Radiation therapy works by damaging the DNA of cells. The damage is caused by a photon, electron, proton, neutron, or ion beam directly or indirectly ionizing the atoms that make up the DNA chain. Indirect ionization happens as a result of the ionization of water, forming free radicals, notably hydroxyl radicals, which then damage the DNA. In the most common forms of radiation therapy, most of the radiation effect is through free radicals. Because cells have mechanisms for repairing DNA damage, breaking the DNA on both strands proves to be the most significant technique in modifying cell characteristics. Because cancer cells generally are undifferentiated and stem cell-like, they reproduce more, and have a diminished ability to repair sublethal damage compared to most healthy differentiated cells. The DNA damage is inherited through cell division, accumulating damage to the cancer cells; as a result, cells either die or reproduce more slowly.
The response of a cancer to radiation is described by its radiosensitivity. Modest doses of radiation rapidly kill highly radiosensitive cancer cells. These include leukemias, most lymphomas, and germ cell tumors. The majority of epithelial cancers is only moderately radiosensitive and requires a significantly higher dose of radiation (60–70 Gy) to achieve a radical cure. Some types of cancer are notably radioresistant, that is, much higher doses are required to produce a radical cure than may be safe in clinical practice. Renal cell cancer and melanoma are generally considered to be radioresistant.
Total Body Irradiation
Total body irradiation (TBI) is used primarily as part of the preparative regimen for AlloHSCT. As the name implies, TBI involves irradiation of the entire body, although in modern practice the lungs are often partially shielded to lower the risk of radiation-induced lung injury. TBI in the setting of AlloHSCT serves to destroy or suppress the recipient’s immune system, preventing immunologic rejection of transplanted donor stem cells. Additionally, high doses of TBI can eradicate residual cancer cells in the transplant recipient, increasing the likelihood that the transplant will be successful.
Doses of TBI used in AlloHSCT typically range from 10 to12 gray (Gy). At these doses, TBI both destroys the patient’s bone marrow (BM; allowing donor BM to engraft) and kills residual cancer cells. Nonmyeloablative (NMA) AlloHSCT uses lower doses of TBI, typically about 2 Gy, which do not destroy the host BM, but do suppress the host immune system sufficiently to promote donor engraftment.
In modern practice, TBI is typically fractionated. That is, the radiation is delivered in multiple small doses rather than 1 large dose. Early research in BM transplantation (BMT) by E. Donnall Thomas and colleagues [30
] demonstrated that this process of splitting TBI into multiple smaller doses resulted in lower toxicity and better outcomes than delivering a single, large dose.
Mechanisms of Radiation-Induced Cell Death in Leukemia and Lymphoma
Apoptosis is the major form of cell death after irradiation of leukemia and lymphoma cells. This cell death event is a series of cascading events involving cellular damage followed by sensing of cellular damage, signal transduction and checkpoint, activation of cell death regulatory genes and/or proteins, caspase activation and cellular destruction, and removal of apoptotic corpses. Although cell death can eventually occur even when protein synthesis is blocked, the important role of P53 and other transcription factors in mediating apoptosis and determining tissue sensitivity to irradiation strongly argues that transcriptional regulation is a very important in vivo mechanism in mediating irradiation-induced cell death.
Bcl-2 and Bax were among the first group of cell death regulatory genes identified as potential mediators of irradiation-induced cell death. Bax is transcriptionally induced by irradiation, and is often accompanied by a suppression of Bcl-2 expression. The suppression of Bcl-2 and induction of Bax appears to be dependent on P53 function. Consensus P53 binding elements are present in the promoter region of the Bax gene, which are required for its P53 responsiveness. In addition to Bcl-2 and Bax, BH3-only family members are also involved in irradiation-induced cell death. For example, Bid is induced by irradiation in human T cell-lineage-derived cells. Although the mammalian and Drosophila orthologs of Ced-4/apaf-1/hac-1 were identified only recently, it did not take long to find that proteins in this family were also involved in irradiation-induced cell death. Just as caspases are universally expressed, transcription of inhibitor of apoptosis (IAP) genes has been detected in essentially all tissues. The expression of CIAP1 in several human cancer cell lines was significantly altered in response to ionizing irradiation. Further, the expression of XIAP can be regulated at the translation step by irradiation. Low doses of gamma irradiation, such as those used in fractionated TBI, increased XIAP expression through an IRES-mediated translation mechanism, which conferred resistance to irradiation-induced cytotoxicity [32
]. The expression of both the death receptors and their ligands, such as Fas and FasL, TRAIL, and DR5 (a death receptor that binds to TRAIL), can be induced/ increased by irradiation and involved in irradiation-induced cell death [34
Limitation of Radiation Dose Escalation to Mitigate Resistance
Attempts to escalate the dose of TBI from 12 to 15.75 Gy resulted in a lower relapse risk, but it was offset by higher nonrelapse mortality (NRM) and led to no improvement in overall survival (OS) [36
]. These randomized trials highlighted the difficulties in balancing toxicity and disease control when attempting to escalate the dose of TBI as a strategy to improve patient outcomes. NRM may manifest at 0.7 Gy, whereas mild symptoms may be observed with doses as low as 0.3 Gy when high-energy X-rays, gamma rays, or neutrons are used. There are 3 classes of acute radiation syndromes [38
]. BM syndrome usually occurs with a dose between 0.7 and 10 Gy. The primary cause of death is the destruction of the BM, resulting in infection and hemorrhage. Gastrointestinal (GI) syndrome usually occurs with a dose greater than approximately 10 Gy. Destructive and irreparable changes in the GI tract and BM usually cause infection, dehydration, and electrolyte imbalance. Death usually occurs within 2 weeks without intensive therapy. Cardiovascular and central nervous system syndrome usually occur with a dose greater than approximately 50 Gy. Death occurs within 3 days because of collapse of the circulatory system as well as increased intracranial pressure caused by edema, vasculitis, and meningitis.
