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Keywords:

  • prostate carcinoma;
  • radiotherapy;
  • dose escalation;
  • metastasis;
  • mortality

Abstract

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

BACKGROUND

The association of increasing radiotherapy (RT) dose with reduced biochemical failure (BF) is accepted widely. However, there is little direct evidence that dose escalation has an impact on distant metastasis (DM) or overall mortality (OM). These associations were examined in the current study.

METHODS

The outcome of 835 patients who were treated at the Fox Chase Cancer Center (Philadelphia, PA) between 1989 and 1997 using 3-dimensional, conformal RT alone (median dose, 74 Gray [Gy]) was analyzed. Stepwise multivariate Cox proportional hazards regression analyses (MVAs) were performed with RT dose included as a covariate along with log-transformed initial pretreatment PSA level, Gleason score, palpation T status, age, and year of treatment (YOT), where indicated. To minimize the effect of YOT, an analysis was performed on a subgroup of 363 patients who were treated prior to 1994.

RESULTS

With a median follow-up of 64 months, there were 220 PSA failures, 44 distant metastases, and 162 deaths. In MVA, RT dose (as a continuous variable) was a significant predictor for BF, DM, and OM. When YOT was included as a covariate, it was related strongly to all endpoints, and the correlations of RT dose with DM and OM were lost. When the effect of YOT was minimized by limiting the MVA to patients who were treated prior to 1994, RT dose again emerged as a significant predictor of DM.

CONCLUSIONS

Escalation of RT dose reduced the rates of BF, DM, and OM significantly in patients with prostate cancer. The inclusion of YOT had a pronounced effect on these correlations that may confound interpretation. Cancer 2004. © 2003 American Cancer Society.

Radiation dose has been reported to be an important determinant of outcome in prostate cancer patients treated with radiotherapy (RT) in the prostate-specific antigen (PSA) era.1–4 However, the effect of RT dose has been manifest primarily using biochemical failure (BF) as the endpoint. Although the pattern of a rising serum PSA level is regarded as an early predictor of distant metastasis (DM),3, 5–8 the relationship of BF to overall mortality (OM) is less well established.9–11 Moreover, direct evidence that RT dose reduces DM or OM has not been conclusive.3, 12–17

In the current report, the association of RT dose to DM and OM was examined in a population of men with prostate carcinoma who were treated relatively uniformly with three-dimensional (3D)-conformal RT. Because, over the period of the study (1989–1997), stage migration18–21 and Gleason score shift22–24 occurred, both of which complicate retrospective analyses of treatment changes over time, the year of treatment (YOT) was included as a covariate. The effect of the inclusion of YOT on the relationship of RT dose to DM and OM was investigated.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Patient Characteristics

A total of 835 patients treated consecutively between April, 1989 and November, 1997 using 3D-conformal RT comprised the study cohort. Pathologic grading of diagnostic biopsy specimens was performed at Fox Chase Cancer Center (Philadelphia, PA). An initial pretreatment PSA (iPSA) level was available from all patients, and the median iPSA level was 9.0 ng/mL (range, 0.4–191.0 ng/mL). The median age of the study population was 69 years (range, 45–89 years). Table 1 shows palpation T stage and Gleason score subdivided by iPSA level. The majority of patients (n = 470) had an iPSA level < 10 ng/mL. Three hundred seventy-one patients had an iPSA level of < 10 ng/mL and a Gleason score between 2 and 6.

Table 1. Distribution of Patients by Gleason Score and T Stage after Subdivision by Initial Pretreatment Prostate-Specific Antigen Level
iPSAGleason scoreT stageTotal
2–67–10T1/T2a–bT2c/T3/T4
  1. iPSA: initial pretreatment prostate-specific antigen level.

< 10 ng/mL3719940070470
≥ 10 ng/mL2689727194365
Total639196671164835

Treatment

Patients were immobilized in a custom-made alpha-cradle cast, supine, before computed tomographic-simulation. The target volumes and technique of RT planning have been described previously.25, 26 In general, a four-field technique using megavoltage irradiation was used. For favorable-risk patients with a T1/T2 classification and a Gleason score of 2–6, only the prostate was treated. For the remaining intermediate- to high-risk patients, the prostate and periprostatic tissues were treated to 46–50 Gray (Gy), then a reduction was made to the prostate and seminal vesicles. A 1-cm planning target volume (PTV) margin was placed around the clinical target volume structures, and a 0.5-cm margin around the PTV typically was placed for penumbra. The median ICRU reference point dose was 74 Gy, with a range of 62.6–84.1 Gy (99% of patients received ≥ 68 Gy), with ≤ 5% variation across the PTV. None of the patients received neoadjuvant or adjuvant androgen deprivation as part of their treatment.

