
Timothy
Dawson
b and
Francis L.
Martin
*a
aCentre for Biophotonics, Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK. E-mail: f.martin@lancaster.ac.uk; Tel: +44 (0)1524 510206
bLancashire Teaching Hospitals NHS Trust, Royal Preston Hospital, Sharoe Green Lane North, Preston, Lancashire, UK
First published on 6th September 2012
The most common initial treatment received by patients with a brain tumour is surgical removal of the growth. Precise histopathological diagnosis of brain tumours is to some extent subjective. Furthermore, currently available diagnostic imaging techniques to delineate the excision border during cytoreductive surgery lack the required spatial precision to aid surgeons. We set out to determine whether infrared (IR) and/or Raman spectroscopy combined with multivariate analysis could be applied to discriminate between normal brain tissue and different tumour types (meningioma, glioma and brain metastasis) based on the unique spectral “fingerprints” of their biochemical composition. Formalin-fixed paraffin-embedded tissue blocks of normal brain and different brain tumours were de-waxed, mounted on low-E slides and desiccated before being analyzed using attenuated total reflection Fourier-transform IR (ATR-FTIR) and Raman spectroscopy. ATR-FTIR spectroscopy showed a clear segregation between normal and different tumour subtypes. Discrimination of tumour classes was also apparent with Raman spectroscopy. Further analysis of spectral data revealed changes in brain biochemical structure associated with different tumours. Decreased tentatively-assigned lipid-to-protein ratio was associated with increased tumour progression. Alteration in cholesterol esters-to-phenylalanine ratio was evident in grade IV glioma and metastatic tumours. The current study indicates that IR and/or Raman spectroscopy have the potential to provide a novel diagnostic approach in the accurate diagnosis of brain tumours and have potential for application in intra-operative diagnosis.
The aetiology of brain tumours remains largely obscure. The only two aetiological factors thus far proven are familial syndromes (Li-Fraumeni syndrome, neurofibromatosis, Von Hippel-Lindau syndrome, Turcot syndrome, and Gorlin syndrome) and ionizing radiation.5 Studies into potential neurocarcinogens like electromagnetic field exposure, cell phone use, tobacco, and environmental causes have failed to prove any causative link.6 Autoimmune disorders, asthma, and allergies appear to have a protective role in gliomas.7
Brain tumours, particularly high-grade (grades III and IV), have poor prognosis and patient survival is associated with age and histological type.7 In 1999, the 5-year survival rate for brain tumours in the US for all ages and types was reported as 20% (95% CI, 18–22%), without significant improvement in the preceding 30 years.8 Patients with glioblastoma multiforme (GBM) consistently have the poorest survival. Benign meningioma patients have good overall survival rates, in the range of 81% at 2 years and 69% at 5 years, but for malignant meningioma the 5-year survival rate drops to 54.6%.9 Other factors that predict overall and progression-free survival include the tumour location and the extent of resection.10
In UK, about 25% of brain tumours in adults are meningiomas.2 The vast majority (≈80%) are grade I and are commonly treated with surgical resection alone. There is a fairly well-defined relationship between completeness of resection and likelihood of recurrence of meningioma.11 About half of all primary brain tumours are gliomas, which are further classified into common subtypes such as low-grade astrocytoma (grade I and II), anaplastic astrocytoma (grade III) and glioblastoma multiforme (grade IV). This grading can be very subjective and particular tumours often do not fit neatly into any given grade.12
Metastatic brain tumours are the most frequently occurring intracranial neoplasms in adults with the US annual incidence being ≈200000 cases.13 The majority of metastases originate from primary cancers in lung (40–50%) or breast (15–25%), or from melanoma (5–20%),14 while in about 15% of patients the primary site remains unidentified.15 Treatment for metastatic brain tumours is centred around surgical and/or radiation therapy. Surgery is a viable option for patients with only one or a small number of lesions located in accessible regions of the brain and can result in rapid relief of symptoms.14,16
Currently, there are two important challenges in the management of brain tumours. The first is accurate diagnosis with determination of grade to guide treatment and predict survival whilst the second is precision in defining intra-operative surgical margins. The current approach for diagnosis and histological grading is to obtain tumour sections by biopsy or cytoreductive surgery, stain with haematoxylin and eosin (H&E) as well as applying an array of immunohistochemical neuronal marker proteins. Immunohistochemical detection of isocitrate dehydrogenase (IDH) expression is considered a useful marker in gliomas.17 However, these methods have limitations, which include subjective interpretation.18 With the recognition of an increasing number of brain tumour phenotypes as well as newly evolving variants, there appears to be a need for the development of more robust and accurate diagnostic tools.
