PhD Thesis Defenses

PhD thesis defence : Out-of-field doses from proton therapy and doses from CBCT imaging

The use of ionizing radiation for treatment of cancer diseases is continuously increasing as patient survival is improving
and new treatment techniques are emerging. While this development is beneficial for curing primary tumors, concerns have
been raised regarding the unwanted dose contribution to healthy tissues of patients and the associated risk of radiationinduced
second cancer (RISC). This is especially important for younger patients receiving radiotherapy more often than
before and for whom the risk of developing RISC is elevated in comparison to the typical adult radiotherapy patient. In
order to estimate the risk of RISC associated with modern radiotherapy and imaging, the associated radiation doses must
be determined.
Patients undergoing radiotherapy receive in-field doses from the primary beam but also out-of-field doses originating
from secondary radiation produced in the beamline and within the patient. Over the last years, the use of proton pencil
beam scanning (PBS) therapy has rapidly increased due to its potential to reduce the in-field doses to healthy tissues in
comparison to photon therapy. One of the drawbacks with proton therapy is the production of neutrons capable of travelling
large distances and depositing out-of-field doses to organs located far from the primary treatment field. The dose reduction
associated with proton PBS therapy could consequently be affected by the out-of-field doses originating from secondary
The sharp dose gradients associated with modern treatment techniques, such as photon intensity-modulated radiotherapy
(IMRT) and proton PBS therapy require more frequent and accurate patient imaging in comparison to conventional
treatment techniques such as three-dimensional conformal radiotherapy (CRT). Setup verification images could be acquired
with cone-beam computed tomography (CBCT) producing three-dimensional patient images at the cost of an increased
patient dose in comparison to planar x-ray imaging. Concerns have been raised regarding the cumulative patient doses
from repeated CBCT imaging versus the dose-saving benefits associated with modern radiotherapy techniques like IMRT
and proton PBS.
In this thesis, a study on the in-field and out-of-field doses to healthy tissues from photon IMRT and CRT treatments
of head and neck tumors showed that the risk of RISC was unaffected by the employed treatment technique and indicated
that the lifetime risk of cancer induction was of the order of 1-2%.
Results from measurements and Monte Carlo simulations showed that the out-of-field absorbed doses and equivalent
doses associated with proton PBS treatments of brain tumors were up to 60 μGy/Gy and 150 μSv/Gy, respectively. The
risk of RISC associated with these out-of-field doses was in the range of approximately one induced cancer in ten thousand
treated patients. A simulation study on the doses from a proton gantry-mounted CBCT system showed that repeated CBCT
imaging could result in cumulative organ doses of almost 2 Gy. The conclusion from these studies is that the dose-sparing
effects of proton PBS therapy are not overshadowed by the out-of-field doses originating from secondary radiation for
brain tumor treatments, but that the cumulative doses from repeated CBCT imaging could have a relevant impact on the
overall dose reduction.