Follow-Up and Endpoints

Patients typically were seen at 3 months and then every 6–12 months. PSA values were obtained at 3 months and every 6–12 months thereafter. Radiologic evaluation, as part of the work-up for metastasis, was performed when indicated based on clinical assessment or PSA values. BF, DM, and OM were the endpoints for the study. The study population had a median follow-up of 64 months (range, 9–147 months). BF was defined according to the American Society of Therapeutic Radiology and Oncology (ASTRO) criteria, which include three consecutive rises in serum levels of PSA after reaching a nadir.27 DM was defined as radiologic confirmation of hematogenous metastasis.

Statistical Analysis

Estimates of BF, DM, and OM were calculated using the Kaplan–Meier product-limit method.28 Univariate comparisons of outcome were accomplished using the log-rank test.29 Multivariate analysis (MVA) using stepwise Cox proportional hazards regression models was used to evaluate predictors of outcome.30 RT dose was used as a continuous variable in MVA and was dichotomized at 76 Gy for univariate comparisons. The 76 Gy cutoff point was chosen because most retrospective series have identified the greatest effects from doses at or above this level.2, 3, 26, 31 Other covariates for MVA included log-transformed iPSA level (continuous), T stage (T1/T2a,b vs. T2c/T3/T4), Gleason score (2–6 vs. 7–10), age (continuous), and YOT (continuous). Log-transformed iPSA(log[iPSA]) values were used because the log-transformation resulted in a Gaussian distribution.6 To minimize the potential confounding effect of YOT as a covariate, the statistical analysis was repeated on the subgroup of 363 patients who were treated between 1989 and 1993; this population had mature follow-up (median, 92 months; range, 11–147 months) and was treated prior to an observed shift in Gleason scoring by pathologists that we determined had occurred after 1994.32

RESULTS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

There were 220 BFs, 44 DMs, and 162 deaths (OM) within the study follow-up period. Kaplan–Meier estimates of BF, DM, and OM at 8 years are shown in Table 2. On univariate analysis, Gleason score, iPSA level, and T stage were significant correlates of BF, DM, and OM. Doses of RT > 76 Gy were associated with reduced rates of BF and OM. RT dose was not related to DM in univariate analysis (P = 0.17). For patients with tumors ≥ T3 and Gleason scores ≥ 7, there was a trend toward a lower DM rate with doses ≥ 76 Gy (P = 0.08). The lack of association between RT dose and DM in univariate analysis may have been caused by the cut-point that was chosen (76 Gy was used because previous studies have indicated that this is an important threshold level) and by an unequal distribution of other factors. MVAs that incorporated RT dose as a continuous variable were performed to address these issues.

Table 2. Relationship of Various Prognostic Factors to 8-Year Kaplan–Meier Estimates of Biochemical Failure, Distant Metastasis, and Overall Mortality
VariableNo. of patientsBF (%)DM (%)OM (%)
  • BF: biochemical failure; DM: distant metastases; OM: overall mortality; iPSA; initial pretreatment prostate-specific antigen level; RT: radiotherapy; Gy: Gray.

  • a

    P < 0.05.

  • b

    P = 0.17.

Gleason score    
 2–663930525
 7–1019644a16a38a
T stage or category    
 T1/T2a–b67131525
 T2c/T3/T416446a16a37a
iPSA (ng/mL)    
 0.0–9.947022623
 10.0–19.924535632
 ≥ 2012073a15a33a
RT dose (Gy)    
 < 7648038832
 ≥ 7635529a6b22a

Tables 3–5 show the results of the MVAs without YOT included as a covariate. RT dose was significantly associated with BF, DM, and OM. The higher the RT dose, the lower the risk of BF, DM, or OM. The other significant covariates were log(iPSA), T stage, and Gleason score. Age was related significantly to OM, but not to BF or DM.

