Effects of Charged Particles on Human Tumor Cells

Abstract

The use of charged particle therapy in cancer treatment is growing rapidly, in large part because the exquisite dose localization of charged particles allows for higher radiation doses to be given to tumor tissue while normal tissues are exposed to lower doses and decreased volumes of normal tissues are irradiated. In addition, charged particles heavier than protons have substantial potential clinical advantages because of their additional biological effects, including greater cell killing effectiveness, decreased radiation resistance of hypoxic cells in tumors, and reduced cell cycle dependence of radiation response. These biological advantages depend on many factors, such as endpoint, cell or tissue type, dose, dose rate or fractionation, charged particle type and energy, and oxygen concentration. This review summarizes the unique biological advantages of charged particle therapy and highlights recent research and areas of particular research needs, such as quantification of relative biological effectiveness (RBE) for various tumor types and radiation qualities, role of genetic background of tumor cells in determining response to charged particles, sensitivity of cancer stem-like cells to charged particles, role of charged particles in tumors with hypoxic fractions, and importance of fractionation, including use of hypofractionation, with charged particles.

Keywords: altered fractionation; cancer stem cells; carbon-ion therapy; charged particles; clustered DNA damage; hypoxic radioresistance; proton therapy; relative biological effectiveness.

Figure 1

A “New Biology” of proton beam therapy. (A) Illustration of how FA/BRCA defects may sensitize cells to proton irradiation. Left, clustered DNA damages after equal physical doses of X-rays and mid-SOBP protons are slightly different despite similar LET (2–2.5 keV/μm) and identical RBE in repair-proficient cells (~1.1). In the presence of a FA/BRCA defect that affects the repair of replication forks encountering clustered damages, there will be greater unrepaired damage after proton-irradiation, as marked by an increased number and relative size of repair-related protein accumulations of DNA double-strand break markers. Representative immunofluorescence microscopy images showing nucleus (DAPI) and 53BP1 foci (green) in FANCD2-mutant cells are shown on the right. (B) Summary of RBE estimates relative to Co60 photons as a function of defects in the FA/BRCA pathway (44, 45). Other, taken from unpublished data (Willers et al.); CoEq, Co60 equivalent; SF, surviving fraction; CL, confidence limits.

Figure 2

RBE versus LET for human tumor cell lines for various endpoints. RBE values are all based on colony formation assays. (A) RBE values calculated as the ratio of isoeffect doses at 10% survival (D10). (B) RBE values calculated as ratios of doses for D0, D30, D50, and D75. D0 was calculated by fitting the survival curve to the single-hit multi-target (SHMT) model: S/S0 = 1 − (1 − e^−D/D0)ⁿ. (C) RBE values calculated as the ratio of doses at the level of photon doses of 2 Gy (SF2) or 3 Gy (SF3). (D) RBE values calculated as the ratios of the alpha parameters of survival curves.

Figure 3

RBE versus LET for human tumor cell lines for chromatin breaks and apoptosis. (A) is from data on unrejoined chromatin breaks in cells after premature chromosome condensation (PCC). Residual unrejoined chromatin breaks were detected using Giemsa staining in cells after chromatin condensation. (B) is RBE values for apoptosis.

Figure 4

RBE at 10% survival (D10) versus LET for human tumor cell lines exposed to various charged particles heavier than protons. RBE values derived at 10% survival from clonogenic survival curves from all available literature are shown as a function of LET for human tumor cells exposed to (A) carbon ions; (B) helium ions; (C) neon ions; (D) boron ions; (E) silicon ions; and (F) argon and iron ions. The RBE values showed substantial variation at any given LET, independent of ion species, but in all cases the RBE increased with LET to a maximum, then decreased at high-LET levels.

Figure 5

RBE at 10% survival versus LET for cells from various types of human tumors exposed to carbon ions. RBE values as a function of LET for carbon-ion beam only, calculated using D10 and sorted by tumor type, are shown. Tumor types included are: (A) brain tumor; (B) lung cancer; (C) head and neck squamous cell carcinoma; (D) melanoma; (E) salivary gland tumor; (F) hepatoma; (G) cervical cancer; (H) pancreatic cancer and (I) chordoma. The graphs include data only for LET < 100 keV/μm. The number of data points, or cell lines, varies greatly with tumor type. The slopes of the RBE versus LET curves are calculated for each cell line and tumor type.

Figure 6

RBE at 10% survival versus LET for cells from human adenocarcinomas and squamous cell carcinomas exposed to carbon ions. RBE values as a function of LET for carbon ion beam only, calculated using D10 for (A) adenocarcinomas and (B) squamous cell carcinomas. The graphs show data only for LET < 100 keV/μm. The slopes of the curves are shown for each cell line.

Figure 7

A model for carbon ion-induced apoptosis and autophagy through the enhancement of death signals and the depression of survival signals. The model is based on AKT survival signaling as shown in our work (126). An arrow “→” indicates enhancement; a sidewise “⊣” indicates depression.

Figure 8

The dependence of survival curves on oxygen concentration typically observed after exposure to X-rays and carbon ions. The importance of the oxygen effect is reduced with high-LET carbon-ion irradiation as is apparent in the small separation of the survival curves compared to that seen with X-rays. The large difference between the cell response in air and hypoxia for X-rays results in a RBE_hypoxic that is greater than RBE_oxic.

Figure 9

α and β terms for mouse normal tissues and tumors plotted against LET. The comparison of survival parameters of Ando et al. (red) (148) and our study (153) using crypt survival and tumor growth delay assays (black). The solid lines show NFSa tumor and broken lines show the normal tissues (crypt or skin).