Prediction of Pituitary Gland Complications by LKB and Log-Logistic Radiobiological Models in 3D Conformal Radiation Therapy of Head and Neck Tumors

The pituitary gland is frequently irradiated during radiation therapy of head and neck tumors which can influence the quality of life of the patients after radiation therapy. This study aimed to estimate the normal tissue complication probability (NTCP) for the pituitary gland in head and neck cancers using two radiobiological models.

53 patients including 20 cases with nasopharyngeal cancer and 33 cases with brain tumor were studied. The dosimetric properties of each plan including minimum, mean and maximum doses were extracted from the dose-volume histogram curve. For estimation of pituitary gland response for each patient, the BIOPLAN software was used to calculate NTCP by LKB model and Matlab software was applied to calculate NTCP by equivalent uniform dose   (EUD) model. Models’ parameters including TD50, , and‘a’ were extracted from a previous study of radiobiological modeling of pituitary gland response to radiation therapy. For statistical analysis, the T-test was used to compare two models.

The average mean doses of 30.42 and 51.29 (Gy) of the pituitary gland were obtained for nasopharyngeal and brain tumor patients, respectively. The average NTCPs of the pituitary gland for nasopharyngeal patients estimated by LKB and Log-logistic models were 3.84 and 3.91%, respectively. In brain tumors, the average NTCP was 16.33% for LKB and 16.41% for Log-logistic models. The results showed that the log-logistic and LKB models provided comparable results and no statistically significant difference (P-value< 0.05) was found between two models.

The NTCP results indicated that the average NTCP of the pituitary gland for nasopharyngeal patients was approximately four times lower than that of brain tumors. Finally, implementation of follow up studies and modeling investigations are recommended for accurate estimation of pituitary gland complications following radiation therapy.