Comparison of calculated I-131 lung absorbed dose by quantitative 2 dimensional scan images and OLINDA dosimetry program

Introduction: The accuracy of radioiodine-131 absorbed dose to lungs was important for proper administered dose to the patient. In order to get a curative treatment as well as to control disease progression with the lowest radiation risk. This proposed study aimed to compare radioiodine-131 lung absorbed doses by manual calculation from quantitative two-dimensional scan images and OLINDA dosimetry program and to study the effect of lung mass on calculated absorbed dose. Materials and Methods: Retrospective study of 10 well differentiated thyroid cancer with lung metastases patients who had first treatment of radioactive iodine-131 were studied. Calculation of radioactive absorbed dose in lungs by manual calculation from two-dimensional scan images and OLINDA dosimetry program did under MIRD principle. Two sets of adjusted S-value from SAF, lungs mass of Cristy-Eckerman-Stabin phantoms and Thai’s lung mass were used for calculation by MIRD formula. The lung absorbed dose data from both calculation methods were analyzed by the percentage of differences, student pair t-test and correlation coefficients. Results: No significant difference of lung absorbed dose calculated by manual calculation and OLINDA dosimetry program, p>0.05. An excellent correlation between manual calculation and OLINDA dosimetry program was found by using the S-value of Cristy–Eckerman–Stabin phantoms as well as by using the adjusted S-value with Thai’s lung mass for absorbed dose calculation, r = 1.0. The percentage of difference between manual calculation and OLINDA dosimetry program by Cristy–Eckerman–Stabin S-value and adjusted S-value with Thai’s lung mass was lesser than 0.5 and 0.3, respectively. Furthermore, a higher lung absorbed dose was found in adjusted S-value of Thai’s lung mass than S-value of Cristy-Eckerman-Stabin phantom. The percentage of absorbed dose difference by these two S-value sets were between 20.4-38.3. Conclusion: The absorbed doses in lungs calculated by these methods were not different when using the same S-value data set. Thus, both calculation methods could be compatible. In addition, changing of organ mass directly affected on S-value and organ absorbed dose that should be carefully consider in MIRD calculation. To define organ mass by a high-quality of medical images such as MRI, CT, US were recommended to improve accuracy and precision of absorbed dose calculation by these studied methods.