Three-dimensional external beam radiotherapy for prostate cancer increases the risk of hip fracture

Authors

  • Sean P. Elliott MD,

    Corresponding author
    1. Department Urologic Surgery, School of Medicine, University of Minnesota, Minneapolis, Minnesota
    2. Division of Health Policy and Management, School of Public Health, University of Minnesota, Minneapolis, Minnesota
    • Department Urologic Surgery, University of Minnesota, 420 Delaware Street SE, MMC 394, Minneapolis, MN 55455
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    • Fax: (612) 626-0428

  • Stephanie L. Jarosek RN,

    1. Division of Health Policy and Management, School of Public Health, University of Minnesota, Minneapolis, Minnesota
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  • Shaheen R. Alanee MD, MPH,

    1. Department Urologic Surgery, School of Medicine, University of Minnesota, Minneapolis, Minnesota
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  • Badrinath R. Konety MD, MBA,

    1. Department Urologic Surgery, School of Medicine, University of Minnesota, Minneapolis, Minnesota
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  • Kathryn E. Dusenbery MD,

    1. Department of Therapeutic Radiology, School of Medicine, University of Minnesota, Minneapolis, Minnesota
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  • Beth A. Virnig MPH, PhD

    1. Division of Health Policy and Management, School of Public Health, University of Minnesota, Minneapolis, Minnesota
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  • This study used the linked Surveillance, Epidemiology, and End Results (SEER)-Medicare database. The authors are solely responsible for the interpretation and reporting of these data. We acknowledge the efforts of the Applied Research Program, National Cancer Institute (NCI); the Office of Research, Development and Information, Centers for Medicare and Medicaid Services (CMS); Information Management Services (IMS), Inc; and the SEER Program tumor registries in the creation of the SEER-Medicare database.

Abstract

BACKGROUND:

Hip fracture is associated with high morbidity and mortality. Pelvic external beam radiotherapy (EBRT) is known to increase the risk of hip fractures in women, but the effect in men is unknown.

METHODS:

From the Surveillance, Epidemiology, and End Results (SEER)-Medicare database, 45,662 men who were aged ≥66 years and diagnosed with prostate cancer in 1992-2004 were identified. By using Kaplan-Meier methods and Cox proportional hazards models, the primary outcome of hip fracture risk was compared among men who received radical prostatectomy (RP), EBRT, EBRT plus androgen suppression therapy (AST), or AST alone. Age, osteoporosis, race, and other comorbidities were statistically controlled. A secondary outcome was distal forearm fracture as an indicator of the risk of fall-related fracture outside the radiation field.

RESULTS:

After covariates were statistically controlled, the findings showed that EBRT increased the risk of hip fractures by 76% (hazards ratio [HR], 1.76; 95% confidence interval [CI], 1.38-2.40) without increasing the risk of distal forearm fractures (HR, 0.80; 95% CI, 0.56-1.14). Combination therapy with EBRT plus AST increased the risk of hip fracture 145% relative to RP alone (HR, 2.45; 95% CI, 1.88-3.19) and by 40% relative to EBRT alone (HR, 1.40; 95% CI, 1.17-1.68). EBRT plus AST increased the risk of distal forearm fracture by 43% relative to RP alone (HR, 1.43; 95% CI, 0.97-2.10). The number needed to treat to result in 1 hip fracture during a 10-year period was 51 patients (95% CI, 31-103).

CONCLUSIONS:

In men with prostate cancer, pelvic 3-D conformal EBRT was associated with a 76% increased risk of hip fracture. This risk was slightly increased further by the addition of short-course AST to EBRT. This risk associated with EBRT must be site-specific as there was no increase in the risk of fall-related fractures in bones that were outside the radiation field. Cancer 2011. © 2011 American Cancer Society.