During surgery, complete removal of the tumour is one of the most important factors for prediction of recurrence-free survival.19 Various intra-operative diagnostic imaging techniques are available to delineate the excision border; however, no single technique has the spatial resolution to the level required by surgeon-working precision. Despite the use of neuronavigational guidance tools, the precise resection of the brain tumour is hampered due to “brain shift” in stereotactic surgery.20 Brain shift refers to intra-operative brain deformation as a result of changes in tumour volume, cerebrospinal fluid drainage, intracranial pressure or the use of brain retractors that render preoperative neuronavigation registration inaccurate.21 Dedicated high-field intra-operative MRI (iMRI) systems show promising results but the major limitation is cost.22 Stummer and colleagues developed a tumour-specific fluorescent marker, 5-aminolevulinic acid (5-ALA) that allows more accurate discrimination of infiltrating tumour from normal brain parenchyma.23 The limitation of this technique is the limited penetration depth of blue light (mm). Also, non-enhancing tumours do not fluoresce well and the view is often obscured by blood products.24 Another method to delineate tumour margins is to obtain an intra-operative smear or biopsy which provides cell-level information. This is of limited usefulness as it is purely an ex vivo technique and even with rapid staining protocols, at least 20 min are needed to deliver a diagnosis. The results are also only as good as the chosen area of smear or biopsy by the operating surgeon.
Vibrational spectroscopy has potential as a bio-analytical tool for diagnosing cancer because it can probe the chemical composition and molecular structure of normal and pathological tissue.25 Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy determines the fingerprint structure of several molecules including proteins, carbohydrates, DNA/RNA or fatty acids. Infrared (IR) spectroscopic imaging measures a large number of spectral profiles from particular tissue subtypes; then computational algorithms identify potentially relevant spectral markers and facilitate classification.26 Such methods offer a means to identify robust diagnostic spectral patterns, even with substantial intra-class variability.27 Examples of such methods are principal component analysis (PCA), linear discriminant analysis (LDA) and fuzzy cluster analysis, each of which have been employed for the analysis of IR/Raman spectra derived from biological tissues.28
ATR-FTIR spectroscopy provides spectra from intact cells recorded within a few seconds and spectral images of tissue sections within minutes. However, it is an ex vivo technique and requires a dry specimen. Advantages of Raman spectroscopy include intra-operative, in vivo diagnosis with non-destructive, real-time analyses. Unlike IR spectroscopy, water does not influence Raman spectra. Raman spectroscopy has the potential to delineate tumour margins and identify tumour remnants while preserving normal tissue.29,30 Fibre-optic probes give spatial flexibility and spatial resolution may be chosen according to surgical need.30
In this study, we hypothesized that interrogation of brain tissues with IR and/or Raman spectroscopy will allow diagnostic segregation of tumours. Given that protein and DNA conformational changes occur in most pre-cancer or cancer lesions, spectroscopy techniques allow their detection. Following spectroscopic analyses, spectral data were analyzed using LDA. When appropriate, PCA was used to reduce the dataset dimensions before LDA was employed to reveal clustering. Scores plots generated following LDA were used to discriminate tumour subtypes, where closeness between clusters reveals spectral similarities and segregation indicates dissimilar classes. The cluster vector approach allowed determination of any identifiable spectral biomarkers segregating tumour subtypes. Additionally, we tested whether spectroscopic identification of subtypes of glioma was more successful than classification based on immunohistochemical biomarkers like isocitrate dehydrogenase-1 (IDH1) and p53.
