Proceedings of a National Cancer Institute workshop: MR spectroscopy and tumor cell biology

Postoperative and postradiation head and neck: role of magnetic resonance imaging

Interpretation of head and neck imaging after treatment for malignancy poses a challenge even for the experienced neuroradiologist. While computed tomography is often the preferred modality for assessment of the head and neck due to its faster acquisition, magnetic resonance imaging (MRI) is superior in the evaluation of nasopharyngeal, sinonasal and skull base tumors. In this article, we review pretherapy imaging protocols, common surgical approaches and reconstructions, postsurgical and postradiation MRI appearance and complications, MRI criteria for tumor recurrence and clinical applications of advanced MRI techniques as applicable to head and neck tumors.

Multimodal assessment of early tumor response to chemotherapy: comparison between diffusion-weighted MRI, 1H-MR spectroscopy of choline and USPIO particles targeted at cell death

The aim of this study was to determine the value of different magnetic resonance (MR) protocols to assess early tumor response to chemotherapy. We used a murine tumor model (TLT) presenting different degrees of response to three different cytotoxic agents. As shown in survival curves, cyclophosphamide (CP) was the most efficient drug followed by 5-fluorouracil (5-FU), whereas the etoposide treatment had little impact on TLT tumors. Three different MR protocols were used at 9.4 Tesla 24 h post-treatment: diffusion-weighted (DW)-MRI, choline measurement by (1) H MRS, and contrast-enhanced MRI using ultrasmall iron oxide nanoparticles (USPIO) targeted at phosphatidylserine. Accumulation of contrast agent in apoptotic tumors was monitored by T(2) -weighted images and quantified by EPR spectroscopy. Necrosis and apoptosis were assessed by histology. Large variations were observed in the measurement of choline peak areas and could not be directly correlated to tumor response. Although the targeted USPIO particles were able to significantly differentiate between the efficiency of each cytotoxic agent and best correlated with survival endpoint, they present the main disadvantage of non-specific tumor accumulation, which could be problematic when transferring the method to the clinic. DW-MRI presents a better compromise by combining longitudinal studies with a high dynamic range; however, DW-MRI was unable to show any significant effect for 5-FU. This study illustrates the need for multimodal imaging in assessing tumor response to treatment to compensate for individual limitations.

Biologic imaging of head and neck cancer: the present and the future

While anatomic imaging (CT and MR imaging) of HNC is focused on diagnosing and/or characterizing the disease, defining its local extent, and evaluating distant spread, accurate assessment of the biologic status of the cancer (cellularity, growth rate, response to nonsurgical chemoradiation therapy, and so forth) can be invaluable for prognostication, planning therapy, and follow-up of lesions after therapy. The combination of anatomic and biologic imaging techniques can thus provide a more comprehensive evaluation of the patient. The purpose of this work was to review the present and future clinical applications of advanced biologic imaging techniques in HNC evaluation and management. As part of the biologic imaging array, we discuss MR spectroscopy, diffusion and perfusion MR imaging, CTP, and FDG-PET scanning and conclude with exciting developments that hold promise in assessment of tumor hypoxia and neoangiogenesis.

Proton MR spectroscopy of the brain with a focus on chemical issues

Among the vast number of metabolites in living tissues, metabolites detectable by in vivo MR spectroscopy are limited to those present in high concentrations, and the actual number is only 10 to 20. None is disease-specific. Interpretation of MRS data, therefore, must be based on general knowledge of biochemical processes in association with pathological changes. Each spectrum is a window on the actual biochemical changes taking place within the living tissues, but the reality entails a wide and confusing variance. Continuous expansion of the knowledge may reduce the uncertainty of interpreting MRS data.

Magnetic resonance spectroscopy of head and neck neoplasms

Magnetic resonance spectroscopy (MRS) is a validated noninvasive method for evaluation of possible malignant tumor and lymph nodes of the head and neck. From its roots as a budding research application, it has made the critical transition to a widespread clinical tool. MRS analyzes the tissue at a molecular level and searches for the presence of specific metabolites, which are markers for malignancy. Differentiation of benign from malignant neoplasm, detection of recurrence of malignant tumor and noninvasive treatment monitoring of treated or untreated tumor are some of the important utilities of MRS. One dimensional 1H-MRS is the most popular and promising technique for spectroscopic analysis while P-31 MRA and two-dimensional correlated spectroscopy (2D COSY) have also showed some promise. This article describes the application of magnetic resonance spectroscopy for evaluation of malignant tumors of the neck.

Newer MR imaging techniques for head and neck

Dynamic and functional imaging techniques are being developed to improve the evaluation of various pathologic processes of the head and neck region. These techniques include dynamic contrast-enhanced MR imaging for evaluating soft tissue masses and cervical lymph nodes, the use of ultrasmall superparamagnetic iron oxide contrast agent, and functional techniques such as in vivo and in vitro MR spectroscopy of head and neck cancer and lymph nodes and apparent diffusion coefficient mapping of parotid glands. These techniques can help to differentiate nonmalignant tissue from malignant tumors and lymph nodes and can aid in differentiating residual malignancies from postradiation changes. From methodological development, they are making the critical transition to preclinical and clinical validating methods and eventually to widespread clinical tools.