Hip fracture is associated with high morbidity and significant mortality.1, 2 Osteoporosis is the single greatest risk factor for fractures in general and hip fracture in particular. Factors associated with osteoporosis include age, body mass index, multiple comorbidities, glucocorticoid therapy, postmenopausal status in women, and androgen suppression therapy (AST) for prostate cancer in men.3 Fragility fractures, such as distal forearm and hip fracture, are uniquely associated with falls.4 It is well accepted that pelvic external beam radiotherapy (EBRT) for gynecological malignancies and anorectal carcinoma in women increases the risk of hip fractures5, 6; the mechanism is understood to be radiation-induced osteonecrosis that can occur as the beam path passes through the bone to reach the target site.7-9

The effect of pelvic EBRT on hip-fracture risk in men is unknown. Prostate cancer is the most common solid-organ malignancy in men, affecting nearly 200,000 men annually. In 2006, 28% of all men with newly diagnosed prostate cancer received EBRT within 6 months of diagnosis.10 Elderly men with prostate cancer are more commonly treated with EBRT, with or without AST, than younger men.11, 12 Elderly men are also the subpopulation at highest risk for hip fracture on the basis of their age and comorbidities. Randomized clinical trials have demonstrated improved survival with EBRT plus 6 months to 36 months of AST (EBRT + AST) compared with either treatment alone in locally advanced prostate cancer,13, 14 and 50% of high risk prostate cancer patients treated with EBRT also receive AST.15 Moreover, recent evidence shows that men with high-risk prostate cancer treated with lifelong androgen deprivation have improved survival with the addition of EBRT.16

The prevalence of prostate cancer combined with high use of EBRT, particularly in the elderly, plus the frequent combination of EBRT + AST intensify the need for understanding the effect of EBRT alone or in combination with AST on prostate cancer patients' risk of developing hip fractures, especially among the elderly.

Our hypotheses were that compared with radical prostatectomy (RP), pelvic EBRT increases the risk of hip fractures, but not fragility fractures outside the radiation field (distal forearm fractures), and that EBRT + AST would have a higher risk of hip fractures than EBRT alone. Our analysis was limited to 3-D conformal EBRT (henceforth termed EBRT) and did not include intensity-modulated radiotherapy (IMRT).

MATERIALS AND METHODS

Data Sources

After approval from the University of Minnesota Institutional Review Board, data were obtained from the Surveillance, Epidemiology, and End Results (SEER) cancer registry linked to Medicare enrollment and utilization data (SEER-Medicare). SEER contains patient and tumor characteristics as well as complete treatment information through 6 months after cancer diagnosis. The 17 geographic regions composing the SEER registry account for 26% of the US population. An elderly subset of the SEER population can be followed beyond their initial year after diagnosis by linking their SEER information to claims data available through Medicare.

Study Subjects

Men aged 66 years or older who received a first diagnosis of nonmetastatic prostate cancer in the years 1992 through 2004 were identified in SEER (n = 232,071; Fig. 1). To ensure complete information, we limited our analysis to those most likely to have complete claims; we excluded patients who were not enrolled in both Part A and Part B Medicare for the 12 months before prostate cancer diagnosis or throughout the study period follow-up, whose prostate cancer had been diagnosed on autopsy or on a death certificate, and those for whom the month of diagnosis was unknown (n remaining = 151,867). Because they might have received treatment that could put them at risk for fracture, 17,123 men with a prior cancer were excluded. We limited our comparisons to those men managed with 1 of 4 nonoverlapping treatment strategies within 6 months of prostate cancer diagnosis: 1) radical prostatectomy (RP) without EBRT or AST, 2) EBRT without AST, 3) EBRT + AST, or 4) AST without EBRT. EBRT was limited to 3-dimensional conformal radiotherapy because IMRT was not used in significant quantity in the Medicare population until 2003, and the short follow-up available (through 2007) would be inadequate to draw conclusions about the long-term risk of hip fracture. We excluded men who received any of these therapies for the first time after 6 months to yield a uniform risk group who were all “exposed” to the etiology of interest soon after cancer diagnosis. Among those receiving EBRT + AST, we included only those who received a range (6 months to 36 months) of AST doses supported by evidence. These limitations excluded men who initiated AST later in their disease course (after 6 months for the RP and EBRT groups and after 36 months for the EBRT + AST group), thus minimizing the possibility of new metastatic disease after 6 months. Because AST as a sole therapy in nonmetastatic disease is not supported by evidence, we did not place an upper limit on the length of therapy but did require that at least 3 months (commonly, 1 depot injection) be given. This yielded a final cohort of 45,662. Of these men, 7854 also received brachytherapy, cryotherapy, or thermal ablation. We alternately included, excluded, and censored these 7854 men and assessed for a change in outcomes.