BTNW code | Age (years) | Sex | Histology | WHO grade/primary site of origin of metastatic tumour | ATR-FTIR spectroscopy (number of spectra) | Raman spectroscopy (number of spectra) |
---|---|---|---|---|---|---|
3 | 24 | F | Glioblastoma multiforme | Grade 4 | 20 | 50 |
4 | 63 | M | Glioblastoma multiforme | Grade 4 | 20 | 50 |
5 | 47 | F | Glioblastoma multiforme | Grade 4 | 20 | 50 |
12 | 64 | M | Glioblastoma multiforme | Grade 4 | 20 | 50 |
2 | 68 | F | Glioblastoma multiforme | Grade 4 | 20 | 50 |
38 | 48 | M | Anaplastic astrocytoma | Grade 3 | 20 | 50 |
173 | 50 | M | Anaplastic astrocytoma | Grade 3 | 20 | 50 |
458 | 71 | F | Anaplastic astrocytoma | Grade 3 | 20 | 50 |
515 | 65 | M | Anaplastic astrocytoma | Grade 3 | 20 | 50 |
126 | 66 | M | Anaplastic astrocytoma | Grade 3 | 20 | 50 |
20 | 26 | M | Low-grade astrocytoma | Grade 2 | 20 | 50 |
203 | 42 | F | Low-grade astrocytoma | Grade 2 | 20 | 50 |
365 | 42 | F | Low-grade astrocytoma | Grade 2 | 20 | 50 |
422 | 25 | M | Low-grade astrocytoma | Grade 2 | 20 | 50 |
680 | 59 | M | Low-grade astrocytoma | Grade 2 | 20 | 50 |
1 | 39 | F | Meningioma | Grade 1 | 20 | 50 |
36 | 59 | M | Meningioma | Grade 1 | 20 | 50 |
84 | 57 | M | Meningioma | Grade 1 | 20 | 50 |
88 | 57 | F | Meningioma | Grade 1 | 20 | 50 |
91 | 56 | F | Meningioma | Grade 1 | 20 | 50 |
95 | 71 | F | Meningioma | Grade 1 | 20 | 50 |
99 | 55 | F | Meningioma | Grade 1 | 20 | 50 |
139 | 56 | F | Meningioma | Grade 1 | 20 | 50 |
143 | 65 | F | Meningioma | Grade 2 | 20 | 50 |
145 | 31 | F | Meningioma | Grade 1 | 20 | 50 |
148 | 73 | F | Meningioma | Grade 1 | 20 | 50 |
151 | 47 | M | Meningioma | Grade 1 | 20 | 50 |
262 | 75 | F | Meningioma | Grade 1 | 20 | 50 |
291 | 52 | F | Meningioma | Grade 1 | 20 | 50 |
297 | 80 | M | Meningioma | Grade 2 | 20 | 50 |
34 | 83 | F | Metastasis | Unknown primary site | 20 | 50 |
78 | 52 | F | Metastasis | Non-small cell lung cancer | 20 | 50 |
137 | 63 | M | Metastasis | Colon cancer | 20 | 50 |
164 | 54 | M | Metastasis | Oesophageal cancer | 20 | 50 |
181 | 79 | F | Metastasis | Unknown primary site | 20 | 50 |
182 | 70 | F | Metastasis | Non-small cell lung cancer | 20 | 50 |
215 | 34 | M | Metastasis | Lung cancer | 20 | 50 |
253 | 64 | F | Metastasis | Ovarian/breast cancer | 20 | 50 |
295 | 72 | F | Metastasis | Non-small cell lung cancer | 20 | 50 |
313 | 58 | M | Metastasis | Squamous cell carcinoma | 20 | 50 |
379 | 72 | M | Metastasis | Unknown primary site | 20 | 50 |
409 | 84 | F | Metastasis | Bowel cancer | 20 | 50 |
119 | 44 | F | Metastasis | Breast cancer | 20 | 50 |
271 | 81 | F | Metastasis | Previous squamous cell carcinoma | 20 | 50 |
274 | 67 | M | Metastasis | Colon cancer | 20 | 50 |
7 | 66 | M | Normal brain | N/A | 20 | 50 |
83 | 58 | M | Normal brain | N/A | 20 | 50 |
132 | 64 | F | Normal brain | N/A | 20 | 50 |
136 | 51 | F | Normal brain | N/A | 20 | 50 |
625 | 51 | F | Normal brain | N/A | 20 | 50 |
678 | 48 | F | Normal brain | N/A | 20 | 50 |
713 | 39 | F | Normal brain | N/A | 20 | 50 |
Total | 52 (45 tumours, 7 normal) | 1040 | 2600 |