Table 3. Multivariate Cox Proportional Hazards Analysis using Biochemical Failure as the Endpointa
VariableGroupParameter estimateP valueHazard ratio
  • RT: radiotherapy; Log(iPSA): log-transformed initial prostate specific antigen level.

  • a

    Two hundred twenty of 835 patients in the analysis experienced biochemical failure.

RT doseContinuous−0.1098< 0.00010.90
Log(iPSA)Continuous0.7157< 0.00012.05
Gleason score2–6 vs. 7–100.6385< 0.00011.89
T stageT1/T2a–b vs. T2c/T3/T40.40140.0131.49
Table 4. Multivariate Cox Proportional Hazards Analysis using Distant Metastasis as the Endpointa
VariableGroupParameter estimateP valueHazard ratio
  • RT: radiotherapy; Log(iPSA): log-transformed initial prostate-specific antigen level.

  • a

    Forty-four of 835 patients had distant metastasis.

RT doseContinuous−0.16000.0050.85
Log(iPSA)Continuous0.41390.0111.51
Gleason score2–6 vs. 7–101.2506< 0.00013.49
T stageT1/T2a–b vs. T2c/T3/T41.20780.00023.35
Table 5. Multivariate Cox Proportional Hazards Analysis using Overall Mortality as the Endpointa
VariableGroupParameter estimateP valueHazard ratio
  • RT: radiotherapy.

  • a

    There were 162 deaths overall among the 835 patients in the analysis.

RT doseContinuous−0.06680.0130.94
Gleason score2–6 vs. 7–100.41670.0191.52
T stageT1/T2a–b vs. T2c/T3/T40.48770.0061.63
AgeContinuous0.0659< 0.00011.07

When YOT was included in the MVAs as a continuous variable, it was associated significantly with BF (P < 0.0001), DM (P < 0.0001), and OM (P < 0.0001). RT dose remained related independently to BF (not shown) but not to DM (Table 6) or OM (Table 7). Log(iPSA) continued to be related significantly to BF but not to DM or OM.

Table 6. Multivariate Cox Proportional Hazards Analysis using Distant Metastasis as the Endpoint and Including the Year of Treatment as a Covariatea
VariableGroupParameter estimateP valueHazard ratio
  • YOT: year of treatment.

  • a

    Forty-four of 835 patients had distant metastasis.

Gleason score2–6 vs. 7–101.3308< 0.00013.78
T stageT1/T2a–b vs. T2c/T3/T41.11230.00043.04
YOTOrdinal−0.4622< 0.00010.63
Table 7. Multivariate Cox Proportional Hazards Analysis using Overall Mortality as the Endpoint and Including the Year of Treatment as a Covariatea
VariableGroupParameter estimateP valueHazard ratio
  • YOT: year of treatment.

  • a

    There were 162 deaths overall among the 835 patients in the analysis.

Gleason score2–6 vs. 7–100.41740.01791.52
T stageT1/T2a–b vs. T2c/T3/T40.35600.04431.43
YOTContinuous0.1909< 0.00010.83
AgeContinuous0.0594< 0.00011.06

Because YOT was significantly related to RT dose (P = 0.0001 analysis of variance) and, therefore, could confound the association of RT dose to patient outcome, a subgroup of patients treated between 1989 and 1993 (n = 363) was investigated. In this subgroup, there were 132 BFs, 28 DMs, and 116 deaths (OM). Median follow-up was 92 months. In MVA, RT dose, Gleason score 7–10, and T3 category were related significantly to DM (Table 8); YOT was not significant and was not included in the final model. The only significant predictor for OM was age.

Table 8. Multivariate Cox Proportional Hazards Analysis using Distant Metastasis as the Endpoint in the Subset of 363 Patients who were Treated before 1994a
VariableGroupParameter estimateP valueHazard ratio
  • RT: radiotherapy.

  • a

    Twenty-eight of the 363 patients had distant metastasis.