Figure 1.

The cohort creation is depicted.

Outcomes Defined

Our primary outcome was the cumulative incidence of hospitalization for hip fracture through 2007, identified by Medicare inpatient claims. The cumulative incidence of distal forearm fracture was determined from Medicare Part A and B (ie, hospital and physician claims) claims for fracture of the radius and/or ulna. We excluded fractures that occurred in the year of diagnosis to allow for a washout of differences in baseline risk and because such radiation effects are thought to be delayed in onset.

Demographic and Cancer Characteristics

Age was defined as the age at diagnosis of prostate cancer and was categorized in 5-year increments. A modification of the Charlson comorbidity index for use with Medicare claims data was used.17 Charlson score was categorized as 0, 1, ≥2. Disease grade and stage were obtained from the SEER database. Grade was categorized by World Health Organization strata. Stage was categorized by using the 1997 modification (ie, 2 substages within T2) of the TNM classification because more detailed stage information as reported in the 1992 modification (ie, 3 substages within T2) is not available in SEER for the years 1998-2003. Race, year of diagnosis, and SEER registry (ie, geographic location) were determined from SEER. A history of osteoporosis was identified from diagnosis codes on Medicare Part A or B claims in the year before cancer diagnosis. Demographic and cancer characteristics were compared across primary cancer treatment groups, and significant differences were determined by chi-square analysis.

Cumulative Incidence of Hip and Distal Forearm Fracture

Kaplan-Meier time-to-event analysis was used to measure the unadjusted cumulative incidence of hip and distal forearm fracture, stratified by primary cancer treatment group. Fracture rates were compared across treatment groups by using the log-rank test. Men were censored at diagnosis of a second cancer, death, or end of study.

Multivariate Model

Multivariate time-to-event analyses were performed with the use of Cox proportional-hazards regression. Separate models were constructed for the outcomes of hip and distal forearm fracture. The primary etiology of interest was the type of prostate cancer treatment. We were, thus, primarily interested in the hazard ratio of hip or distal forearm fracture in those managed with EBRT versus RP, AST versus RP, and EBRT + AST versus RP. We also report the risk of hip and distal forearm fracture after treatment with EBRT + AST versus EBRT. Other covariates in the model included age, race, year of diagnosis, registry, comorbidities, osteoporosis, tumor stage, and tumor grade. To provide an absolute measure of the impact of EBRT on hip fracture, the number of men who must be treated with EBRT to result in 1 hip fracture was calculated. This was performed by calculating the inverse of the difference between the adjusted risk of hip fracture in the RP-treated and EBRT-treated men at 10 years. All analyses were 2-sided. Type 1 error rate was set at P < .05. All analyses were performed with SAS version 9.1 (SAS Institute, Cary, North Carolina).

RESULTS

The final cohort of 45,662 consisted of 8146 men managed with RP, 13,396 managed with EBRT, 6974 managed with EBRT + AST, and 17,146 managed with AST (Table 1). Median follow-up for the total cohort was 49 months and varied by treatment group (RP, 54 months; EBRT, 62 months; EBRT + AST, 46 months; AST, 38 months; Table 1). The 10-year probability of overall survival likewise varied across treatment groups with RP-treated men having the highest likelihood of survival (77%) followed by EBRT (48%), EBRT + AST (40%), and AST (29%). Of the 45,662 men in the cohort, 1636 (3.6%) developed hip fracture; distal forearm fracture occurred in 451 (1%; Table 1).

Table 1. Cohort Demographics and Clinical Characteristics Stratified by Prostate Cancer Treatment and Incidence of Hip and Forearm Fracture Stratified by Demographic and Clinical Characteristics
 RPEBRTEBRT+ASTASTTotalHip FractureForearm Fracture
  1. Abbreviations: AST, androgen suppression therapy; EBRT, external beam radiation therapy; RP, radical prostatectomy; WHO, World Health Organization.

