Association between C-reactive protein and radiotherapy-related pain in a tri-racial/ethnic population of breast cancer patients: a prospective cohort study

Data for the current analysis was obtained from a prospective cohort study (University of Miami, FL, USA) where the goal was to examine the disparity of RT-induced early adverse skin reactions in a racially and ethnically diverse population of breast cancer patients. Briefly, the study recruited breast cancer patients from the Radiation Oncology clinics at the University of Miami Sylvester Comprehensive Cancer Center and Jackson Memorial Hospital in Miami, Florida, between December 2008 and August 2014. Patients were followed up for up to 12 months after the completion of RT. At the time of enrollment, each participant completed a self-administered baseline questionnaire. In addition, participants completed QOL questionnaire on the first day before initiation of RT, on the last day immediately after completion of RT, and at each follow-up visit (1, 2, 6, and 12 months). The current study only used QOL data collected on the first day of RT (i.e., pre-RT) and on the last day of RT (i.e. post-RT). The treating radiation oncologist met patients each week during the radiation treatment and evaluated adverse skin reactions at week 3 (mid-treatment), at week 6 (completion of RT), and at each follow-up visit. We collected blood samples (20 mL) at pre- and post-RT for biomarker data. Blood samples were processed within 2 h of phlebotomy, and the aliquoted plasma samples were stored at − 80 °C until assay. The study was approved by Institutional Review Boards of the University of Miami and Jackson Memorial Hospital, and all patients provided written informed consent.

The inclusion criteria were adult (≥ 18 years old at the time of diagnosis) female patients, newly diagnosed with breast cancer (AJCC stage 0–III) who had undergone BCS and planned to receive adjuvant RT to the whole breast with or without regional lymph nodes (total dose ≥ 40 Gy, dose per fraction ≥ 2.0 Gy). Other criteria included patients belonging to one of three racial/ethnic groups [self-reported non-Hispanic whites (NHW), black/African Americans (AA), and Hispanic whites (HW)] and being able to speak English or Spanish. The exclusion criteria were patients diagnosed with stage IV breast cancer and those that received partial breast irradiation and/or concurrent chemoradiation. Patients with missing pain score and/or CRP level at pre- or post-RT were excluded. To increase the validity of RT-related change in pain score, patients who reported pain due to other acute health conditions unrelated to cancer or radiation (such as shingles or fracture) were also excluded from the analysis after medical record verification.

RT was delivered using standard or partially wide photon tangents using 6 and/or 10MV photons with forward planned field-in-field technique to maximize dose homogeneity. Patients received RT to the whole breast ± regional lymph nodes with conventional fractionation (2.0 Gy/day over 5–6 weeks, mostly 50 Gy in 25 fractions) or hypo-fractionation (> 2.0 Gy/day over 3 weeks, most commonly 42.4 Gy in 16 fractions). An additional boost dose of 10–20 Gy without bolus was delivered to the tumor-bed site in most patients. Radiation oncologists contoured target volumes, including the breast and lumpectomy cavity. The treatment plan was completed on the Eclipse or Pinnacle planning systems.

All women enrolled in the study filled out the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-39/RTOG 0413 protocol QOL questionnaire pre- and post-RT. This questionnaire measured QOL relating to breast cosmesis, fatigue, treatment-related symptoms, and perceived convenience of care. The section pertaining to treatment-related symptoms included four pain severity items, which were extracted from the Brief Pain Inventory (BPI): “Rate your pain at its worst, at its least, on average in the past four weeks, and now (0 = no pain to 10 = pain as bad as you can imagine).” The pain score was measured as a mean of these four pain severity items; a pain score of 4–10 was used to define the presence of clinically relevant pain because pain ≥ 4 indicates a moderate to severe level of pain, as used in previous studies [7, 24, 25]. In addition, patients who reported an increase in pain level from pre- to post-RT (i.e., pain score changed from < 4 to ≥ 4) was defined as having RT-related pain as previously reported [11] and compared to patients with pain score < 4 at both pre- and post-RT.

Plasma CRP levels were measured using a high-sensitivity CRP enzyme-linked immunosorbent assay (ELISA) kit (Calbiotech, Spring Valley, CA) according to the manufacturer’s protocol, as previously described [16]. A standard curve was generated for each batch of samples based on CRP concentrations, which ranged from 0.2 to 10.0 mg/L. To ensure that the diluted samples were within the linear range of the standard curve, we re-ran the assays by adjusting the dilution ratio if samples were outside the detection range. The average coefficient of variation was 8.3%, and the inter-assay variation was less than 10%. The cut-off value of CRP level was determined based on clinical usage and literature review where CRP ≥ 10.0 mg/L is a prognostic biomarker for breast cancer survival [26]. For CRP change, we used 1.0 mg/L as the cut-off value because it has been significantly associated with RT-induced skin toxicity in the same patient population [16]. Considering that CRP is an acute-phase protein with a half-life of 18 h, we collected post-RT blood samples immediately after RT on the last day consistently among all sample patients.

Demographic information, self-reported race and ethnicity, comorbidities, and smoking history/status were obtained from a self-administered baseline questionnaire at the time of enrollment. A high correlation was found between the comorbidities reported on the questionnaires and those extracted from medical records [27, 28]. Tumor characteristics, such as tumor stage, estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and detailed information on treatments were ascertained from medical records.

