Very Early Post-Treatment PET/CT Scan Performed One Week after First Chemotherapy Course Compared Against the Standard PET after Third Course in Histologically Aggressive Lymphoma (NHL).

The FDG-PET/CT scan has become a standard tool in the management of NHL. Failure to achieve an early metabolic CR (mCR) is associated with a poor outcome and this information can be used to modify subsequent management. However, several issues still need to be resolved, including: 1-What is the best time to perform a post-treatment PET scan? Is very early (i.e. 1 week) after the first chemotherapy superior to the traditional 3^rd course PET scan? 2-What criteria can we use to define mCR? Is there an SUV threshold on the very early PET scan that best defines mCR? In order to answer these questions we designed a study in which patients with histologically aggressive NHLs had 3 sequential PET scans: 1^st one at baseline, a 2^nd one on week 1 after the 1^st course of chemotherapy (W_{1 PET}) and the 3^rd one after course 3 of chemotherapy (C_{3 PET}). We have entered 32 patients of which 30 are currently assessable. Histologies were: DLCL=26, FLCL=4. IPI was ≥2 in 69%. At baseline, median SUV_max=17.14 (range 5.6–47.1). In the 30 cases who had a W_{1 PET,} the mean SUV_max dropped to 4.4 (0.7–21.3, corrected for background). In the 24 who have completed their C_{3 PET} the mean SUV_max was 1.55 (0–15.2) and only 6 patients had an SUV >0: 0.9, 3.7, 3.9, 5.1, 8.3, 15.2. We then examined the correlation between the SUV_max of W_{1 PET} with SUV_max of C_{3 PET}. 24 cases have already had both a W_{1 PET} and a C_{3 PET.} Table 1 shows that the W_{1 PET} correlated well with 17 (71%) of the 24 C_{3 PETs.} All 14 who achieved SUV_max <4.0 on W_{1 PET} had an SUV_max <4.0 on C_{3 PET} (all of the latter achieved SUVs <1.0). However, of 10 cases who failed to attain SUVmax <4.0 on W_{1 PET,} 7 went on to achieve SUV_max <4.0 on C_{3 PET,} and 2 of these 7 relapsed. For the purpose of determining the best SUV cut-off that can be used to define mCR, we examined the failure free survival (FFS) at various cut-offs ranging from 0 to 8.0 for both W_{1 PETs} and C_{3 PETs}. The best cut-off for both W_{1 PET} and C_{3 PET} was observed at 4.0 (table 2). We thus defined mCR as a post-treatment SUV_max <4.0. Those who achieved mCR on the W_{1 PET} had a significantly superior FFS (Kaplan-Meier method) than those who didn’t reach mCR (table 2). The results for the C_{3 PET} didn’t reach statistical significance (table 2).

Conclusions: 1- Striking improvements in the PET scan are seen as early as 1 week after the first chemotherapy course. 2-The results of the W_{1 PET} anticipated correctly 71% of C_{3 PET} results. 3- Patients who attained an SUV_max <4.0 on the W_{1 PET} had a remarkably superior outcome than those with SUV _max ≥4.0 and was associated with a 91% FFS. Thus an SUV cut-off of <4.0 at W_{1 PET} can be used to define mCR. This finding should be confirmed in an independent set of patients. 4- The W_{1 PET} is more sensitive and specific than the C_{3 PET} and is preferable to the C_{3 PET} in predicting clinical outcome.