Abstract
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 (0001). . Doctoral dissertation, Texas A & M University. . Available electronically from
https : / /hdl .handle .net /1969 .1 /173720.