Abstract Symptom clusters are stable groups of interrelated symptoms that occur simultaneously. Research on symptom clusters may yield findings that can be used to improve symptom assessment and management. The purposes of this dissertation research were: to evaluate for differences in symptom clusters in a homogeneous sample of oncology patients who underwent radiation therapy (RT) using both the occurrence rates and severity ratings from the Memorial Symptom Assessment Scale (MSAS), as well as to evaluate for differences in symptom factor severity scores between patients with breast and prostate cancer at the end of RT, and to determine the number and types of symptom clusters at three time points (i.e., at the middle of, at the end of, and 1 month after the completion of RT).

A sample of 160 patients who underwent RT was evaluated in this study. Patients completed a clinical questionnaire that obtained information on demographic and clinical characteristics, as well as the MSAS that measured the multidimensional experience of symptoms. Exploratory factor analyses of symptoms were done for selected time points. While the specific symptoms within each symptom cluster were not identical, three very similar symptom factors (i.e., "mood-cognitive" symptom cluster, "sickness-behavior" symptom cluster, "treatment-related" symptom cluster) were identified regardless of whether occurrence rates or severity ratings were used to create the symptom clusters at the end of RT. However, the factor solution derived using the severity ratings fit the data better. Significant differences in all three symptom severity scores were found between patients with breast and prostate cancer. For all three symptom factors, the patients with breast cancer had higher symptom severity factor scores than the patients with prostate cancer.

Although the number and specific symptoms within each cluster were not identical, three distinct symptom clusters were found across the three time points: "mood-cognitive" symptom cluster, "sickness-behavior" symptom cluster, and "treatment-related" or "pain" symptom cluster. However, differences over time were noted in the "treatment-related" or "pain" symptom clusters. Specifically, at the middle of RT, problem with urination and diarrhea clustered together, while problem with urination and skin problems associated with treatment clustered together at the end of RT. Furthermore, a new symptom cluster (i.e., pain) emerged at 1 month after completion of RT. Future research needs to confirm these findings as well as consider an evaluation of homogeneous samples of patients in terms of both cancer diagnoses and treatments.