The clinical reality that oncology patients experience multiple symptoms as a result of their disease and its treatment fostered the need to do research on multiple symptoms and symptom clusters. In 2001, two papers in the oncology literature presented compelling evidence of the deleterious effects of symptom clusters on patient outcomes.1,2
In addition, as part of the National Institutes of Health (NIH) State of the Science Conference on Symptom Management in Cancer: Pain, Depression, and Fatigue,3
the concept of a symptom cluster was explored in terms of its occurrence, assessment, and treatment. This research, as well as the NIH conference, stimulated a series of studies on symptom clusters (for reviews see4-6
A variety of instruments and approaches were used to assess multiple symptoms in oncology patients and to derive symptom clusters from these assessments. The three most commonly used instruments in these symptom cluster studies were the M. D. Anderson Symptom Inventory (MDASI7
), the Symptom Distress Scale (SDS8
), and the Edmonton Symptom Assessment Scale (ESAS9
). However, a comparison of the symptom clusters identified across the studies that used the MDASI,7,10-13
and the ESAS15-17
is difficult for two reasons. First, the number of symptoms evaluated by these instruments ranged from 9 for the ESAS to 13 for the MDASI. In fact, only five symptoms (i.e., pain, fatigue, nausea, lack of appetite, shortness of breath) are common across all three instruments. Second, while the MDASI and the ESAS evaluate symptom severity, the SDS evaluates symptom distress. Therefore, it is not surprising that inconsistent results are found across these studies in terms of the number of clusters identified as well as the specific symptoms within each cluster.
Another factor that contributes to the difficulty in making comparisons of symptom clusters across studies is the heterogeneity of the samples that were evaluated in terms of cancer diagnoses, stage of disease, and cancer treatments. About half of the studies7, 10-12, 16, 17
used heterogeneous samples that ranged in age from 50 to 68 years. In these cross-sectional studies, the patients underwent a variety of cancer treatments and 24% to 100% had metastatic disease. In the studies that evaluated for symptom clusters in homogeneous samples in terms of cancer diagnosis, four assessed patients with lung cancer,13,14,18,19
one focused on patients with brain tumors,20
one on patients with prostate cancer,21
and one evaluated patients with brain metastasis.15
However, even in these homogeneous samples, patients were at various stages of their disease and underwent different treatments. In addition, the instruments used to evaluate symptom clusters varied across these studies.
Finally, a variety of analytic procedures (e.g., factor analysis, cluster analysis, multiple dimensional scaling) were used to identify symptom clusters with both heterogeneous and homogeneous samples of patients in terms of their cancer diagnoses. The majority of the studies used factor analysis to derive between one and four symptom cluster.7,11-16,18, 20-22
In the four studies that used the MDASI with heterogeneous samples,7,11-13
two to three symptom clusters were derived using factor analysis. While different symptom clusters or factors were reported across these four studies, they represent combinations of the following domains: a “general” symptom factor, a “gastrointestinal” symptom factor, and an “emotional” symptom factor. The commonality in the symptom factors across these four studies is encouraging and may be related to the use of the same instrument despite differences in patients' cancer diagnoses and treatments.
In contrast, in the three studies that evaluated symptom clusters in patients with lung cancer using factor analysis,14,18,19
while one to four symptom clusters were identified, commonalities in the clusters were not as evident. Differences in the number of clusters as well as differences in the composition of the clusters may be related to differences in the instruments used to assess the symptoms (i.e., SDS8
) versus Physical Symptom Experience Scale23
), the number of symptoms assessed (i.e., 13 versus 37), or the dimensions of the symptom assessed (i.e., distress versus severity). In the study that evaluated patients with prostate cancer at 8 months following treatment,21
two distinct symptom clusters were identified (i.e., fatigue and emotional cluster; sexual dysfunction, bowel dysfunction, and pain cluster).
Because of the numerous methodologic differences across the studies of symptom clusters done to date, it is difficult to draw definitive conclusions regarding the number and types of symptom clusters that occur in oncology patients with a specific cancer diagnosis or in those who undergo a specific cancer treatment. In addition, it is interesting to note that none of the studies used the Memorial Symptom Assessment Scale (MSAS) to evaluate for symptom clusters. This omission is serious because the MSAS is the most comprehensive multidimensional symptom inventory (i.e., 32 symptoms) available with well established validity and reliability.24
Finally, as Miaskowski and colleagues noted,25
studies are needed that compare the number and types of symptom clusters based on whether the symptom clusters are derived using ratings of symptom occurrence (i.e., present or absent) or symptom severity. In addition, symptom cluster studies need to be done with homogeneous samples of patients in terms of cancer diagnosis and/or treatment.
Given the numerous methodologic issues across the symptom cluster studies done to date, this study focused on a homogeneous sample of oncology patients in terms of cancer treatment (i.e., radiation therapy (RT)) and on a comparison of symptom clusters derived using occurrence and severity ratings. The purposes of this study, in a sample of oncology patients at the end of RT, were to: identify the number and types of symptom clusters using the yes/no responses from the MSAS; identify the number and types of symptom clusters using the severity scores from the MSAS; compare the identified symptom clusters derived using the MSAS severity scores to those derived using the occurrence ratings; and evaluate for differences in symptom cluster severity scores between patients with breast and prostate cancer.