No. (% of total)8146 (18)13396 (29)6974 (15)17146 (38)456651636 (3.6)451 (1.0)
Median follow-up, mo5462463849  
10-Year overall survival81%49%49%34%49%  
10-Year incidence of hip fracture3%8%9%16%9%  
Cohort characteristicsColumn PercentageOverall Percentage
 Age, y       
  66-6955.321.419.512.925.41.50.6
  70-7437.439.937.022.132.32.80.9
  75-796.730.032.228.025.44.11.1
  80-840.57.59.723.212.56.61.5
  85+0.11.11.613.85.87.91.2
 Charlson score       
  085.074.872.069.074.03.30.9
  112.118.119.220.118.04.41.2
  2+2.97.18.810.98.14.41.1
 Race       
  White82.282.480.174.579.13.81.1
  Black6.79.77.18.78.42.40.5
  Hispanic7.04.37.011.67.93.30.8
  Asian3.93.45.34.54.22.50.8
  Other0.30.20.50.70.42.02.4
 WHO grade       
  13.09.32.34.05.15.81.5
  282.073.857.964.869.43.20.9
  314.413.737.628.222.94.11.1
  Undifferentiated0.63.22.23.02.55.10.9
 N0M0 tumors by T category       
  11.532.430.132.126.43.51.0
  283.263.461.763.666.83.61.0
  314.13.77.03.35.93.50.9
  41.20.51.11.00.94.30.5
 Osteoporosis at baseline       
  098.598.798.097.798.23.61.0
  11.51.32.02.31.85.31.0
 Radical prostatectomy       
  00.098.799.095.379.84.21.1
  1100.01.31.04.720.21.10.6
 Subsequent cancer       
  094.987.091.494.291.83.71.0
  15.113.08.65.88.22.80.3
 Brachytherapy       
  0100.084.577.879.084.24.01.1
  10.015.522.221.015.81.20.5
 High dose rate brachytherapy       
  099.996.795.399.198.03.61.0
  10.13.34.70.92.01.40.4
 Thermal ablation       
  0100.099.799.9100.099.93.61.0
  10.00.30.10.00.14.72.3
 Cryotherapy       
  0100.099.999.998.299.33.61.0
  10.00.10.11.80.70.30.3

Demographic and Cancer Characteristics Stratified by Cancer Treatment

Men treated with RP were the youngest and healthiest group and had the best 10-year overall survival, whereas those managed with AST were the oldest and were more likely to have multiple comorbidities and the lowest 10-year overall survival (Table 1). Men treated with EBRT or EBRT + AST were of intermediate age, had intermediate levels of comorbidities, and had intermediate survival. Known osteoporosis at baseline was rare in all groups (1.3%-2.3%). Of both RP-treated and EBRT-treated men, 14% had high-grade disease, whereas the rate of high-grade disease in those receiving EBRT + AST was 38% and in those receiving AST was 28%. Less than 5% of men in the EBRT, EBRT + AST, and AST groups also underwent RP. By definition, no one in the RP group received brachytherapy. Brachytherapy was used in 15%-22% of the other groups. High dose-rate brachytherapy was used in 3% and 5% of men in the EBRT and EBRT + AST groups, respectively, and was rare in the other groups. Thermal ablation and cryotherapy were rare in all groups (<1% in most cases).

Unadjusted Cumulative Incidence of Hip or Distal Forearm Fracture

The cumulative incidence of hip fracture at 10 years was 2.6% in men undergoing RP and 8.4% in men undergoing EBRT (P < .001; Fig. 2a and Table 1). The addition of AST (mean, 11 monthly AST doses) to EBRT slightly increase the risk of hip fracture above that with EBRT alone (8.7% vs 8.4%, P = .014). Hip fracture risk was highest in those managed with AST alone (16.2%; mean, 19 monthly AST doses). Except for EBRT versus EBRT + AST, all other pair-wise comparisons by log-rank test (ie, EBRT vs RP, EBRT vs AST, AST vs RP, AST vs EBRT + AST, and RP vs EBRT + AST) were statistically significant at P < .0001.