MRI of the tumor microenvironment

The microenvironment within tumors is significantly different from that in normal tissues. A major difference is seen in the chaotic vasculature of tumors, which results in unbalanced blood supply and significant perfusion heterogeneities. As a consequence, many regions within tumors are transiently or chronically hypoxic. This exacerbates tumor cells' natural tendency to overproduce acids, resulting in very acidic pH values. The hypoxia and acidity of tumors have important consequences for antitumor therapy and can contribute to the progression of tumors to a more aggressive metastatic phenotype. Over the past decade, techniques have emerged that allow the interrogation of the tumor microenvironment with high resolution and molecularly specific probes. Techniques are available to interrogate perfusion, vascular distribution, pH, and pO(2) nondestructively in living tissues with relatively high precision. Studies employing these methods have provided new insights into the causes and consequences of the hostile tumor microenvironment. Furthermore, it is quite exciting that there are emerging techniques that generate tumor image contrast via ill-defined mechanisms. Elucidation of these mechanisms will yield further insights into the tumor microenvironment. This review attempts to identify techniques and their application to tumor biology, with an emphasis on nuclear magnetic resonance (NMR) approaches. Examples are also discussed using electron MR, optical, and radionuclear imaging techniques.

Prediction of treatment response of head and neck cancers with P-31 MR spectroscopy from pretreatment relative phosphomonoester levels

RATIONALE AND OBJECTIVES

Combinations of chemotherapy and fractionated radiation therapy are the currently preferred nonsurgical treatment methods for squamous cell carcinoma of the head and neck, but to the authors' knowledge there is no reliable marker for predicting therapeutic response. Early identification of nonresponders would allow prompt replacement of ineffective, toxic therapy by alternative, potentially more effective procedures. Frequent regional node involvement facilitates surface coil investigation with phosphorus-31 magnetic resonance spectroscopy.

MATERIALS AND METHODS

P-31 magnetic resonance spectra were acquired from 12 patients before radiation therapy or chemotherapy. In vivo three-dimensional localized P-31 nuclear magnetic resonance chemical shift imaging was performed with a 1.5-T clinical imager and a dual-tuned H-1/P-31 surface coil. Proton decoupling and nuclear Overhauser enhancement were used to improve sensitivity and resolve overlapping signals in the phosphomonoester region of the spectrum.

RESULTS

The average pretreatment ratio of phosphomonoester to beta-nucleoside triphosphate was significantly smaller in complete responders (n = 4) than in incomplete responders (partial responders plus nonresponders, n = 8) (0.0 +/- 0.0 vs 1.22 +/- 0.17 [P = .004]).

CONCLUSION

Results of this preliminary study suggest that H-1-decoupled P-31 magnetic resonance spectroscopy may prove to be a useful predictor of therapeutic response in head and neck cancers.

How can we measure substrate, metabolite and neurotransmitter concentrations in the human brain?

Cerebral injury and disease is associated with fundamental derangements in metabolism, with changes in the concentration of important substrates (e.g. glucose), metabolites (e.g. lactate) and neurotransmitters (e.g. glutamate and y-aminobutyric acid) in addition to changes in oxygen utilization. The ability to measure these substances in the human brain is increasing our understanding of the pathophysiology of trauma, stroke, epilepsy and tumours. There are several techniques in clinical practice already in use and new methods are under evaluation. Such techniques include the use of cerebral probes (e.g. microdialysis. voltammetry and spectrophotometry) and functional imaging (e.g. positron emission tomography and magnetic resonance spectroscopy). This review describes these techniques in terms of their principles and clinical applications.

Magnetic resonance spectroscopy of the malignant prostate gland after radiotherapy: a histopathologic study of diagnostic validity

PURPOSE

Accurate spatial representation of tumor clearance after conformal radiotherapy is an endpoint of clinical importance. Magnetic resonance spectroscopy (MRS) can diagnose malignancy in the untreated prostate gland through measurements of cellular metabolites. In this study we sought to describe spectral metabolic changes in prostatic tissue after radiotherapy and validate a multivariate analytic strategy (based on MRS) that could identify viable tumor.

METHODS AND MATERIALS

Transrectal ultrasound-guided prostate biopsies from 35 patients were obtained 18-36 months after external beam radiotherapy. One hundred sixteen tissue specimens were subjected to 1H MRS, submitted to histopathology, and analyzed for correlation with a multivariate strategy specifically developed for biomedical spectra.

RESULTS

The sensitivity and specificity of MRS in identifying a malignant biopsy were 88.9% and 92% respectively, with an overall classification accuracy of 91.4%. The diagnostic spectral regions identified by our algorithm included those due to choline, creatine, glutamine, and lipid. Citrate, an important discriminating resonance in the untreated prostate gland, was invisible in all spectra, regardless of histology.

