For chemotherapy, an increased risk of acute myeloid leukemia within 10 years of childhood cancer treatment has been well established for alkylating agents, epipodophyllotoxins, and anthracyclines. 18 , 19 The role of chemotherapy in the etiology of solid cancers is less clear. Alkylating agents and anthracyclines were linked to an increased risk of subsequent sarcoma. 11 , 14 , 20 , 21 In addition, a recent Childhood Cancer Survivor Study among nonirradiated CCSs showed dose-dependent increases in breast cancer risk for these two classes of agents, particularly after childhood sarcoma and leukemia. 22 , 23 Not much is known, however, about the effects of specific chemotherapy agents on breast cancer and sarcoma risk. To examine the role of specific chemotherapeutic agents, we evaluated the long-term risk of SMNs in the Dutch Childhood Cancer Oncology Group–Long-Term Effects After Childhood Cancer (DCOG LATER) cohort of 5-year CCSs.

Cumulative incidences of SMNs were estimated in the presence of death as a competing risk. 28 Cumulative incidences of any SMN after 15 years were compared for childhood cancer diagnosis period with use of pairwise Pepe-Mori tests. 29 Expected cumulative incidence was derived from expected cancer incidence adjusted for overall mortality in the general population. The effects of potential risk factors for all solid cancers combined and for the two most frequent solid cancers (breast cancer and sarcoma) were analyzed by using multivariable Cox proportional hazards regression models, with attained age as the time scale because cancer incidence varies by age. 30 All variables with P < .1 in univariable analyses were tested in multivariable models, except for chemotherapy agents with fewer than five exposed cases for the outcome of interest (Appendix Table A1 , online only). Agents with at least 10 exposed cases were additionally categorized according to dose tertiles. Radiotherapy exposure was evaluated as yes/no variables and in prescribed dose categories if at least 10 exposed cases for the outcome of interest occurred (Appendix). All variables that remained significantly associated with the outcome of interest in multivariable analyses ( P < .05) or that considerably changed the effect of other variables in the model were included in the final model. Besides specific chemotherapeutic agents, we tested categories of chemotherapy exposure. 31 , 32 Tests for trend were based on the likelihood ratio–based P value for a model with the respective continuous variable on the basis of exposed patients only, unless otherwise specified. The assumption of proportional hazards was checked in all models and was not violated. Childhood cancer treatments for initial treatment and all recurrences were summed in analysis variables. The default childhood cancer type categories were leukemia, lymphoma, CNS tumor, bone sarcoma, soft tissue sarcoma, and other. Analyses on subsequent sarcoma used leukemia, bone and soft tissue sarcoma, and other. Finally, we assessed the effect of treatment-related risk factors separately for survivors of Li-Fraumeni syndrome (LFS)–associated childhood cancers (leukemia, CNS tumor, and sarcoma [except Ewing sarcoma] 33 - 35 ) versus that in other CCSs as a surrogate for genetic susceptibility for second cancers. All analyses were performed with Stata 13 software (StataCorp, College Station, TX).

Time at risk started 5 years after childhood cancer diagnosis and ended on the date of SMN diagnosis, date of death, date of emigration, date of loss to follow up, or end of study (January 1, 2013), whichever occurred first. SMNs diagnosed ≥ 5 years after childhood cancer diagnosis were included as the outcome of interest. For survivors with multiple SMNs, only the first SMN was counted in the analyses of all SMNs, and follow-up ended at the diagnosis of the first SMN. In cancer site–specific analyses, survivors contributed time at risk until the diagnosis of interest occurred, irrespective of possible preceding cancers. Standardized incidence ratios (SIRs) were calculated as the ratio of observed and expected number of SMNs. Excess absolute risks (EARs) were calculated as observed minus expected number of SMNs per 10,000 person-years of follow-up. Age-, sex-, and calendar year–specific rates from the Eindhoven Cancer Registry until 1988 and the NCR from 1989 onward were used to calculate expected numbers. 25 , 27

