CT measurement of lung density: the role of patient position and value for total body irradiation

Electron density of tissues and breast cancer radiotherapy: a quantitative CT study

PURPOSE

To obtain more accurate data on the electron density of tissues to be used in the treatment planning of breast cancer patients.

METHODS AND MATERIALS

Single kVp quantitative computed tomography was applied in 70 women, 20 to 77 years old, to study the electron density of the breast, the thoracic wall close and parallel to the breast, and the lung parenchyma.

RESULTS

The electron density of the entire breast decreases with increasing age in premenopausal women and remains practically constant in postmenopausal women (8% less than that of water). No difference was found in the electron densities of the right and left breast. The electron density of the lung parenchyma in proximity to the breast is lower than the density in the entire lung parenchyma.

CONCLUSIONS

Whenever no accurate data is available on individual patients, the electron density values to be used in treatment planning for breast and thoracic wall have to take into account both age and menstrual status. The regional differences in electron density of the lung also have to be considered.

Influence of radiation therapy on the lung-tissue in breast cancer patients: CT-assessed density changes and associated symptoms

The relative electron density of lung tissue was measured from computer tomography (CT) slices in 33 breast cancer patients treated by various techniques of adjuvant radiotherapy. The measurements were made before radiotherapy, 3 months and 9 months after completion of radiation therapy. The changes in lung densities at 3 months and 9 months were compared to radiation induced radiological (CT) findings. In addition, subjective symptoms such as cough and dyspnoea were assessed before and after radiotherapy. It was observed that the mean of the relative electron density of lung tissue varied from 0.25 when the whole lung was considered to 0.17 when only the anterior lateral quarter of the lung was taken into account. In patients with positive radiological (CT) findings the mean lung density of the anterior lateral quarter increased 2.1 times 3 months after radiotherapy and was still increased 1.6 times 6 months later. For those patients without findings, in the CT pictures the corresponding values were 1.2 and 1.1, respectively. The standard deviation of the pixel values within the anterior lateral quarter of the lung increased 3.8 times and 3.2 times at 3 months and 9 months, respectively, in the former group, as opposed to 1.2 and 1.1 in the latter group. Thirteen patients had an increase in either cough or dyspnoea as observed 3 months after completion of radiotherapy. In eleven patients these symptoms persisted 6 months later. No significant correlation was found between radiological findings and subjective symptoms. However, when three different treatment techniques were compared among 29 patients the highest rate of radiological findings was observed in patients in which the largest lung volumes received the target dose. A tendency towards an increased rate of subjective symptoms was also found in this group.

The use of CT densitometry in the assessment of radiation-induced damage to the rat lung: a comparison with other endpoints

Changes in rat lung following irradiation have been assessed on the basis of survival, histopathology, function (breathing rate assay), and density changes measured by computed tomography (CT) scanning. CT densitometry is a non-invasive procedure which may be used without modification to assess lung damage in experimental animals and in man. An increase in the breathing rate is seen following irradiation of the thorax, the time of onset and severity of which are dose dependent and correlate with histopathological changes occurring at the same time. Lung density changes occurring after irradiation are more complex. For the lowest dose used (10.75 Gy) no density increase was observed and in fact density decreased with time after irradiation to a slightly greater extent than in non-irradiated controls. A post-irradiation increase in lung density was seen for rats given 13.0 Gy, but values fluctuated with maxima at 50 and 225 days after irradiation. Higher radiation doses (14.5, 16.0 Gy) were followed by a transient decrease in density before a dose-related density increase was observed. Density averaged over the whole lung proved to be a sub-optimal index of radiation-induced lung damage because of the focal nature of radiation-induced lung lesions and because of the apparently anomalous changes in post-irradiation damage which are observed. Further studies are being made to determine if regional density values will provide a more sensitive index.

The use of CT densitometry to predict lung toxicity in bone marrow transplant patients

Total body irradiation (TBI) is considered an integral part of the preparation of patients with hematological malignancies for marrow transplantation. One of the major causes of death following bone marrow transplantation is interstitial pneumonia. Its pathogenesis is complex but radiation may play a major role in its development. Computed tomography (CT) has been used in animal and human studies as a sensitive non-invasive method for detecting changes in the lung following radiotherapy. In the present study CT scans are studied before and up to 1 year after TBI. Average lung densities measured before TBI showed large variations among the individual patients. On follow-up scans, lung density decreases were measured for patients who did not develop lung complications. Significant lung density increases were measured in patients who subsequently had lung complications. These lung density increases were observed prior to the onset of respiratory complications and could be correlated with the clinical course of the patients, suggesting the possibility for the usage of CT lung densitometry to predict lung complications before the onset of clinical symptoms.