Fig. 1 Microscopic images of normal and different tumour subtypes of the brain. Haematoxylin and eosin (H&E) staining of normal brain (Nor) tissue is shown in (A); low-grade tumours like meningioma (Men) in (B); Glioma WHO grade II or low-grade astrocytoma (LA) in (C); high-grade tumours like Glioma WHO grade III or anaplastic astrocytoma (AA) in (D); Glioma WHO grade IV or glioblastoma multiforme (GBM) in (E); and, metastatic brain tumours (Mets, primary colon cancer) in (F). Scale bar = 50 μm. |
Fig. 2 Comparative analysis of glioma tumours using immunohistochemistry (IDH1 and p53 staining) vs. vibrational spectroscopy. (A) shows characteristic staining for IDH1-R132H; and, (B) for p53 in low-grade glioma. LD1 scores plots from PCA-LDA representing spectra from LA, AA and GBM are shown in (C) for IR spectra and in (D) for Raman spectra. P-values from scores plot results using ANOVA test show that LA tumours were statistically significant from other gliomas (*, P ≤ 0.05; **, P ≤ 0.01). |
IDH1 staining | Grades of glioma | ||
---|---|---|---|
LA (n = 5) | AA (n = 5) | GBM (n = 5) | |
Positive (33%) | 3 | 1 | 1 |
Negative (67%) | 2 | 4 | 4 |
Grades of p53 staining | Grades of glioma | ||
---|---|---|---|
LA (n = 5) | AA (n = 5) | GBM (n = 5) | |
Negative (27%) | 2 | 1 | 1 |
0–25 % | 1 | 3 | 1 |
26–50% | 1 | 0 | 1 |
51–75% | 1 | 1 | 0 |
>75% | 0 | 0 | 2 |
IR and Raman spectra of LA, AA and GBM were analyzed using the PCA-LDA method and LD1 scores plots were obtained. As seen in Fig. 2, the LD1 scores of IR and Raman spectra for LA is significantly different from AA and GBM.
Fig. 3 Compares the discriminating power of IR and Raman spectroscopy for normal brain tissue vs. brain tumours. (A) shows the average absorbance spectra of the biochemical-cell fingerprint regions for IR spectroscopy (1800 cm−1 to 900 cm−1); and, (B) for Raman spectroscopy (1750 cm−1 to 800 cm−1). (C and D) shows LD1 scores plot for IR and Raman spectroscopy respectively and represents the spectra from normal brain compared to brain tumours. (E and F) shows the mean ± SD of the spectral points. The difference of the spectral points for normal vs. brain tumour tissue is statistically significant (***, P ≤ 0.0001) for both techniques. |
The sub-grouped spectra of all tumour types show notable differences in the wavenumber regions 1100 cm−1 to 1050 cm−1 (Fig. S1A and C; see ESI†). This region corresponds to carbohydrate moieties and these differences may indicate variations in the ganglioside profile of human gliomas with increasing degree of malignancy.42,43 The 2nd-most prominent divergent spectral trend is seen in the region of 1550 cm−1 to 1400 cm−1, which tentatively is associated with protein moieties while the spectral alterations in the region of 1760 cm−1 to 1720 cm−1 may arise from the lipid constituents (e.g., phospholipids and possibly others).43
Fig. 4A and B shows the 3-D scores plot derived after LDA of IR and Raman spectra, comparing all categories of brain tissues [i.e., Normal (Nor), meningioma (Men), low-grade astrocytoma (LA), anaplastic astrocytoma (AA), glioblastoma multiforme (GBM) and metastatic tumours (Mets)]. Good separation is observed between the normal brain tissue and different types of tumours in all LDs following ATR-FTIR spectroscopy, whereas following Raman spectroscopy a degree of overlap is observed with all tissue types except for LA in LD2 and LD3.
