The applications of positron emission tomography to oncology

PET FDG studies in oncology

Positron emission tomography (PET) studies of cancer with the glucose analog 2-[F-18]fluoro-2-deoxy-D-glucose (FDG) have emerged as both a useful research and clinical method for detecting (diagnosing), staging, and monitoring treatment responses in a variety of neoplasms, including tumors of the brain, head and neck, lung, breast, gastrointestinal and genitourinary systems, lymphatic system, musculoskeletal system, and other organ systems. In addition to FDG, many other positron emitting radiopharmaceuticals are under investigation and development in oncology, but the largest set of clinically relevant results to date has been acquired with FDG. Because most aggressive neoplasms have high glycolytic rates, neoplasms throughout the body may potentially be visualized with PET, using both standard transaxial imaging methods and techniques such as whole body PET imaging for surveying the entire body.

Conformal radiotherapy at the Royal Marsden Hospital (UK)

Conformal radiotherapy seeks to allow increased intensity of radiation by reducing the volume of normal tissues within the treatment volume. Techniques have developed secondary to improvements in three-dimensional imaging and accessible treatment technology is based on computer-controlled multileaf collimators to create an irregular radiation beam shape. Preliminary clinical work in the Royal Marsden Hospital seeks to quantify the toxicity reduction achievable by conformal techniques in the context of a prospective randomized pelvic radiotherapy trial which has now recruited 240 patients. The data accumulated during this trial will allow comparison of conformal and conventional radiotherapy and also analysis of the impact of dose and volume of a particular organ on both acute and late toxicity. Assessments have revealed that conformal techniques reduced significantly the treatment volume of normal tissues, e.g. by a mean of 54% for rectum and 42% for bladder. However, a relationship between volume and acute toxicity has not been established. Late toxicity is currently being analysed. Dose escalation trials in thoracic and in pelvic tumours are planned.

Tracer imaging in lung cancer

Lung cancer is the leading cause of death from cancer in males. Adequate staging is essential if proper treatment is to be administered. Current morphological imaging modalities are confronted with problems in the staging of lung cancer, in the evaluation of treatment response, and in establishing whether a residual mass is due to fibrosis, residual tumors or local recurrence. Nuclear medicine imaging techniques have advanced from planar gallium-67 citrate scans in the 1970s to multihead-detector single-photon emission tomography for 67Ga, Thallium-201 chloride, technetium-99m SestaMIBI, monoclonal antibodies, and octreotide compounds. Results of positron emission tomography (PET) with fluorine-18 deoxyglucose or carbon-11 methionine are very promising. PET units are now employed in centers all over the world and the recently introduced whole-body PET units will be ideal for the correct staging of malignant diseases. The current status of various nuclear medicine imaging procedures is reviewed. The problems and advantages of each scintigraphic procedure are discussed. It appears that many of the problems that confront morphological imaging will be solved by nuclear medicine techniques in the future.

Perfusion of colorectal liver metastases and uptake of fluorouracil assessed by H2(15)O and [18F]uracil positron emission tomography (PET)

Perfusion and fluorouracil (FU) accumulation were assessed using positron emission tomography (PET) with H2(15)O and 18FU in 36 patients with colorectal liver metastases. The tracers were injected intravenously and via the hepatic artery. Standard uptake values (SUV) were calculated using a region of interest (ROI) technique. The perfusion of non-tumorous liver tissue was similar after intravenous (i.v.) and intra-arterial (i.a.) assessment [mean of 2.67 (s = 0.61) and 2.2 (s = 0.45)]. Metastases were found to be hypoperfused compared to normal liver tissue after i.v. examinations [mean 1.73 (s = 0.77)]; i.a. injections revealed greater perfusion in metastases [mean 6.41 (s = 5.47)]. Single metastases showed up to 10 times greater perfusion with the i.a. injection route than with the i.v. one. However, lesions with no change or lower perfusion were also observed. Generally, accumulation of 18FU in metastases after i.v. infusion was less than after i.a.. Correlation of i.v. perfusion and uptake was moderate (r = 0.54, P = 0.0001); i.a. correlation was only slightly better (r = 0.61, P = 0.008). Perfusion as measured by H2(15)O-PET does not generally predict uptake of 18FU in colorectal liver metastases. To measure FU uptake using PET and 18F seems to be the most accurate method. It would allow one to identify individual patients with considerably greater accumulation of 18FU following i.a. administration who should profit from a cross-over to intrahepatic chemotherapy.

Metabolic response of AH13r rat tumours to cyclophosphamide as monitored by pO2 and pH semi-microelectrodes

The composition of the microenvironment has an important influence on the cellular response to cytotoxic agents. Using pH and pO2 semi-microelectrodes, we have monitored metabolic changes in AH13r rat tumours as a function of time after subcurative chemotherapy. Prior to therapy, tumours contained large areas considered hypoxic (mean pO2 approximately 4 mmHg) and are characterised by a marked accumulation of acidic metabolites (mean pH 6.65). Administration of cyclophosphamide (40 mg/kg body weight) resulted in tumour regression to 15% of pretreatment volumes and a growth delay of 12 days. Concomitant with volume reduction, tumours became reoxygenated (mean pO2 approximately 7 mmHg), with maximum values being reached within 2-4 days, paralleled by a shift of pH to more alkaline values (0.17 U on average). These changes coincided with the development of subtotal necrosis. During early tumour regrowth, the pH and pO2 histograms returned to control values. These data corroborate and extend the results of previous studies in which noninvasive techniques had been applied for the monitoring of treatment-induced metabolic changes in malignant tumours in vivo. In addition, these results support the notion that the effectiveness of anticancer therapy might be improved by selecting and scheduling therapeutic agents in consideration of physiological changes caused by preceding courses of treatment.

The role of positron emission tomography in oncology and other whole-body applications

Imaging and quantifying biochemical and physiological processes with PET clearly has major potential significance for all organ systems and many disease states. Although the full utility and potential of emerging new applications of PET in organs other than the heart and brain must be demonstrated in basic and clinical research studies, the rapidly accumulating aggregate experience in oncology in particular, and in other organ systems and disease states as well, indicates that PET is now truly becoming a modality of both clinical and investigative use for the body as a whole as well as for specific organ systems. Whole-body PET FDG imaging (Fig 9) illustrates the potential of biochemical imaging to map the distribution of cancer throughout the body. With the growing list of radiopharmaceutical and quantitative techniques applicable to cancer studies with PET, this field will continue to realize significant growth.

Radiotherapy update

Images