Dose and Risk Assessment Methods for Epidemiologic Studies Investigating Long-term Side Effects of Proton Therapy
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Proton therapy has been substantially growing in use and acceptance for the treatment of cancer in the past years. However, the long-term risks of secondary malignancies possibly associated with proton therapy have not been studied in detail due to the lack of well-established methodology and available patient information. Organspecific radiation dosimetry in proton therapy is complicated but crucial in epidemiological investigations of radiotherapy patients. Scatter neutrons generated in the treatment head and inside the patient via nuclear interactions need to be taken into account in normal tissue dose assessment. The present study establishes the dosimetry methods for patient-specific organ dose calculations from proton therapy and methods to improve dose reconstruction of organ doses in out-of-field regions not typically covered by radiographic images. First, using limited radiographic data from Computed Tomography (CT) modalities combined with the library of body size-dependent computational human phantoms, methods were created to generate a full-body patient-specific phantom. This allows for full-body neutron dose calculations using Monte Carlo radiation transport methods. Second, a computer simulation model was developed to reconstruct a patientspecific proton therapy treatment within Monte Carlo radiation transport codes using already available patient radiotherapy log files and reference measurements from the Maryland Proton Treatment Center (MPTC). The method will allow for calculation of normal tissue dose in patients at MPTC and will be extended to other proton therapy machines installed at other proton therapy facilities. Finally, based on the normal tissue doses calculated for the pediatric phantoms, the risk of second neoplasms was calculated based on the BEIR VII risk models. Two pediatric computational phantoms were imported into the treatment planning system at MPTC. Full-body organ dose calculations were carried out using the Monte Carlo calculation modules developed in the current study and the radiation weighting factors reported by the International Commission on Radiological Protection (ICRP) followed by the assessment of the risks of developing second neoplasms. Lifetime attributable risk were low, maximum being for the lungs of 0.111% for the one-year-old case and 0.108% for the five-year-old case. In conclusion, key methods for normal tissue dosimetry in proton therapy patients were developed and validated. Methods established in this dissertation will be applied to support ongoing international epidemiologic studies of pediatric patients undergoing proton therapy.
Kuzmin, Gleb Andrejevich (2018). Dose and Risk Assessment Methods for Epidemiologic Studies Investigating Long-term Side Effects of Proton Therapy. Doctoral dissertation, Texas A & M University. Available electronically from