Figure 2.

Kaplan-Meier estimates of probability of freedom from (a) hip fracture or (b) distal forearm fracture are stratified by primary prostate cancer treatment (RP, radical prostatectomy; EBRT, external beam radiotherapy; AST, androgen suppression therapy). All pair-wise comparisons are significant by log-rank test except freedom from distal forearm fracture with RP versus EBRT (Fig. 2b).

The 10-year incidence of distal forearm fracture was 1.6% for men treated with either RP or EBRT (P = .6313) but was 2.5% in those undergoing EBRT + AST and was 4.4% in those on AST (Fig. 2b and Table 1). Except for EBRT versus RP, all other pair-wise comparisons by log-rank test (ie, EBRT vs AST, EBRT vs EBRT + AST, AST vs RP, AST vs EBRT + AST, and RP vs EBRT + AST) were statistically significant at P < .002.

Multivariate-Adjusted Cox Models

After we controlled for measurable confounding variables, men managed with EBRT were at a 76% increased risk of hip fracture compared with men managed with RP (HR, 1.76; 95% CI, 1.38-2.40) but at no increased risk of distal forearm fracture (HR, 0.80; 95% CI, 0.36-1.78) (Fig. 3 and Table 2). EBRT + AST increased the risk of hip fracture by 145% relative to RP (HR, 2.45; 95% CI, 1.88-3.19) and increased the risk of distal forearm fracture by 43% relative to RP (HR, 1.43; 95% CI, 0.97-2.10). Finally, AST tripled the risk of hip fracture (HR, 2.97; 95% CI, 2.32-3.80) and doubled the risk of distal forearm fracture (HR, 2.02; 95% CI, 1.43-2.85). EBRT + AST increased the risk of hip fracture by 40% compared with EBRT alone (HR, 1.40; 95% CI, 1.17-1.68). Other factors associated with hip and distal forearm fracture included age, comorbidities, race, and a baseline diagnosis of osteoporosis (see Table 2 for details). Year of diagnosis was associated with hip fracture but not distal forearm fracture. There was no change in the hazard ratio of hip or distal forearm fracture associated with the various treatment groups when patients who received brachytherapy, cryotherapy, or thermal ablation were excluded or censored at the time of therapy rather than included. The number of patients needed to treat to result in 1 hip fracture during 10 years was 51 (95% CI, 31-103).

Figure 3.

Multivariate-adjusted hazard ratio of hip or wrist fracture is stratified by primary prostate cancer treatment (RP, radical prostatectomy; EBRT, external beam radiotherapy; AST, androgen suppression therapy). 95% confidence interval represented by whiskers.

Table 2. Complete Results of Cox Proportional Hazards Models Predicting the Time to Hip or Distal Forearm Fracture
CovariatesHip FractureDistal Forearm Fracture
 OR95% CIPOR95% CIP
  1. Abbreviations: AST, androgen suppression therapy; CI, confidence interval; EBRT, external beam radiation therapy; OR, odds ratio; RP, radical prostatectomy; WHO, World Health Organization.

  2. Significant findings are marked in bold. The outcomes of interest are the 3 treatment-group variables (EBRT vs RP, EBRT+AST vs RP, and AST vs RP), and they also appear in the main body of the article. The remainder (of the variables) was controlled for in the model but was not the focus of this article and is found only here in the table.