SMNs until January 1, 2013, were identified as follows: We linked the cohort to the population-based Netherlands Cancer Registry (NCR), with nationwide coverage since 1989, 25 and, for the pre-1989 era, to the nationwide network and registry of histo- and cytopathology in the Netherlands (PALGA [Dutch Pathology Registry]). 26 Furthermore, we reviewed hospital medical records. Discrepancies between SMN sources were resolved by review of pathology reports. SMNs excluded myelodysplastic syndrome and basal cell carcinoma of the skin (not systematically ascertained by the NCR). We did include ductal carcinomas in situ in multivariable models on female breast cancer risk.

Vital and emigration status were obtained by tracing CCSs through the Central Office for Genealogy register (which keeps records of Dutch decedents) and in the digital Municipal Personal Records Database that includes personal information of all inhabitants of the Netherlands since 1994. CCSs without a Municiple Personal Records Database record were traced through the last known municipality of residence before 1994.

Details on prior cancer diagnosis and treatment of primary tumor and all recurrences were collected by trained data managers. Chemotherapy details included start and end dates, drug names, and cumulative doses. For radiotherapy, details on prescribed dose, field, and boost/surdosage were recorded. In addition, names of other drugs and details on hematopoietic cell transplantation (HCT) were recorded. In case of missing chemotherapy doses when we knew the agent was given, we imputed mean doses of the agents administered to survivors within the same treatment protocol or to survivors from the same diagnosis group and diagnosis period (Appendix, online only).

The DCOG LATER cohort includes 5-year CCSs treated before the age of 18 years in one of seven Dutch pediatric oncology and stem cell transplant centers between January 1, 1963, and December 31, 2001. Eligible 5-year CCSs were identified from prospective registries (Emma Children’s Hospital/Academic Medical Center 24 ) and listings of pediatric patients with newly diagnosed cancer; the clinic-specific starting year varied (1963 to 1977) on the basis of completeness of the source. The study protocol was declared exempt from review of medical intervention research by institutional review boards of all participating centers.

We performed separate analyses for leukemia, CNS tumor, and sarcoma survivors (except Ewing sarcoma; n = 3,578) because combinations of these cancer types are specific for LFS, and many of these survivors (especially of sarcoma) were exposed to high-dose cyclophosphamide and/or doxorubicin (Appendix Table A6 , online only). The doxorubicin dose-response trend for breast cancer was stronger ( P difference = .008) among female survivors of potentially LFS-associated childhood cancers ( P trend < .001) versus other CCSs ( P trend = .94; Table 4 ). The cyclophosphamide dose-response trend for subsequent sarcoma was not materially different ( P difference = .98).

For all subsequent solid cancers, treatment effects were observed for TBI (HR, 4.7; 95% CI, 2.7 to 8.4); any radiotherapy other than TBI (HR, 1.9; 95% CI, 1.4 to 2.6); and doxorubicin dose, with HRs of 0.8 (95% CI, 0.5 to 1.2), 1.8 (95% CI, 1.1 to 2.9), and 2.5 (95% CI, 1.6 to 3.9) for ≤ 270, 271 to 443, and > 443 mg/m 2 , respectively ( P trend < .001). For ifosfamide, some evidence of increased risk was observed in the highest dose tertile, with dose-tertile–specific HRs of 1.8 (95% CI, 0.9 to 3.8), 1.4 (95% CI, 0.7 to 2.6), and 2.5 (95% CI, 1.2 to 5.0), respectively ( P trend = .15). Additional sensitivity analyses are reported in Appendix Table A5 (online only). We found no significant CED dose response ( P trend = .09), but we did for the anthracycline agents as a group ( P trend = .006). Women whose childhood cancer treatment included anthracycline ≥ 250 mg/m 2 had a significantly higher risk than those treated without anthracyclines (HR, 1.9; 95% CI, 1.3 to 2.7; Table 3 , solid cancer model 2).

