Jessica Sofía Riback1,2,3, Julia Sabrina Viglietti1,3,4, Agustina Mariana Portu1,3, Gustavo Santa Cruz1,2, Sara Josefina González1,2
1National Scientific and Technical Research Council (CONICET), Buenos Aires, Argentina;
2Computational Dosimetry and Treatment Planning Group, Boron Neutron Capture Therapy Department, Gerente de Área Aplicaciones Nucleares a la Salud (GAANS), National Atomic Energy Commission (CNEA), Buenos Aires, Argentina;
3National University of San Martín (UNSAM), Buenos Aires, Argentina;
4Radiobiology Department, National Atomic Energy Commission (CNEA), Buenos Aires, Argentina
Correspondence to: Jessica Sofía Riback, PhD student. National Scientific and Technical Research Council (CONICET), Buenos Aires, Argentina; Computational Dosimetry and Treatment Planning Group, Boron Neutron Capture Therapy Department, Gerente de Área Aplicaciones Nucleares a la Salud (GAANS), National Atomic Energy Commission (CNEA), Av. Gral. Paz 1499, B1650 Villa Maipú, Buenos Aires, Argentina; National University of San Martín (UNSAM), Buenos Aires, Argentina. Email: jessicariback@cnea.gob.ar; jessicariback@gmail.com.
Background: The present work is accomplished in the context of the Argentine Boron Neutron Capture Therapy project that involves a phase II clinical trial for the treatment of skin cancer, carried out by the National Atomic Energy Commission (CNEA) of Argentina in collaboration with medical institutions. Argentina has an open clinical protocol for the treatment of cutaneous melanoma of extremities. Irradiations are carried out at the RA-6 reactor. Over 100 lesions were treated following this protocol. An overall response was observed in 69.3% of the nodules, while 30.7% of the tumors remained stable. The main objective of this work is to improve dosimetry for the organ at risk, the skin, and to prevent potential early radiotoxic effects. The calculation of the absorbed dose calculation in the main organ at risk, i.e., the normal skin [as a result of the energy deposited by low linear energy transfer (LET) electrons, intermediate LET protons, and particles alpha and high LET 7Li nuclei], is usually performed on the basis of two approximations: (I) all the kinetic energy transferred to the charged particles is deposited locally [kinetic energy released in materials (KERMA) approximation]; and (II) the distribution of sources of charged particles produced by uncharged ones is spatially uniform in both tumor and normal tissue. For certain conditions these assumptions are not satisfied. Therefore, estimates of the dose by more detailed calculations are required.
Methods: The skin dosimetry is addressed following two approaches: the detailed computational simulation of the skin irradiation, and the experimental determination of the distribution of neutron-generated secondary particle sources in the tissue. In the first case, the approach is carried out by simulations of therapeutic irradiations and calculations of the detailed dosimetry, with the transport code PHITS. The construction and modelling of a more complex geometry than the one usually used in skin calculations is proposed, that involved a complete study of the anatomy and histology of human skin, considering different material compositions and processes that determine the heterogeneous accumulation of 10-B. In the second case, the neutron autoradiography technique is used to estimate the microdistribution and boron uptake in different skin structures and thus, to include this information to the developed computational model. Biological samples from a preclinical study on a large animal were used for this purpose.
Results: Preliminary results show that, on average, the dermis absorbs twice as much boron as the epidermis. Basal cells, a critical dermal population related to early effects on radiation, receive about 10 Gy more than the macroscopical skin model.
Conclusions: This study shows the importance of a more realistic skin model in the dosimetric analysis of skin treatments and explains the differences between dose calculation and effects observed on the tissue. More analysis is necessary.
Keywords: Computational dosimetry; skin cancer; neutron autoradiography