We first examined the distributions and frequencies of patient-, tumor-, and treatment-related characteristics overall and by race/ethnicity using the Pearson’s chi-square test or the Fisher’s exact test. The analysis of variance (ANOVA) was used to compare CRP levels by patient characteristics. The Pearson's chi-square test or the Fisher's exact test was used to compare the frequencies of elevated CRP or pain by patient characteristics. Univariable and multivariable logistic regression analyses were used to test whether elevated pre-RT CRP and/or obesity (BMI ≥ 30 kg/m²) were significantly associated with RT-related pain. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were reported. In addition, we performed the receiver operating characteristics (ROC) curve analysis to evaluate whether pre-RT CRP level and/or obesity contribute to RT-related pain. A two-tailed P value < 0.05 was considered statistically significant, and all statistical analyses were performed using SAS 9.4 (SAS Institute, Cary, NC, USA).

The study population consisted of 366 breast cancer patients: 64% HW, 20% AA, and 16% NHW. The mean ± standard deviation (SD) of age was 56.0 ± 9.1 years. As shown in Table 1, AA women were more likely to have BMI ≥ 30 kg/m², advanced stage or triple-negative tumors, larger volume (cc) of the breast, diabetes mellitus, and hypertension compared to HW or NHW women. HW women were more likely to receive hormone therapy (HT) with aromatase inhibitors prior to RT compared to other racial/ethnic groups. For breast cancer surgery, 68% of patients received BCS with or without sentinel lymph node biopsy (SLNB), and 32% received BCS with axillary lymph node dissection (ALND). For systemic therapy, about half of the patients received chemotherapy, 44% initiated HT prior to RT, and 7% began HT during RT. For RT, 84% of patients received conventional fractionation with a mean total dose of 58.2 ± 4.8 (SD) Gy, including an additional boost to the lumpectomy cavity, and 16% were treated with hypo-fractionated regimens. There were no significant differences in RT treatment regimens across the three racial/ethnic groups. Overall, patients reported a significantly higher pain score at post-RT (mean ± SD = 2.8 ± 2.5) compared to pre-RT (mean ± SD = 1.7 ± 2.1). In general, AA and HW patients had significantly higher pre-RT and post-RT pain scores compared to NHW patients.

As shown in Table 2, there was no significant difference between pre- (mean ± SD = 6.5 ± 9.3) and post-RT (mean ± SD = 6.1 ± 8.9) plasma CRP levels. The CRP levels were significantly higher in obese patients at both pre- and post-RT. Pre-RT CRP levels were significantly higher in patients with pre- or post-RT pain score ≥ 4. Post-RT CRP levels were significantly higher in patients with smoking history, post-RT pain score ≥ 4, larger breast volume, and tamoxifen treatment during RT.

As shown in Table 3, the proportion of patients who reported clinically relevant pain (pain score ≥ 4) increased from 17% at pre-RT to 30% at post-RT. Pre-RT pain was more prevalent in patients with AA or HW race/ethnicity, BMI ≥ 30 kg/m², HER2-positive tumor, received trastuzumab alone or taxane+trastuzumab, received ALND, or pre-RT CRP ≥ 10 mg/L, compared to their respective comparison groups. Post-RT pain was more prevalent in patients with AA or HW race/ethnicity, age < 50 years, BMI ≥ 30 kg/m², at least 2 comorbid conditions, conventional RT fractionation, total RT dose ≥ 60 Gy, or pre-RT CRP ≥ 10 mg/L, compared to their respective counterparts. About 23% of patients had RT-related pain, and it was more frequent in patients with AA or HW race/ethnicity, at least 2 comorbid conditions, conventional RT fractionation, or RT-induced CRP change > 1 mg/L.

In Table 4, we summarize CRP levels in 4 or 8 groups of patients and identified significantly higher CRP levels (mean ± SD = 10.8 ± 12.1) in 34 patients with pain scores ≥ 4 at both pre- and post-RT. We have also identified 20 patients with pain score ≥ 4 at pre-RT but < 4 at post-RT. In stratified analysis by obesity, we identified 11 non-obese patients with high pre-RT CRP also had pain scores ≥ 4 at both pre- and post-RT. Therefore, we limited subsequent data analysis of RT-related pain to only two groups of patients with pre-RT pain score <  4 and post-RT score either < 4 (no) or ≥ 4 (yes).

In Table 5, we evaluated the association of elevated pre-RT CRP (≥ 10 mg/L) and/or obesity with RT-related pain. In multivariable model, there was a significant association between high pre-RT CRP and RT-related pain (OR = 2.44, 95% CI = 1.02, 5.85) regardless of obesity status. In obese patients, there was a stronger association between high pre-RT CRP and RT-related pain (OR = 3.71, 95% CI = 1.05, 13.09) than in non-obese patients (OR = 1.36, 95% CI = 0.35, 5.39). Therefore, we conducted a combined analysis to show that patients with BMI ≥ 30 kg/m² and pre-RT CRP ≥ 10 mg/L had 4.73-fold elevated risk for RT-related pain (95% CI = 1.41, 15.81) compared to patients with BMI <  30 kg/m² and pre-RT CRP < 10 mg/L. All models were adjusted for age and race/ethnicity.

We also present ROC curves of high pre-RT CRP and/or obesity in predicting RT-related pain for (A) all, (B) NHW, (C) HW, and (D) AA patients and their corresponding area under the curve (AUC). The gray line represents the theoretical performance of the variable equivalent to a coin toss. The blue line is for obesity (BMI ≥ 30 kg/m²), the red line is for pre-RT CRP ≥ 10 mg/L, and the green line shows the combined effect of obesity and pre-RT CRP ≥ 10 mg/L. The results show some improvements of AUC in the combined BMI and pre-RT CRP model for NHW (AUC = 0.6540) and AA (AUC = 0.6524) patients (see Additional file 1: Figure S1).