Optimization of brachytherapy treatment planning and radiation field studies in the vicinity of the Leksell Gamma Knife

by Lucilio dos Santos Matias (Stockholm University)

Friday 25 Oct 2024, 10:00 → 13:00 Europe/Stockholm

AlbaNova FB42 (AlbaNova Main Building)

Abstract
Carcinoma of the cervix is a global problem, and brachytherapy is one of the main radiation therapy components used in
the treatment of patients with this type of cancer. With the advent of scientific and technological developments in treatment
planning, inverse optimization in brachytherapy and its thorough comparison with traditional manual optimization methods
is warranted. In this work, the physical parameters; minimum dose received by 98% and 90% of the target volume
represented by D98 and D90, respectively, were used to evaluate the treatment plans with respect to the target while the
minimum dose received by 2cm3 volume, D2cm3, was used to investigate complications in organs at risk. The conformity
index, COIN, was used to describe the coverage of the target by the prescribed dose and the fraction of each organ
at risk volume that receives the critical dose, which may cause complications. The treatment plan evaluation was also
performed in terms of the radiobiological parameter complication-free tumor control probability, P+. The physical and
radiobiological evaluation corresponding to two brachytherapy inverse planning algorithms have been compared with
homologous manual graphical optimization plans. The main observations from this study are that well tuned class solutions
of inverse optimization methods may produce similar dose volume histograms to those produced with manual graphical
optimization plans, and inverse methods have the potential to spare organs at risk while delivering an acceptable dose to the
target. In addition, radiobiological indexes such as the P+ can be useful complements to the physical parameters in treatment
plan evaluation. The Elekta Leksell Gamma Knife® unit has been successfully utilized in the management of intracranial
malignancies for more than half a century. As required by national and international regulation, adequate knowledge of
risks posed by ionizing radiation instrumentation is necessary in order to protect the patient, workers, public, and the
environment. In that perspective, the Nuclear Physics research group at the Department of Physics of Stockholm University
(Stockholm, Sweden) in collaboration with Elekta Instrument AB (Stockholm, Sweden) have conducted investigation on
the radiation field in the vicinity of the Gamma Knife utilizing the High Purity Germanium (HPGe) detector. As part of
the ongoing research, the principal objective of the present work was to improve the modeling and characterization of the
radiation field around the Gamma Knife to interrogate the efficacy of the National Council on Radiation Protection and
Measurements (NCRP) methodology for structural shielding design and evaluation for the Leksell Gamma Knife treatment
room. Acquisition of high-resolution γ-ray spectra and ambient dose equivalent H*(10) in the field of the Gamma Knife-
PerfexionTM took place at the Division of Neurosurgery, Radiumhemmet (Department of Oncology), Karolinska University
Hospital (Stockholm, Sweden). A p-type coaxial HPGe detector and a satellite survey meter were utilized to acquire the
γ-ray spectra and H*(10), respectively. The measured configurations were simulated on Pegasos Monte Carlo system. A
phase space on a cylindrical surface enclosing the Gamma Knife with open doors and the phantom assembled was used
as the source of radiation. About 4·107 γ photons were collected on the phase space corresponding to 2·1012 decays. With
the Gamma Knife doors open, most of the radiation was measured in the forward direction up to θ = 45o in relation to
the z axis. The Monte Carlo simulations reproduced the measured results and; therefore, good agreement was achieved
between response measurements and simulated spectra. The recent Gamma Knife models PerfexionTM, IconTM, and EspritTM
are dosimetricaly equivalent and represent more than half the total number of Gamma Knife systems installed around
the globe. Given the increasing importance of these machines in the treatment of patients with intracranial malignancies,
research and development of tools for radiological protection around the Gamma Knife unit is topical and will continue
to evolve, especially with the advent of new models or versions of Gamma Knife units integrating novel technology such
as the satellite cone-beam computed tomography (CBCT) imaging system. The Monte Carlo method is a powerful tool
for simulation of radiation transport in matter. It reproduced the measured results and may be used in structural shielding
design calculations for γ-photons from the Gamma Knife.