The postoperative, oxaliplatin-based adjuvant chemotherapy for stage III CC patients demonstrated an improvement in patient outcome and has become the standard of care. In clinical trials, FOLFOX/CAPOX in adjuvant setting has been shown to cause a statistically significant improvement in DFS and OS over fluorouracil-based chemotherapy (4-7% and 2-6%, respectively) [1-6]. Further, in the MOSAIC and NO16968 study an improvement in OS was detected after a longer, approximately seven-year, follow-up [2, 6].
Unfortunately, adverse events caused by oxaliplatin often lead to premature termination of therapy and thus a reduction in the number of cycles or dose, and consequently the CD and RDI [2-6]. In the MOSAIC, NASBP-C07 and NO16968 trials, approximately 30% of patients receiving oxaliplatin did not complete the planned treatment due to adverse events, mainly burdensome OXIPN [1-3]. Similarly, in our study about twenty percent of patients discontinued treatment, mainly due to OXIPN which occurred in various grades in 212 patients (58.1%). Therefore, the number of adjuvant oxaliplatin-based chemotherapy cycles that may be enough in this group of patients remains an open question. Tsai et al. have shown that at least eight FOLFOX cycles are needed to have OS benefit, and seven to ensure DFS [19]. Moreover, in the International Duration Evaluation of Adjuvant Therapy (IDEA) project in stage III CC patients, the non-inferiority of FOLFOX/CAPOX regimens used for 3 vs. 6 months was not demonstrated [20-22]. However, in the lower risk group (pT1-3/N1), the 3-month (4 cycles) CAPOX was as effective as the 6-month (8 cycles) treatment and the 3-year rate of DFS was 74.6% and 75.5%, respectively [20-22]. Importantly, the shorter treatment was associated with a lower risk of adverse events, including OXIPN.
To the best of our knowledge, the impact of RDI-O in adjuvant therapy among stage III colon cancer patients on early recurrence has not been systematically addressed, either in a prospective or a retrospective fashion. The correlation between the RDI of systemic therapy and clinical outcomes has been demonstrated mainly for diffuse lymphoma [23-25] and metastatic solid tumours that are relatively sensitive to anti-tumour drugs [26-28]. In stage III CC, a retrospective study of 367 patients treated with fluoropyrimidine-based chemotherapy mainly without oxaliplatin between 2003 and 2008 at 19 VA medical centers in the USA showed an RDI of chemotherapy above 70%, improving 5-year OS [29]. It should be noted that in this study RDI was calculated for each drug within each regimen: i.e. 5-fluorouracil, capecitabine and leucovorin [29]. One study directly evaluated the prognostic impact of oxaliplatin dose reduction in the adjuvant setting, but in stage II and III colorectal cancer [30]. In this study of South Korean patients it was observed that more than 60% of standard dose of oxaliplatin should be administered to achieve no difference in 5-year DFS and OS [30].
In our study of homogenous Caucasian stage III CC patients treated mainly with FOLFOX-4 (N = 336, 92%), no relationship was found between numbers of cycles: ≤6 vs. > 6 cycles and DFS or OS. However, it should be observed that the majority of patients received more than 6 cycles of chemotherapy (N = 320, 88%). Due to the oxaliplatin dose reduction in subsequent chemotherapy cycles, the median CD-O was 936.86 mg/m2, and in the group of patients who completed 12 cycles, 1012.33 mg/m2. However, the CD-O in our group treated in clinical practice was higher compared to the doses in the MOSAIC and NASBP-C07 studies (respectively 810 and 677 mg/m2) but the clinical outcomes were slightly worse, with a 3-year DFS 61.64% and median DFS 43,86 months [1-3]. The results in patients participating in clinical trials are generally better than in patients treated in everyday practice for a variety of reasons. However in our study, half of patients met the criteria of low-risk of relapse (189, 51.8%) but only 165 patients (45.2%) in the whole cohort had at least twelve lymph nodes removed during surgery. The small number of examined nodes may be “under-staged” and affect the prognosis. However, some reports suggest that the total number of lymph nodes analyzed in stage III CC is not a prognostic indicator of cancer-specific and DFS [31, 32].
Among CC patients, 80% of recurrences become apparent within the first 3 years and an additional 15% between the 3rd and 5th year of curative surgery [8-10, 33, 34]. Previous studies showed that stage III CC patients were more prevalent in the early-recurrence group than in the late-recurrence group, and had worse clinical outcomes [8, 10, 11, 35]. In our study, the factors associated with early recurrence (within 18 months) were tumour grade, the number of positive and harvested lymph nodes, and RDI-O <60%. Interestingly, the risk of early recurrence in patients with RDI-O <60% concerned the low-risk group in particular. These results should be referred to the aforementioned IDEA project and CAPOX efficacy in low-risk group [20-22]. It seems that RDI-O may be a more accurate reflection of the true patient-relevant benefit of adjuvant chemotherapy among stage III CC. However, patients with early recurrence showed worse overall survival regardless of the RDI-O.
We are aware that the retrospective nature of the study may have influenced our findings. Moreover, in our study we focused only on RDI-O not each drug within regimen and the risk of early recurrence. Further, we did not collect data on molecular abnormalities in primary tumour, e.g. the presence of activating mutations in the KRAS, NRAS and BRAF, or microsatellite instability and defective DNA mismatch repair (dMMR). However, previous research suggests that dMMR seems to be important only among stage II patients being considered for single-agent, fluoropyrimidine-based therapy [36]. Further research should focus on better biomarkers to assess the likelihood of chemotherapy response, based on molecular biology and pharmacokinetic analyses to reduce toxicity and improve treatment outcomes.