PET/CT as Tool in Assessing Response to Neoadjuvant CRT for Rectal Cancer
PET/CT as Tool in Assessing Response to Neoadjuvant CRT for Rectal Cancer
Abstract & Commentary
Synopsis: Neoadjuvant chemoradiotherapy has become the standard approach for patients with locally advanced rectal adenocarcinoma. For patients who achieve a complete response determined preoperatively, less than maximally aggressive surgery has been contemplated. However, improved accuracy of the preoperative assessment needs to be established. In the current prospective evaluation, assessments of response were made with and without the use of PET/CT in a consecutive series and it was found that the additional scans were useful adjuncts to comprehensive clinical examination.
Source: Perez RO, et al. Accuracy of positron emission tomography/computed tomography and clinical assessment in the detection of complete rectal tumor regression after neoadjuvant chemoradiation. Long-term results of a prospective trial. Cancer 2012;118:3501-3511.
The current standard approach for locally advanced rectal cancer includes neoadjuvant chemoradiation therapy (CRT) followed by surgical resection. Optimal surgery often includes total mesorectal excision (TME),1 a highly effective approach when considering oncologic outcomes, but one with substantial risk for postoperative urinary, fecal, and sexual dysfunction.2
Neoadjuvant CRT has resulted in significant down-staging for many patients, raising the possibility that alternatives to TME might produce comparable oncologic outcomes with fewer adverse consequences. In fact, in some series, CRT has produced complete pathological responses (pCR) in as many as 40% of patients,3 and such patients are known to have improved local control and overall survival when compared to patients who are found to have residual disease upon surgical resection.4 For those thought to be without residual cancer, less than aggressive or even no surgery might be contemplated. Of course, this would hinge upon a comprehensive assessment performed at a defined interval after completion of CRT, and careful follow-up during the subsequent months and years.
To address the role of PET/CT in determining (clinical) complete response (cCR), Perez and his Brazilian colleagues conducted a prospective trial in which PET/CT scans were utilized in addition to comprehensive physical examination, proctoscopy, CT, and high-resolution MRI, prior to and 6 weeks and 12 weeks after completion of CRT.
Consecutive patients treated at the University of Sao Paulo with clinical T2 (cT2) through cT4NxM0 rectal adenocarcinoma were enrolled. Neoadjuvant therapy consisted of 54 Gy of radiation and 5-fluorouracil-based chemotherapy. Baseline PET/CT studies were obtained before CRT followed by PET/CT studies at 6 weeks and 12 weeks after the completion of CRT. Clinical assessment as described above was performed at 12 weeks after CRT completion. PET/CT results were compared with clinical and pathologic data.
In total, 99 patients were included in the study. Twenty-three patients were complete responders (16 had a complete clinical response, and seven had a complete pathologic response). The PET/CT response evaluation at 12 weeks indicated that 18 patients had a complete response, and 81 patients had an incomplete response. There were five false-negative and 10 false-positive PET/CT results. PET/CT for the detection of residual cancer had 93% sensitivity, 53% specificity, a 73% negative predictive value, an 87% positive predictive value, and 85% accuracy. Clinical assessment alone resulted in an accuracy of 91%. PET/CT information may have detected misdiagnoses made by clinical assessment alone, improving overall accuracy to 96%.
Commentary
There have been several prior studies addressing the role of PET/CT in assessing tumor response to CRT with varying results.5-8 In general, these were retrospective analyses with small numbers, and several examined PET/CT at short intervals (4 or 6 weeks) after CRT completion. The current report was of a larger series examined prospectively in a systematic way and at a sufficient interval (12 weeks) such that PET-detected increased metabolic activity more likely would reflect residual tumor than the inflammatory response to radiation, which would be more likely on earlier studies.
The current report demonstrated that findings from PET/CT alone would be insufficient to base critical decisions, such as which patients would be candidates for less-aggressive surgery or no surgery at all. The comprehensive clinical evaluation alone also was less than perfect in identifying which patients would turn out to have a pCR. However, by adding the data from the 12-week PET/CT and the comprehensive clinical examination, accuracy was shown to be significantly improved. Patients with a negative PET/CT at that time who were independently felt to be in cCR had a 96% chance of being shown to be free of residual tumor (pCR) upon resection. If this experience can be corroborated, the incorporation of PET/CT findings may indeed provide a useful additional tool with good overall accuracy for the selection of patients who may avoid unnecessary radical resection after achieving a complete clinical response.
One point gleaned from this study worth emphasizing is that PET/CT obtained at 6 weeks post-CRT was remarkably inaccurate in predicting which patients would be found to be true CRs (< 50%).
References
1. Martling A, et al. Br J Surg 2005;92:225-229.
2. Lezoche G, et al. Surg Endosc 2008;22:352-358.
3. Sanghera P, et al. Clin Oncol (R Coll Radiol) 2008;20:176-183.
4. Maas M, et al. Lancet Oncol 2010;11:835-844.
5. Capirci C, et al. Int J Radiat Oncol Biol Phys 2009;74:1461-1469.
6. Konski A, et al. Int J Radiat Oncol Biol Phys 2009;74:55-59.
7. Kristiansen C, et al. Dis Colon Rectum 2008;51:21-25.
8. Martoni AA, et al. Ann Oncol 2011;22:650-656.
Neoadjuvant chemoradiotherapy has become the standard approach for patients with locally advanced rectal adenocarcinoma. For patients who achieve a complete response determined preoperatively, less than maximally aggressive surgery has been contemplated. However, improved accuracy of the preoperative assessment needs to be established. In the current prospective evaluation, assessments of response were made with and without the use of PET/CT in a consecutive series and it was found that the additional scans were useful adjuncts to comprehensive clinical examination.Subscribe Now for Access
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