Incidence of subsequent sarcoma (Appendix Table A4 , online only) was increased in survivors who received cyclophosphamide, with HRs of 0.2 (95% CI, 0.0 to 0.9), 1.7 (95% CI, 0.7 to 4.2), and 3.1 (95% CI, 1.5 to 6.0) for ≤ 4,800, 4,801 to 9,400, and > 9,400 mg/m 2 , respectively ( P trend = .01). Ifosfamide was also associated with increased risk of subsequent sarcoma (any v none HR, 2.6; 95% CI, 1.3 to 5.2) without a clear dose effect ( P trend = .24). Other risk factors were TBI (HR, 4.5; 95% CI, 1.5 to 13.4) and any radiotherapy other than TBI (HR, 2.7; 95% CI, 1.5 to 4.9). The separate evaluation of risk for bone sarcoma and soft tissue or other extraosseous sarcoma showed that cyclophosphamide was associated with an increased risk of subsequent bone sarcoma, with HRs of 2.6 (95% CI, 0.6 to 11.1) and 8.2 (95% CI, 3.1 to 21.5) for medium and high dose, respectively (no low-dose–exposed survivors; P trend = .007), but not with an increased risk of other sarcomas, with HRs of 0.3 (95% CI, 0.1 to 1.2), 1.4 (95% CI, 0.5 to 4.3), and 1.1 (95% CI, 0.3 to 3.6) for low, medium, and high dose, respectively ( P trend = 0.58; data not shown). We then restricted the cohort to CCSs treated without radiotherapy and found that sarcoma risks associated with cyclophosphamide dose were lower, with HRs of 0.9 (95% CI, 0.1 to 7.0) and 1.3 (95% CI, 0.2 to 10.1) for median and high dose, respectively ( P trend = .69; data not shown). A model with the CED showed a significant alkylating agent dose response ( P trend = .003; Table 3 , sarcoma model 2).

Risk factors for subsequent breast cancer (n = 49, including five ductal carcinomas in situ), sarcoma (n = 55), and all solid tumors combined (n = 230) were evaluated in multivariable Cox proportional hazard regression analyses ( Table 3 ). Doxorubicin was associated with a dose-dependent increased risk of female breast cancer, with hazard ratios (HRs) of 1.1 (95% CI, 0.4 to 2.9), 2.6 (95% CI, 1.1 to 6.5), and 5.8 (95% CI, 2.7 to 12.5) for ≤ 270, 271 to 443, and > 443 mg/m 2 , respectively ( P trend < .001). Ifosfamide was also associated with breast cancer risk (any v none HR, 3.4; 95% CI, 1.3 to 8.8). Furthermore, both total body irradiation (TBI; HR, 10.6; 95% CI, 3.7 to 30.2) and chest radiotherapy (HR, 2.5; 95% CI, 1.3 to 4.9) were risk factors for female breast cancer. When we restricted the cohort to 2,451 female CCSs who had no chest radiotherapy or TBI (n = 31 with breast cancer), HRs for doxorubicin dose tertiles were 1.3 (95% CI, 0.3 to 6.1), 5.6 (95% CI, 1.9 to 16.2), and 9.9 (95% CI, 4.2 to 23.8), respectively ( P trend = .002), whereas risk associated with ifosfamide was no longer significant (HR, 2.3; 95% CI, 0.6 to 8.0; data not shown). We found a significant dose-related risk for anthracyclines ( P trend = .004) but not for the cyclophosphamide equivalent dose (CED; P trend = 0.99; Table 3 , female breast cancer model 2).