CONCLUSIONS

Although the spectral features of prostate tissue markedly change after radiotherapy, MRS combined with multivariate methods of analysis can accurately identify histologically malignant biopsies. MRS shows promise as a modality that could integrate three-dimensional measures of tumor response.

Cerebral mucormycosis: proton MR spectroscopy and MR imaging

Proton magnetic resonance spectroscopy (MRS) was integrated with magnetic resonance imaging (MRI) in the evaluation of a case of cerebral mucormycosis. MRS showed markedly elevated lactate, depleted N-acetyl aspartate and metabolite resonances attributable to succinate and acetate. The spectroscopy profile is essentially similar to that of bacterial abscess but without the commonly seen resonances of the amino acids valine, leucine and isoleucine. Our extensive literature review did not yield any reports of MRS findings on cerebral mucormycosis. MRS prospectively limited the differential diagnoses given the otherwise nonspecific and complex MR imaging findings in our immunosuppressed patient.

Localized in vivo 1H magnetic resonance spectroscopy and in vitro analyses of heterogeneous brain tumors

Results of magnetic resonance spectroscopic (MRS) studies of the chemical patterns in brain tumors have been inconsistent. Actual biochemical correlations are needed. In 2 patients with heterogeneous intracranial tumors, in vivo 1H MRS and in vitro biochemical analyses were correlated. Histology confirmed the tumor heterogeneity. Choline was elevated in the cellular portion of both tumors but decreased in the necrotic or cystic portions. Creatine was diffusely decreased while lactate was elevated in all regions of both tumors. Furthermore, the increase in the choline peak on 1H MRS appeared to be due to increases in water-soluble choline compounds. This study illustrates the value of small localized voxels for differentiating regional chemical differences in tumors.

Clinical applicability of human in vivo localized phosphorus-31 magnetic resonance spectroscopy of bone and soft tissue tumors

BACKGROUND

Magnetic resonance imaging (MRI) is of restricted value for the in vivo characterization of tumor types. The applicability of phosphorus-31 (31P) magnetic resonance spectroscopy (MRS) in the diagnosis of bone and soft tissue tumors is unknown.

METHODS

A total of 191 consecutive patients (85 females and 106 males; mean age 41 years, range 1-80) with a well-defined bone or soft tissue tumor on MRI were analyzed for additional 31P spectroscopy. Histology and/or cytology was obtained from all tumors. Because of low sensitivity of the 31P nucleus and the contamination of surrounding tissue, only large, superficially located tumors accessible to the surface coil could be accepted for MRS.

RESULTS

Twenty-one patients (11%) could be included in the study. From this remaining group only 12 studies (57%) produced spectra with well resolved phosphorus peaks and an acceptable signal-to-noise ratio. However, these spectra did not allow differentiation between the benign and malignant nature of the lesions. The other 9 studies showed spectra with poor signal intensities and/or poorly defined peaks, making tumor differentiation impossible.

CONCLUSION

Only 6% of the bone and soft tissue tumors produced well defined spectra, which implies that localized 31P MRS cannot be considered as a routine technique in the diagnostic and treatment evaluation of bone and soft tissue tumors.

Modern techniques in radiological imaging related to oncology

BACKGROUND

Radiological imaging has existed for about 100 years and there have been significant advances in computer technology during the last 25 years. The ideal investigation should be non-invasive, repeatable, and have a sensitivity (detection of the lesion) and specificity (ability to predict the absence of disease) of 100%.

TECHNIQUES

Recent advances in ultrasonography, computed tomography (CT) scanning and magnetic resonance imaging (MRI) have enabled a more accurate demonstration of anatomical structures and better spatial resolution. This has led to the detection of smaller lesions and faster scan times, and thus new or recurrent disease is demonstrated at an earlier stage and motion artefacts are reduced. Advances in imaging techniques have also enabled percutaneous manoeuvres, ranging from diagnostic biopsy of suspicious lesions to therapeutic stenting of malignant strictures, to be performed. Magnetic resonance spectroscopy (MRS) and positron emission tomography (PET) represent a different concept. They assess in vivo tissue metabolism and provide a physiological approach which allows comparison of normal and abnormal tissue, such as tumour. The measurement of certain metabolites or isotope tracers correlates with tumour metabolism, and the response to treatment can be predicted by quantitative changes. Thus, PET scanning and MRS will probably be valuable in assessing new chemotherapeutic agents in animals and patients in vivo. Patients likely to respond to a certain drug can be selected and further studies may help determine ideal dosing regimens. Lack of fine anatomical detail on PET images can be overcome by 'fusing' the 'slice' with the corresponding CT or MRI slice, allowing accurate anatomical and physiological information to be displayed.

CONCLUSION

As more specific indicators of disease and the response to therapy become available, the combination of anatomical and functional imaging modalities will enable treatment to be undertaken at an earlier stage with the potential for increased survival.