354 reached. In total, 38 patients were enrolled. In 24, the dose could not be escalated beyond 73.6 Gy based on TGF- 1 criteria. In the remaining patients, eight received 80 Gy and six received 86.4 Gy. The latter dose was determined to be the maximally tolerated dose. Thus, this study determined that it was feasible to use TGF- 1 to guide radiation dose selection for patients with lung cancer. Given the changes in the treatment approach to lung cancer since that study was conducted (i.e., ubiquitous use of combined chemora- diotherapy and increased use of stereotactic radiation ther- apy for smaller tumors), the study needs to be repeated using a more current treatment regimen. In a follow-up re- port, the authors noted that serious long-term toxicity (grade 3–5) occurred only in the control arm, further sup- porting the potential of this approach to treatment individ- ualization [72]. It should be noted, however, that these higher doses of radiation did not translate into longer sur- vival. The study was not designed to test this endpoint, and further work is needed before any conclusions can be drawn in this regard. WHY IS TGF- 1 ELEVATED IN CANCER PATIENTS EVEN BEFORE TREATMENT BEGINS? As hinted at above, circulating TGF- 1 levels in many can- cer patients are elevated at the time of diagnosis, before treatment, as compared with people without cancer, and the tumor is thought to be the source of TGF- 1 [63, 73]. In addition to contributing to the risk for normal tissue injury from cancer treatment, TGF- 1 also promotes metastasis [74] and suppresses the immune system [75], so that the presence of high levels of TGF- 1 in cancer patients may have an adverse impact on prognosis [76 –78]. Animal stud- ies support this conclusion. For example, in transgenic mouse models, induction of active TGF- increased the in- cidence of lung metastases from breast cancer [79], whereas blocking TGF- activity reduced the metastatic potential [80]. Recent evidence has also suggested that re- ceptor tyrosine kinase inhibitors that block the binding of TGF- to its transmembrane receptors prevent the activa- tion of the TGF- signaling pathway and reduce the inci- dence of metastases in a breast cancer model [81]. Also, tumors that express TGF- 1 are more resistant to chemo- therapy, and sensitivity to cisplatin can be restored with TGF- 1 inhibition [82, 83]. These studies support the ra- tionale for targeting the TGF- pathway as a potential an- ticancer therapy. The reason for the presence of higher levels of TGF- 1 in these cancer patients appears to be related to both greater production and altered bioavailability of this cytokine [84 – 86]. Kong et al. [62] studied a group of women suspected to have breast cancer on the basis of an abnormal mammo- Targeting TGF- 1 to Prevent Complications gram. In these patients, blood was obtained for cytokine measurements before and after removal of the tumors by lumpectomy. Following surgery, there was a significant re- duction in plasma TGF- 1 levels in these patients, suggest- ing that the tumor was the source of the TGF- 1 production. Those authors also studied the tumor speci- mens using immunohistochemistry to stain for TGF- 1 ex- pression and in situ hybridization to look for the message to produce the TGF- 1 protein. In comparison with regions of normal breast tissue in the biopsy specimens, there was sig- nificantly greater expression of both messenger RNA and TGF- 1 in the tumor stroma, but not in epithelial cells. As noted above, many malignant epithelial cells lose the ability to respond to the growth inhibitory effects of TGF- 1, and it appears that the loss of this growth inhibitory feedback loop leads to increased production of TGF- 1 in a futile at- tempt to stop the proliferation of these malignant epithelial cells [33, 47, 74, 87, 88]. In terms of bioavailability, the mannose-6-phosphate/insulin-like growth factor receptor type II (M6P/IGF2R) plays a critical role in this process [89]. The gene coding for this receptor has been shown to be a tumor suppressor gene [90]. Binding of latent TGF- 1 to M6P/IGF2R facilitates its activation [85], but it also en- ables the molecule to be presented to lysosomes for degra- dation [91]. Thus, loss of function of this tumor suppressor gene could lead to a decreased ability to dispose of latent TGF- 1, which could be activated by other mechanisms (e.g., free radicals) and lead to an increased risk for normal tissue injury. Kong et al. [92] studied a group of lung cancer patients to test this hypothesis. They found that if the patients had loss of heterozygosity in M6P/IGF2R, and consequently a nonfunctioning receptor, they were sig- nificantly more likely to have higher plasma TGF- 1 levels and to develop radiation-induced lung injury than those without loss of heterozygosity in the receptor [93]. Thus, it appears that bioavailability of TGF- 1 plays an important role in the risk for normal tissue injury in at least a subset of cancer patients. SUSTAINING THE PHENOTYPE OF NORMAL TISSUE INJURY AFTER TREATMENT IS COMPLETED The development of normal tissue injury results from a complex interplay of contributions to risk from the treat- ment, the tumor, and the patient (Figure 5). Radiation con- tributes numerous insults beyond merely dose and volume, although these two factors no doubt impact the risk for in- jury. For example, radiation produces free radicals, dam- ages vasculature, creates a cascade of local and systemic cytokine and chemokine expression, elicits an inflamma- tory response, and causes loss of parenchymal cells. Che- motherapy may increase radiosensitization through various