In total, 261 survivors developed at least one SMN, including 24 with two SMNs and three with three SMNs. Risk of any SMN was significantly elevated compared with cancer incidence in the general population (SIR, 5.2; 95% CI, 4.6 to 5.8), with 20.3 excess cancers per 10,000 person-years ( Table 2 ). EARs ≥ 2.0/10,000 person-years were observed for female breast, thyroid, soft tissue sarcoma, and CNS malignancies. SIRs decreased with increasing age at childhood cancer diagnosis and decreased gradually with increasing time since diagnosis, although the SIR was still significantly increased after > 30 years (SIR, 3.8; Appendix Table A2 , online only). Conversely, EARs increased with increasing follow-up time. SIRs and EARs for solid cancers followed the time pattern for all SMNs, whereas those for hematologic malignancies peaked 5 to 9 years since childhood cancer and dropped afterward (Appendix Table A3 , online only). The cumulative incidence of SMNs 25 years after childhood cancer was 3.9% (95% CI, 3.3% to 4.5%). For all solid cancers, female breast cancer, and sarcoma, 25-year cumulative incidences were 3.4% (95% CI, 2.9% to 4.0%), 1.5% (95% CI, 1.0% to 2.2%), and 1.0% (95% CI, 0.7% to 1.3%), respectively. The cumulative incidence of any SMN 15 years after childhood cancer for CCSs diagnosed in the 1990s was similar to that for CCSs treated in earlier decades (1.4%, 1.7%, and 1.6% for 1963 to 1979, 1980 to 1989, and 1990 to 2001, respectively; Fig 1 ).

The cohort included 6,165 5-year CCSs who contributed 103,949 person-years of follow-up. Leukemia (34%), lymphoma (16%), and CNS tumors (14%) were the most frequent childhood cancers ( Table 1 ). The median time since childhood cancer diagnosis was 20.7 years (range, 5.0 to 49.8 years); 20% of CCSs were followed for ≥ 30 years. Median attained age at the end of follow-up was 28.1 years (range, 5.3 to 65.1 years). Approximately 10% of the total cohort was deceased. Of all survivors, 48% received chemotherapy without radiotherapy, 8% received radiotherapy without chemotherapy, 33% received a combination of chemotherapy and radiotherapy, and 6% received HCT. The proportion of survivors treated with radiotherapy strongly decreased over time (75%, 43%, and 28% for those diagnosed in 1963 to 1979, 1980 to 1989, and 1990 to 2001, respectively), whereas the contribution of chemotherapy to treatment increased from 72% of survivors in 1963 to 1979 to > 80% after 1980 (data not shown).

DISCUSSION Section: Choose Top of page Abstract INTRODUCTION METHODS RESULTS DISCUSSION << REFERENCES

This study in a well-characterized cohort of Dutch CCSs strongly suggests that chemotherapeutic agents can increase the risk of solid cancers independently of radiotherapy. Doxorubicin was associated with a dose-dependent increased risk of female breast cancer, whereas cyclophosphamide increased subsequent sarcoma risk in a dose-dependent manner. These findings are based on > 6,000 5-year survivors, with a median follow-up of 20.7 years since primary diagnosis, detailed therapy information, and highly complete follow-up for SMNs.

Doxorubicin exposure was associated with female breast cancer in a dose-dependent manner, particularly for women who may have had LFS-associated childhood cancer types. These findings independently validate and extend a recent Childhood Cancer Survivor Study report on a dose-dependent increased breast cancer risk with cumulative anthracycline exposure (relative SIRs, 2.6 and 3.8 for 1 to 249 and ≥ 250 mg/m2, respectively; P trend = .004) in female CCSs treated without chest radiotherapy, which was stronger among survivors of leukemia and sarcoma.22 The current findings of a stronger doxorubicin dose response among survivors of LFS-associated cancer types compared with other CCSs suggest a gene-anthracycline interaction in the development of breast cancer, perhaps with the inclusion of LFS or LFS-like syndromes, as hypothesized by Henderson et al.22 Future collaborative studies that include family history of cancer and/or TP53 status are needed to truly disentangle the role that childhood cancer type, treatment details, and genetic factors play in the development of subsequent female breast cancer. We observed no effects of epirubicin and idarubicin, with only one and zero breast cancers occurring among exposed women, respectively. For daunorubicin (seven cases of breast cancer), risk was only increased in the multivariable model for the subcohort of LFS-associated childhood cancers (HR, 3.8; 95% CI, 1.1 to 13.2). An association between anthracyclines and (breast) cancer risk was previously suggested by smaller follow-up studies, particularly for doxorubicin,36-38 and is supported by studies in rodents.39-44 We observed an increased risk of sarcoma after high-dose doxorubicin treatment in univariable analyses but not after multivariable adjustment that included cyclophosphamide. This observation seemingly contrasts an earlier report on anthracycline-related sarcoma risk, which only crudely adjusted for other chemotherapy and not specifically for alkylating agents.21 In all, the current result of a dose-dependent increased risk of solid cancer, particularly breast cancer, irrespective of radiotherapy treatment suggests that anthracyclines play a role in the etiology of breast cancer and perhaps other solid cancers.