Fig. 4 Comparison of LDA scores plots and cluster vector plots derived from the spectra acquired from normal brain and tumour tissue subtypes. (A) shows scores plot derived from IR spectra (n = 1040 spectra; 20 spectra per patient); (B) Raman spectra (n = 2600 spectra; 50 spectra per patient); and, (C and D) shows corresponding cluster vector plots (peak detection plots) with wavenumbers discriminating tumour subtypes from normal tissue. |
Fig. 5 LDA scores plot of IR spectra showing the inter-class variance. To obtain this scores plot, each patient in the dataset is treated as a class without specifying the histological classes (i.e., normal or tumour). After LDA, individual patients' spectra are given a matching colour and symbol in accordance with their original tissue types. Thus, any observed clustering of patients would be spontaneous suggesting a common underlying biochemical signature. |
Normal vs. tumour type | P-value |
---|---|
Normal vs. Men | P ≤ 0.001 |
Normal vs. LA | P > 0.05 |
Normal vs. AA | P ≤ 0.01 |
Normal vs. GBM | P ≤ 0.01 |
Normal vs. Mets | P ≤ 0.001 |
Fig. 6 (A) Shows 1-D scores plot of lipid-to-protein ratio (based on the intensity at wavenumbers 1740 cm−1/intensity at 1400 cm−1) for normal brain tissue and different types of tumours. The transverse bars represent mean ± SD of spectrally-derived estimations for a particular tissue type. The ratio of lipid to protein is higher in normal brain tissue compared to tumours with significant difference between normal tissue and meningioma (P ≤ 0.001), high-grade gliomas (AA and GBM) (P ≤ 0.01) and metastatic tumours (P ≤ 0.001) but not with low-grade glioma (LA). (B) shows 1-D scores plots of phosphate (1045 cm−1) to carbohydrate (1545 cm−1) ratio for normal brain tissue and brain tumours. Differences in the ratio are apparent between normal tissue and high-grade gliomas (AA and GBM) (P ≤ 0.001). (C) shows ratio of RNA (1121 cm−1) to DNA (1020 cm−1) comparing the spectral points acquired by LDA for normal brain tissue and various tumour grades. The RNA-to-DNA ratio is significantly altered from normal brain tissue compared to meningioma (P ≤ 0.001) and to a lesser extent in metastatic tumours (P ≤ 0.05). |
Fig. 6B shows the ratio of phosphate-to-carbohydrate which is obtained by calculating the ratio of band intensities at 1045 cm−1 and 1545 cm−1 in each IR spectrum. This scale may provide information on metabolic turnover in tissues. The phosphate-to-carbohydrate ratio is reduced in high-grade gliomas (AA and GBM), but there is no significant difference in the ratio between normal tissue compared to meningioma, low-grade glioma or metastatic tumours (Table 4). The ratio of IR peak intensities located at 1121 cm−1 and 1020 cm−1 giving an RNA/DNA ratio can be used as a potential biomarker to predict the cell proliferation in the normal or malignant tissue.44,45 The RNA/DNA ratio in IR spectra was reduced significantly in meningioma (P ≤ 0.001) and to a lesser extent in metastatic tumours (P ≤ 0.05) compared to normal brain tissue (Fig. 6C, Table 5).
