Abstract
Anal squamous cell carcinoma is an uncommon malignancy primarily treated with radiation therapy and concurrent chemotherapy. The use of pre-treatment PET/CT has become common for target delineation and assessment of disease extent, with traditional PET metrics, such as SUVmax, being prognostic of progression free survival (PFS) and overall survival (OS). This study sought to understand the significance of quantitative PET metrics for pre and post-treatment prognostication of treatment response in anal cancer. Databases of a single institution were retrospectively reviewed for patients treated definitively for anal squamous cell carcinoma between 2005 and 2017. Patients were required to have a pre-treatment PET/CT, and a subset of patient had a post-treatment PET/CT within 2-6 months of completion of treatment. Relevant clinical data was extracted for evaluation of baseline patient characteristics, treatment parameters, freedom from local recurrence (FFLR), and OS. PET data was analyzed for metabolic tumor volume (MTV2.5), defined as the extent of tumor with SUV > 2.5, and total lesion glycolysis (TLG2.5), defined as the MTV multiplied by the SUVmean. Both the absolute and relative change in quantitative PET parameters were assessed for association with FFLR, PFS, and OS using univariate and multivariate Cox regression models. With a median follow up of 25 months, 75 patients had a pre-treatment PET/CT, and 59 patients underwent a 2 to 6-month post-treatment PET/CT. The cohort consisted of 49 males and 26 females, 37 were HIV positive and 38 HIV negative, with a median age of 57.4 years (IQR: 50.4-65.5). The 24-month freedom from LR, PFS, and OS were 79.8% (95% CI: 70.9-89.9), 75.4% (95% CI: 65.8-86.4), and 84.2% (95% CI: 75.5-93.8) respectively. On univariate analysis of pre-treatment PET metrics both MTV2.5 and TLG2.5 were significant for FFLR, and PFS. On multivariate analysis only MTV2.5 remained significant for FFLR and PFS with HR 1.017 (95% CI: 1.002-1.033) and 1.018 (95% CI: 1.004-1.032) respectively. On univariate and multivariate analysis of post-treatment PET/CT, MTV2.5 was the most significant metric for FFLR and PFS with HR 1.115 (95% CI: 1.030-1.208) p=0.007, and HR 1.086 (95% CI: 1.012-1.166) p=0.021 respectively. There was no significant association between the relative changes of the quantitative PET metrics and FFLR or PFS. No significant association between these metrics was established for OS. Quantitative PET metrics for anal cancer can be useful for pre-treatment prognostication and post-treatment assessment of disease response. MTV2.5 demonstrated itself to be the most robust metric assessed in this study. Use of these measurements prior to treatment could allow for personalized planning based on patient risks, and in the post treatment setting could stratified patients to intensified surveillance regimens or adjuvant immunotherapy to improve disease outcomes.
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