Alkylating agents have been associated with many different solid cancers among (childhood) cancer survivors.9-11,14,45-52 However, only a few studies identified specific agents (eg, procarbazine for GI cancer45-48,51 and cyclophosphamide for bladder and pancreatic cancer45,53,54). We did not find an increased risk of solid cancer after procarbazine exposure on the basis of data from 17 patients with a GI cancer. Cyclophosphamide, however, showed a dose-response relation with subsequent sarcoma, particularly bone sarcoma, in accordance with previous SMN studies11,14,45,53,54 and experimental data.55,56 Previous treatment with ifosfamide also resulted in increased risks of female breast cancer, sarcoma, and all solid cancers among CCSs. We interpret this finding with caution because the seemingly elevated overall risk was not paralleled by clear dose-response patterns or is supported by other epidemiologic studies on carcinogenicity of ifosfamide, although some evidence from animal studies exists.57,58

We chose to focus on agent-specific dose information because the various agents collated in chemotherapy agent categories on the basis of structure and/or antitumor mechanism may not necessarily have similar carcinogenic properties. Moreover, aggregate measures of chemotherapy exposures are based on acute hematologic toxicity32 or late cardiac toxicity,59 or they are cohort specific, which limits comparability across cohorts.14,60 Although we observed significant associations between anthracycline dose and risk of breast cancer and all solid cancers, they were confined to doxorubicin. For sarcoma, we observed a significant dose response for the CED, but when we examined specific agents, only cyclophosphamide dose seemed to be associated with sarcoma. Radiotherapy was associated with solid cancer risk, with stronger effects for TBI than for other radiotherapy, which may be related to the large volume of the body treated in TBI, a high dose per fraction, or HCT-related immunologic alterations.61

Despite the reduction of radiotherapy exposure over time, the findings indicate that the risk of SMN did not decrease noticeably in patients treated between 1990 and 2001 (median follow-up, 16 years) compared with those treated earlier. SMNs that occur relatively soon after childhood cancer diagnosis conceivably have a genetic basis, whereas treatment-related (mainly solid) SMNs occur later. Longer follow-up is needed to evaluate SMN risk among recently treated patients.

The major strengths of this study are the large cohort size, detailed information on individual treatments, and availability of highly complete SMN follow-up by record linkage and medical information. A limitation of the study is that the number of events for most SMN sites were fairly low as a result of the age distribution at the end of follow-up. In addition, as in other survivorship cohorts, correlations between patient and treatment factors, which reflect clinical reality, sometimes hampered the ability to disentangle effects in multivariable analysis.22 Furthermore, we tested many variables in our models and performed various post hoc tests to validate the findings, so we cannot exclude the possibility that some of the findings are based on chance.

In conclusion, the results strongly suggest that CCSs who received treatment, including TBI, other radiotherapy, doxorubicin, or cyclophosphamide, are at the highest risk for developing subsequent solid cancers. In addition, our observations indicate that genetic susceptibility may influence doxorubicin-associated breast cancer risk. The results of this study will inform future childhood cancer treatment protocols as well as SMN surveillance guidelines for former patients.