Normal vs. tumour type | P-value |
---|---|
Normal vs. Men | P > 0.05 |
Normal vs. LA | P > 0.05 |
Normal vs. AA | P ≤ 0.001 |
Normal vs. GBM | P ≤ 0.001 |
Normal vs. Mets | P > 0.05 |
Normal vs. tumour type | P-value |
---|---|
Normal vs. Men | P ≤ 0.001 |
Normal vs. LA | P > 0.05 |
Normal vs. AA | P > 0.05 |
Normal vs. GBM | P > 0.05 |
Normal vs. Mets | P > 0.05 |
Comparisons | Top six discriminating wavenumbers with tentative biochemical assignments | |||
---|---|---|---|---|
ATR FTIR spectroscopy | Raman spectroscopy | |||
Normal vs. meningioma | 1018 | Glycogen | ≈911 | C–C stretching of proline ring/glucose/lactic acid |
1173 | Carbohydrate | ≈964 | Lipids, proteins (CH3 deformations) | |
1543 | Amide I | ≈1237 | Amide III | |
1582 | Amide II | 1276 | Amide III (α-helix) | |
1620 | Amide I | 1485 | Lipids and proteins (CH2 deformation), purine ring(guanine) | |
1740 | Lipids | 1655 | Amide I/lipids | |
Normal vs. LA | 1103 | ν sPO2− | 800 | Undefined |
1234 | ν asPO2− | 903 | Undefined | |
1470 | CH2 bending of the methylene chains in lipids | ≈999 | Glucose-I-phosphate and symmetric ring breathing mode of phenylalanine | |
1504 | Amide II | 1306 | Lipids, collagen, protein amide III, DNA purine bases, phenylalanine | |
1628 | Amide I | 1446 | Proteins and lipids (CH2 bending mode of proteins and lipids) | |
1686 | Amide I | 1670 | Cholesterol esters, Amide 1 | |
Normal vs. AA | 1018 | Glycogen | 810 | Undefined |
1234 | Asymmetric phosphate | 853 | Tyrosine, proline, glycogen | |
1489 | In-plane CH bending vibration | 911 | C–C stretching of proline ring/glucose/lactic acid | |
1551 | Amide II | 1004 | Lipids and proteins, phenylalanine | |
1628 | Amide I | ≈1455 | Protein δ (CH2/CH3) | |
1701 | Lipid | 1670 | Cholesterol esters, Amide 1 | |
Normal vs. GBM | 1107 | Glycogen | ≈849 | Tyrosine and proline, glycogen |
1393 | COO– symmetric stretching | 904 | Undefined | |
1474 | Proteins? | ≈917 | C–C stretching, glycogen, lactic acid | |
1531 | Amide II, Modified guanine? | 1001 | Phenylalanine (symmetric ring breathing mode) | |
1585 | Amide I | ≈1473 | CH2 deformation | |
1659 | Amide I | 1673 | Lipids, Amide I | |
Normal vs. metastasis | 1173 | Carbohydrate | ≈997 | Phospholipids, glucose-I-phosphate |
1489 | In-plane CH bending vibration | 1077 | Lipids (C–C vibrations) | |
1543 | Amide II | 1241 | Amide III | |
1632 | Amide I | 1446 | Proteins and lipids (CH2 bending mode of proteins and lipids) | |
1659 | Amide I | ≈1460 | Cytosine | |
1740 | CO stretching (lipids) | 1654 | Amide I (CO stretching mode of proteins, α-helix conformation)/CC lipid stretch |
When the ratio of cholesterol esters (1670 cm−1) to phenylalanine (1001 cm−1) for normal brain tumour was compared with tumour subtypes, we observed an increase in the ratio for meningioma, whereas the ratio was reduced for LA, GBM and metastatic tumours. The ratio difference was most noticeable between meningioma and low-grade astrocytoma with reduction in the mean ± SD of spectral intensity ratio (Fig. S8; see ESI†). Additionally, the ratio of intensities at wavenumbers 1654 cm−1 (Amide 1 α-helix) to 1446 cm−1 (CH2 bending mode of proteins and lipids) in Raman spectra showed significant reduction from normal brain tissue to Men, LA, AA and metastatic tumours (Fig. S9; see ESI†). This ratio has been used to differentiate normal and cancerous tissues at different sites including brain, breast and gynaecological tissues.50–52
