Disscussion
Elucidating the pathophysiology and risk of transformation from MGUS to MM is an ongoing endeavor, even 35 years after the end of US involvement in the Vietnam War. Our study sought to understand a relationship between AO exposure, risk of MGUS transforming to MM, and associated mortality in US Vietnam War veterans. The rate of transformation (MGUS progressing to active MM) is well cited at 1% per year.15 Here, we found 12% of our cohort had undergone this transformation over 10 years.
Vietnam War era veterans who were exposed to AO during the Operation Ranch Hand period had 2.4 times greater risk of developing MGUS compared with veterans not exposed to AO.8 Our study was not designed to look at this association of AO exposure and MGUS/MM as this was a retrospective review to assess the difference in outcomes based on AO exposure. We found that AO exposure is associated with a decrease in mortality in contrast to a prior study showing worse survival with individuals with AO exposure.10 Another single center study found no association between AO exposure and overall survival, but it did identify an increased risk of progression from MGUS to MM.11 Our study did not show increased risk of transformation but did show positive effect on survival.
Black individuals have twice the risk of developing MM compared with White individuals and are diagnosed at a younger age (66 vs 70 years, respectively).16 Interestingly, Black race was a protective factor in our study. Given the length of time (35 years) elapsed since the Vietnam War ended, it is likely that most vulnerable Black veterans did not survive until our observation period.
HSCT, as expected, was a protective factor for veterans undergoing this treatment modality, but it is unclear why such a small number (8%) underwent HSCT as this is a standard of care in the management of MM. Obesity was also found to be a protective factor in a prior study, which was also seen in our study cohort.8
Limitations
This study was limited by its retrospective review of survivors among the Vietnam-era cohort several decades after the exposure of concern. Clinician notes and full historical data, such as date of onset for any disorder, were unavailable. These data also relied on the practitioners caring for the veterans to make the correct diagnosis with the associated code so that the data could be captured. Neither AO exposure nor diagnoses codes were verified against other sources of data; however, validation studies over the years have supported the accuracy of the diagnosis codes recorded in the VA EHR.
Conclusions
Because AO exposure is a nonmodifiable risk factor, focus should be placed on modifiable risk factors (eg, nicotine dependence, alcohol and substance use disorders, underlying comorbid conditions) as these were associated with worse outcomes. Future studies will look at the correlation of AO exposure, cytogenetics, and clinical outcomes in these veterans to learn how best to identify their disease course and optimize their care in the latter part of their life.
Acknowledgments
This research was supported by the Central Texas Veterans Health Care System and Baylor Scott and White Health, both in Temple and Veterans Affairs Central Western Massachusetts Healthcare System, Leeds.