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Catch‐up of bone mineral density among long‐term survivors of childhood cancer? Letter to the editor: Response to the article of Gurney et al. 2014

To the editor: the manuscript by Gurney and colleagues [1] that described that bone mineral density (BMD) in 400 adult longterm survivors of childhood acute lymphoblastic leukaemia (ALL) of the St. Jude Lifetime Cohort tended to improve over 8.5 years’ time. We examined BMD-change over time in 188 adult childhood cancer survivors (CCS), including 98 ALL survivors andmost other disease subtypes (median age: 24.5 years), who were treated between 1965–2004 in our hospital [2–4]. Median period between DXA scans (Lunar device) was 3.2 years (range 0.9–10.9 years). Total body BMD (BMDTB) at DXA2 was mean 0.08 SDS (P< 0.01) and BMDLS was 0.06 SDS (P1⁄4 0.03) higher than at DXA1 (Supplemental Table 1). BMDTB increased significantly over time in AML, NHL, and renal tumor survivors, and BMDLS in AML, but not in ALL survivors. Analyzed by gender, BMDTB and BMDLS improved significantly only in males. When we plotted BMD change by age at DXA2, the peak bone mass (PBM) for BMDLS seemed to be reached at about age 23 (Fig. 1a); BMDTB tended to increase till age 26/27 (Fig. 1b). The results of our study confirm the trend shown byGurney et al. that BMD may improve over time, even after a follow-up time of only 3 years. We showed that improvement occurred in both BMDTB and BMDLS, and that it mainly seems to occur in males (Supplemental Figure 1). This may be due to an extended physiological delay in reaching PBM in men [3]. In the manuscript of Gurney and colleagues [1], separate analysis of the changes in men and women, was not performed. In our study, BMDTB improvement occurred until about age 27 years. This indicates that survivors may reach PBM later in life than the usual age of 18–23 years suggesting a delayed regenerative potential of bone remodeling in CCS. Both study cohorts report a selection bias, as only around in 40–50% of the survivors BMD change was assessed. The subset of the St Judes cohort were rather young patients, were less often treated with cranial radiation and used higher doses of MTX and prednisone. Our subset received more often MTX and prednisone. Therefore, we should be careful to generalize these findings to all CCS. We cannot exclude that the improvement in BMD in both studies is partly due to the phenomenon of regression to the mean [5], meaning that when a variable is extremely low or high at DXA1, it will tend to be closer to the average on DXA2. Finally, in both studies BMD change was not adjusted for potential confounders, including vitamin D or physical exercise. To examine if catch-up of BMD occurs, longitudinal studies in which BMD is measured over time with fixed intervals in a representative sample of CCS, aged 18 to about 30 years, are mandatory. We confirm that BMDmay improve over time in CCS, even after the age of physiological PBM has been reached, indicating that survivors may have a potential for reaching PBM later in life than healthy women and men.