As seen in the 3-D scores plot in Fig. 4(A and B), ATR-FTIR spectroscopy is able to distinguish meningioma from various grades of gliomas as well as from metastatic tumours, while Raman spectroscopy was able to distinguish low-grade glioma from high-grade gliomas and from normal brain tissue. From a clinical as well as histological perspective, low-grade gliomas are poorly demarcated and the ability of vibrational spectroscopy to distinguish their extent during surgery could be of potential advantage. Moreover, it is often difficult to determine where the tumour ends and normal tissue begins either by imaging, direct observation during surgery or gross pathological examination.53
LD1 scores plots following PCA-LDA of normal brain compared to individual tumour subtypes for IR (Figs. 2–6, panel A) and Raman spectra (Figs. 2–6, panel B) showed separation of tissue classes with a degree of overlap between them. These between-class similarities and differences could be attributable to the inherent heterogeneity of tumour tissues. Brain tumours may contain various grades of neoplastic tissue, stromal elements, haemorrhage and necrotic tissue, making the spatially-resolved IR spectral data derived from μm-size tissue sample sections non-representative of the tumour class. Despite this limitation important molecular markers were identified by wavenumber assignments resulting in segregation of normal tissue from different tumour types. In derived PCA-LDA scores plots (Fig. 5), marked within-class variation (i.e., heterogeneity) was noted; even so, good discrimination between different grades of glioma was also observed (i.e., towards between-category discriminating biomarkers). This is explained by the fact that the gene and protein expression of morphologically-similar astrocytoma tissues can vary depending on the patients' tumour grade.
The transition of normal brain tissue to neoplastic tissue is connected with qualitative and quantitative changes of lipids. Lipid-to-protein ratio is of particular interest due to its ability to distinguish normal brain tissue and tumour tissue. Tumour tissue shows marked decreases in the bands associated with lipids and subtle changes in the main protein bands. Earlier studies have shown that the magnitude of the lipid-to-protein ratio correlates with the progression of malignancy in gliomas.54 As seen in Fig. 6A, the lipid-to-protein ratio decreases significantly going from normal to glioma tissue as well as in metastatic brain tumours. However the decrease in the band intensity does not appear to reflect the worsening grades of glioma in our study. A plausible explanation for this could be that although astrocytic gliomas are classified in grade II–IV, it is not an absolute classification. A single tissue section of any grade of glioma may still encompass regions characteristic of all four grades of malignancy.43 We also observed reductions in the phosphate-to-carbohydrate ratio in grade III and grade IV gliomas compared to normal brain tissue (Fig. 6B, Table 3), possibly suggesting similar underlying biochemical alterations in grade III and grade IV gliomas. Certainly it is arguable from the above ratio observations that glioma grade III (anaplastic astrocytoma) is not in the middle of the biological spectrum but closer to the highly malignant glioma grade IV (glioblastoma). The ratio of phospholipids to proteins in Raman spectra (Fig. S7; see ESI†) for normal brain and different tumour subtypes has shown a decreasing trend with progression of gliomas. As well as the reduction in lipids with increasing grade of malignancy, relative increases in nucleic acids and proteins in tumour tissues may contribute to the reduction in the ratio.