Reduced-intensity conditioning (RIC) regimens have drawn intensive investigation in the past decade. These RIC regimens were based on either low-dose TBI (2–8 Gy). RIC regimen was feasible for patients normally excluded from AlloHSCT by myeloablative (MA) regimens because of older age (>50 years), reduced performance status, or comorbidities. Thus, selection criteria for AlloHSCT could be “loosened” if an RIC regimen was considered as an alternative to an MA regimen.
Targeting Stem Cell Pathway Posts Promise in Overcoming Resistance
Tumor growth and regrowth after therapy is a property of CSCs; their response to radiation is a critical parameter for curability. Bao et al. [39
] reported radiation resistance of CD133+
cells in glioma. This resistance was attributed to constitutive activation of the DNA repair checkpoint and inhibition of the corresponding kinase radiosensitized CD133+
cells. The observation that CSCs resist radiation has also been reported by several groups [40
]. Radiation response curves of CSCs isolated from breast cancer lines showed a clear radioresistance shoulder. CSCs also failed to phosphorylate H2AX in response to radiation, suggesting diminished damage or alternative mechanisms might be involved [41
Radiation can activate the stem cell signaling pathway, Notch, by upregulating both the Notch ligand Jagged-1 and downstream Hey1. The Notch pathway is involved in stem cell maintenance in breast cancer, and its activation by radiation increased the number of CSCs. Activation of the Notch pathway by radiation suggests that this pathway may contribute to the radiation response of normal and malignant tissues [42
A major obstacle to successful chemotherapy is intrinsic or acquired multidrug resistance (MDR). The most common cause of MDR involves increased drug efflux from cancer cells mediated by members of the ATP-binding cassette (ABC) transporter family [43
]. The regulation of ABC transporters in the context of cancer is poorly understood, and clinical efforts to inhibit their function have not been fruitful. Sims-Mourtada et al. [44
] showed that inhibition of hedgehog (Hh) signaling increases the response of cancer cells to multiple structurally unrelated chemotherapies. Hh pathway activation induces chemoresistance in part by increasing drug efflux in an ABC transporter-dependent manner. Hh signaling regulates the expression of the ABC transporter proteins multidrug resistance protein-1 (MDR1, ABCB1, P-glycoprotein), the breast cancer resistance protein (BCRP), and the ATP-binding cassette (ABCG2), and that targeted knockdown of MDR1 and BCRP expression by small interfering RNA partially reverses Hh-induced chemoresistance [44
]. Similarly, Chen et al. [46
] showed that downregulation of Hh signaling with an inhibitor, cyclopamine, improved tumor control in vivo without increased toxicity.
Diffuse large B-cell lymphoma (DLBCL) expresses sonic Hh more frequently and more intensely than follicular lymphomas or chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma. Dysregulation of Hh signaling pathway was shown in DLBCL. Kim et al. [47
] assessed 67 cases of DLBCL for expression of Hh ligand, GLI1, GLI2, and GLI3 (transcriptional effectors of Hh signaling), and ABCG2. In DLBCL, these Hh markers were expressed in 72% to 91% of cases, and more importantly, expression of ABCG2 was detected in 95% of patients. Patients with DLBCL with high ABCG2 expression showed significantly shorter OS compared with patients with tumors with low or no expression of ABCG2. Hh signaling was positively correlated with expression levels of ABCG2; this association implies likely involvement of Hh in chemoresistance of lymphomas [47
B cell CLL (B-CLL) is characterized by an accumulation of neoplastic B cells because of their resistance to apoptosis and increased survival. Among various factors, the tumor microenvironment is known to play a role in the regulation of cell proliferation and survival of many cancers. However, it remains unclear how the tumor microenvironment contributes to the increased survival of B-CLL cells. Hegde et al. [48
] studied the influence of BM stromal cell-induced Hh signaling on the survival of B-CLL cells, and showed that Hh signaling inhibitor, cyclopamine, inhibits BM stromal cell-induced survival of B-CLL cells, suggesting a role for Hh signaling in the survival of B-CLL cells. Furthermore, selective downregulation of GLI1 by antisense oligodeoxynucleotides (GLI1-ASO) results in decreased BCL2 expression and cell survival, suggesting that GLI1 may regulate BCL2 and, thereby, modulate cell survival in B-CLL. These results suggest that overexpression of the stem cell pathways is associated with aggressive subsets of leukemia and lymphoma, and downregulation of these stem cell signaling pathways posts promise in overcoming resistance to chemotherapy and radiotherapy without increasing NRM.