RT doseContinuous−0.21150.00190.81
Gleason score2–6 vs. 7–101.25310.00213.50
T statusT1/T2a–b vs. T2c/T3/T41.54520.00014.69

DISCUSSION

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES

Many retrospective and prospective studies have confirmed a reduction in BF as a consequence of increasing RT dose.1–4 However, there is little evidence of the impact of RT dose on DM and OM. In the University of Texas M. D. Anderson Cancer Center's randomized radiation dose–escalation prostate carcinoma trial, which compared a 70 Gy to 78 Gy,3 the DM rate was lower for patients with a PSA level > 10 ng/mL who were treated with 78 Gy (2% vs. 12% at 6 years; P = 0.056). No relation between RT dose and OM was seen in that study, because there were few deaths attributable to prostate carcinoma. Also, in a matched-pair analysis that compared ≤ 74 Gy > 74 Gy by Hanks et al.,15 RT dose was associated significantly with DM and cause-specific death for patients who were matched for iPSA, Gleason score, and T stage. Kestin et al.16 also did a matched-pair analysis and found that higher biologic RT doses were associated with reduced OM; however, those authors did not find any relation of dose with DM or cause-specific death. In one of the few studies wherein MVA was performed, Valicenti et al.17 found that RT dose was an independent predictor of cause-specific death and OM for patients who had Gleason scores of 8–10. That analysis involved patients who were treated in Radiation Therapy Oncology Group Trials in the pre-PSA era; and, although there was a range of doses used, 87% of patients were treated with conventional doses ≤ 70.2 Gy. No adjustment was made for time-related changes, such as stage migration or differences in follow-up. To our knowledge, no previous PSA-era multivariate study of patients who were treated mainly with conventional or greater RT doses has investigated the impact of dose on DM or OM and included a covariate (e.g., YOT) to account for such time-related concerns.

The data shown here reveal a strong association between increasing RT dose and reduced DM and OM. Although the MVAs documented that RT dose was independent of T stage, Gleason score, and iPSA, there are concerns about reliability. Over the course of the study (1989–1997), there were notable changes in patient attributes. The use of PSA and new or improved imaging modalities in screening (e.g., ultrasound and endorectal coil magnetic resonance imaging) and transrectal, ultrasound-guided, sextant or greater biopsies contributed to earlier diagnosis. These features, combined with changes in the American Joint Committee on Cancer staging system, such as the inclusion of imaging and biopsy findings, have resulted in stage migration.18–21 A prostate cancer patient with T2b disease today is not the same as a patient with T2b disease 5–10 years ago. Another, perhaps even more significant factor that obscures retrospective studies is the shift in Gleason scoring that has occurred over the study period.22–24 Recently, Chism et al.32 found that pathologists consistently are assigning higher Gleason scores in diagnostic prostate biopsy specimens today compared with the scores assigned prior to 1994. Thus, a tumor may have been assigned a Gleason score of 6 in the early 1990s and a score of 7 in the late 1990s. Such time-related changes would improve the outcome of patients with Gleason 6 and Gleason 7 disease artificially over time.

Distinguishing between the effect of RT dose, which also was altered over time, and follow-up differences (patients who received higher RT doses generally were treated more recently),33 stage migration,18–21 Gleason score shift,22–24 and possibly other factors affecting patient outcome is not possible without the inclusion of a covariate that adjusts for these changes. This is the reason for the inclusion of YOT as a covariate. The influence of the time-dependent changes that occurred in follow-up, stage classification, and Gleason scoring should be negated by YOT; however, because RT dose is associated significantly with YOT, any impact of RT dose on outcome also would be blunted. Indeed, when YOT was included in the MVAs, RT dose no longer was associated with DM or OM. There still was a strong relationship between RT dose and BF.

One way to limit the influence of time-related factors is to perform the MVAs on patients who were treated over a shortened time span. When only patients treated between 1989 and 1993, inclusive, were considered, it was found that RT dose was related to DM. Under those conditions, YOT was not significant in MVA and was not included in the final model (Table 8). These data provide much more convincing evidence that RT dose is effective at reducing DM. The inability to document an association between RT dose and OM in this cohort probably was related to the limited number of patients; death due to intercurrent disease in patients with prostate cancer is common, necessitating large numbers of patients when OM is used as the endpoint.

In summary, increasing RT dose improved outcomes in patients with clinically localized prostate carcinoma. The association with BF is undisputed; however, the association with DM and OM has been less certain. Our data show that RT dose also is related significantly to DM. We were only able to show an association between RT dose and OM without correcting for time-related changes (exclusion of YOT as a covariate); a larger patient cohort will be needed to establish this association more conclusively.

REFERENCES

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. REFERENCES
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