The concentration of minor lipid cholesterol esters can increase up to 100 times in gliomas compared to the trace amounts found in normal brain tissue.55 In a recent study utilizing Raman spectroscopy for grading of astrocytoma, phenylalanine bands appeared to give important contributions discriminating high-grade gliomas (AA and GBM) compared to normal tissue.30 Reduced band intensities for phenylalanine had been reported in dysplastic tissue compared to normal tissue in previous studies.48,56 Koljenović et al. discriminated vital from necrotic glioblastoma tissues by Raman spectroscopy;57 they demonstrated that necrotic tissue contains higher levels of cholesterol than vital tumour tissue. Yamada et al. came to the same conclusions by comparing necrotic and vital carcinoma tissues.58 In our study, the most prominent contribution for distinction between normal brain tissue and gliomas is at 1670–1674 cm−1 in Raman spectra, corresponding to cholesterol and cholesterol esters.30,59–61 The second prominent band resulting into segregation of normal tissue from gliomas is at 1001 cm−1 corresponding to phenylalanine. The ratio of cholesterol esters to phenylalanine (Fig. S8; see ESI†) has the potential to be used as a marker to differentiate between meningioma and low-grade astrocytoma. In Raman spectra, the wavenumber contribution at 850 cm−1 (tyrosine and proline) is able to discriminate normal tissue from glioblastoma in our study. Similar observations were made in a recent study in which the bands at 850 cm−1 in Raman spectroscopy were considered to give evidence of high-grade tumours.30
Isocitrate dehydrogenase 1 (IDH1) mutations have recently been identified as early and frequent genetic alterations in astrocytomas and secondary glioblastomas, whereas primary glioblastomas very rarely contain IDH1 mutations. IDH1 expression is emerging as an important biomarker for gliomas with about 80% low-grade gliomas staining positive for IDH1-R132H mutation.17 In addition, several studies have demonstrated that an IDH1 mutation is associated with good prognosis and can be utilized as marker of prognosis in gliomas.17,35 In our cohort of gliomas (Table 2), IDH1 staining was positive for 60% of LA with an overall positive rate of 33% for all gliomas. A recent study, using the same antibody, has found similar low positive results for IDH1 (23.72%).62 When the glioma samples were analyzed to identify differences in the spectral intensity for both IR and Raman spectra, we found significant (P ≤ 0.01) differences in the spectra between all three grades of gliomas (Fig. 2C and D). This finding shows the ability of biospectroscopy to distinguish different grades of gliomas. Spectral biomarkers appeared to provide more robust identification of aberrant tissue than immunohistochemical markers.
The major limitation of ATR-FTIR spectroscopy lies in the fact that it is an ex vivo technique and can only be performed on fixed tissue. Raman spectroscopy is one of the optical spectroscopy techniques currently under investigation for in vivo endoscopic applications. In contrast to the ex vivo histological analysis, the concept of in vivo Raman spectroscopy is the distinction of tissues within the tumour intra-operatively. Studies using fibre-optic Raman probes under in vivo conditions have been reported for the oesophagus,63 brain,29 pre-cancer lesions in the cervical epithelium,64 lesions of breast tissue,65 and polyps in colon.66
It would be expected that substantial modifications occur at the molecular level during the process of development of brain tumours before visible changes are apparent on histological assessment by conventional H&E staining and microscopic examination for structural changes. ATR-FTIR spectroscopy and/or Raman spectroscopy allows qualitative and quantitative analysis of basic cellular components like lipids, proteins, carbohydrates and nucleic acids within biological tissues. IR/Raman spectroscopy thus has the potential to detect early changes in tissue resulting from the development of the tumour which may not be apparent in tissue sections or cell preparations.67 The information obtained by IR/Raman spectroscopy can be combined with conventional methods like histopathological grading of smears/biopsies from tumour margins or stereotactic biopsies to diagnose and grade brain tumours. This will allow for more accurate planning and execution of surgery and/or radiation therapy resulting in the concept of personalized medicine for individualized treatment with potentially better long-term survival and cure rates.
AA | Anaplastic astrocytoma |
5-ALA | 5-Aminolevulinic acid |
ATR | Attenuated total reflection |
FFPE | Formalin-fixed paraffin-embedded |
FTIR | Fourier-transform infrared |
GBM | Glioblastoma multiforme |
H&E | Haematoxylin and eosin |
IDH1 | Isocitrate dehydrogenase-1 |
IHC | Immunohistochemistry |
iMRI | Intraoperative MRI |
IR | Infrared |
LA | Low-grade astrocytoma |
LDA | Linear discriminant analysis |
Men | Meningioma |
Mets | Metastatic tumours |
Nor | Normal |
PC | Principal component |
PCA | Principal component analysis |
WHO | World Health Organization. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ay25544h |
This journal is © The Royal Society of Chemistry 2013 |