EBRT vs RP1.7551.375-2.239<.00010.7960.559-1.135.2084
EBRT+AST vs RP2.451.881-3.192<.00011.4290.974-2.095.0676
AST vs RP2.9672.315-3.802<.00012.021.43-2.852<.0001
1993 vs 19920.9310.747-1.161.52610.7640.429-1.36.3593
1994 vs 19920.8590.682-1.082.1981.3550.857-2.142.1942
1995 vs 19920.7720.608-0.982.03470.9240.559-1.525.7559
1996 vs 19920.7420.581-0.948.01691.0050.614-1.646.983
1997 vs 19920.6440.5-0.829.00060.7960.47-1.347.3946
1998 vs 19920.6130.476-0.789.00020.9230.56-1.524.7551
1999 vs 19920.630.491-0.809.00030.7810.472-1.292.3357
2000 vs 19920.6850.543-0.865.00150.6070.358-1.029.0637
2001 vs 19920.5150.401-0.663<.00010.9440.593-1.501.8065
2002 vs 19920.5060.387-0.662<.00010.8320.514-1.347.454
2003 vs 19920.6320.467-0.856.00310.7970.479-1.328.384
2004 vs 19920.5740.357-0.925.02240.5720.298-1.099.0935
Age as a continuous variable1.0971.087-1.107<.00010.7920.357-1.757.5669
Charlson 1 vs 01.4041.246-1.583<.00011.3941.116-1.741.0034
Charlson 2+ vs 01.6181.369-1.913<.00011.4381.038-1.992.0288
Grade 2 vs 110.831-1.202.99740.8480.602-1.194.3443
Grade 3 vs 11.1160.911-1.367.28861.010.693-1.472.9579
Grade Unknown vs 11.0380.762-1.414.81270.6980.364-1.338.2788
T2 vs T11.0290.917-1.155.62740.9890.797-1.227.9184
T3 vs T11.1650.923-1.469.19840.880.562-1.378.5769
T4 vs T11.2790.796-2.057.30940.4540.112-1.842.2689
Black vs White0.6630.534-0.823.00020.4820.308-0.755.0015
Hispanic vs White0.9020.74-1.101.31130.7210.485-1.071.1056
Asian vs White0.5450.386-0.77.00060.6790.377-1.222.1967
Other vs White0.5310.199-1.42.20712.2540.926-5.488.0735
Osteoporosis vs None at cancer diagnosis1.6571.225-2.242.00111.0010.495-2.021<.0001

DISCUSSION

We have demonstrated that 3-D conformal pelvic EBRT increases the risk of hip fracture by 76% in men aged older than 65 years who are being treated for prostate cancer. To analyze the site-specific risk modification of the EBRT, we also tested the effect of EBRT on the risk of fall-related fractures to bones outside the radiation field, specifically distal forearm fracture. Indeed, we show that whereas EBRT increased the risk of hip fracture, it did not increase the risk of distal forearm fracture. As reference points, the added hip fracture risk due to EBRT treatment is similar in scale to the added risk from a 7-year increase in age (HR, 1.097 per year); or having a Charlson comorbidity score of ≥2 versus a score of 0 in our model, the risk imparted by a baseline diagnosis of osteoporosis; or the increased risk others have shown due to being a current smoker versus a never smoker.6 In absolute risk terms, 51 men need to be treated with EBRT to induce 1 hip fracture during 10 years of follow-up. Given that more than 28% of the nearly 200,000 men diagnosed with prostate cancer each year receive EBRT, 3-D conformal EBRT may be linked to approximately 1000 hip fractures each year.

We have previously shown that pelvic EBRT is a significant risk factor for hip fractures but not other fractures in women with gynecologic and colorectal malignancies.5 Similarly, the Stockholm trial of short-course radiotherapy for rectal cancer resulted in a doubling of fracture incidence in the radiated group for both men and women.18 The bony pelvis lies in close proximity to genitourinary pelvic organs and their lymphatics. Therefore, when radiation is used to treat the prostate and/or the pelvic lymph nodes, nearby bony structures are also irradiated.

Radiation damage occurs in the bone matrix, at the cellular level and at the vascular level.7 Radiation can lead to death of osteoblasts, osteocytes, and osteoclasts, resulting in a reduction in bone matrix production. In addition, radiation damage to the vascular supply to the bone may lead to further bone loss.9 Radiotherapy has been associated with fractures of the femur, pubic rami and pubic symphysis, acetabular failure, and avascular necrosis of the hip.8, 9, 19 Adding to the complexity of the problem, fractures after radiotherapy are more difficult to treat; hip replacement after radiotherapy is associated with an increased risk of infection and malfunction.8, 20

This study did not analyze the association between IMRT and hip fractures. Prostate IMRT use has increased greatly over the last decade since it was first approved for payment by Medicare in 2002. Whereas some of the 3-D conformal EBRT beams pass through the hip en route to the prostate (Fig. 4), IMRT can allow planners to decrease the dose to the hip or completely avoid the hip when necessary without compromising dose to the prostate or increasing scatter to the rectum and bladder.21 This may create a lower risk for hip fracture with IMRT. However, we did not include IMRT in this analysis because it was introduced late in our study period and because our Medicare claims follow-up would have been too short (3-4 years) to measure the long-term risk of hip fracture in this group.

Figure 4.

In the EBRT treatment plan for prostate cancer, note how lateral beams pass through the hip bones en route to the prostate. (From Kathryn Dusenbery, MD, Department of Therapeutic Radiology, University of Minnesota, Minneapolis, Minnesota).

AST is known to increase the risk of fractures.22 Because EBRT + AST combination therapy in high-risk prostate cancer is known to improve survival compared with either treatment alone,13, 14, 16 we were interested in the combined effect of these modalities on the risk of hip fracture. As all cancer therapy should be aimed at balancing benefits and risks, an examination of the risk of combination therapy is important. As expected, hip and distal forearm fracture risk was highest in the group treated with AST alone (mean, 19 monthly doses). Although the addition of short course AST to EBRT (mean, 11 monthly doses) did increase the risk of hip fracture by 40% compared with EBRT alone, the added effect of AST was not as dramatic as that seen at the higher number of doses used with AST monotherapy. Given that current trial evidence favors 36 months over 6 months of AST when combined with EBRT,14 it would be interesting to know how more AST doses affect hip fracture risk in the EBRT + AST group. However, a low number of events (hip fractures) in each group after stratification by the number of AST doses prevented us from investigating for such a dose-response relation.

Certain limitations deserve mention. As our findings are based on a population of men aged >65 years, conclusions may not be applicable in younger patients. Claims-based research can be an inexact measure of minor events; however, hip fracture nearly always results in acute hospitalization and claims-based methods have been shown to be valid in such disease models.23 It may be argued that the older age and higher number of comorbidities of our EBRT-treated cohort are evidence that men who select EBRT are generally more ill and at higher baseline risk for hip fracture. We have controlled for such a selection bias in 2 ways. First we demonstrate that EBRT-treated patients were not at higher risk for distal forearm fractures. In a way, distal forearm fracture serves as an internal control—if the increase in hip fracture risk seen in the EBRT-treated men was due to selection bias then one would expect their risk of distal forearm fracture to have been higher too. Second, our multivariate model demonstrates that after controlling for these other risk factors (eg, age and comorbidities), EBRT still increases the risk of hip but not distal forearm fracture. Of note, there may be residual confounding due to differences in the prevalence of risk factors that we were not able to measure such as patient weight, smoking, glucocorticoid therapy, alcohol use, and radiation dose to the hip—some of these risk factors may be correlated with the receipt of EBRT. Finally, we were able to exclude metastatic disease only through 6 months after diagnosis. Should new bony metastases occur after 6 months, these may result in fracture. This may confound measurement of our outcome when the likelihood of delayed metastatic disease differs between RP and EBRT-treated men. We controlled for the risk of confounding due to metastases by excluding men who initiated AST or underwent orchiectomy more than 6 months after diagnosis. Furthermore, previous studies have shown that even among men with advanced prostate cancer managed with AST, only 7%-16% of fractures are due to metastases.24, 25 Randomized studies would be needed to confirm these findings. Such studies should have extended follow-up (10 years) and be of sufficient size to be powered to detect a difference in events that occur in <10% of men. These studies should control for the risk factors we were unable to directly measure.

Conclusion

In men with prostate cancer, pelvic 3-D conformal EBRT is associated with a 76% increase in the risk of hip fracture. This risk is slightly increased further by the addition of short-course AST to EBRT. This risk associated with EBRT is site-specific as no increase in the risk of fall-related fractures outside the radiation field is seen. An assessment of baseline bone health may prove useful in men considering EBRT.

FUNDING SOURCES

This study was funded in part by the National Institutes of Health grant no. 5K12-RR023247-03.

CONFLICT OF INTEREST DISCLOSURES

The authors made no disclosures.

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