An important amount of cytotoxic drug may accumulate in the workplace following the breakage of a vial containing an anticancer drug. Thanks to the monthly monitoring of the surface contamination in our compounding unit, a strong increase of cyclophosphamide contamination was highlighted in the storage area following the breakage of the vial, despite application of the emergency procedure. This study presents an analysis of chemical decontamination in the context of massive contamination. Samples were taken on the floor and on the caster of a storage shelf where the vial broke. The residual contamination was measured with a liquid chromatography–mass spectrometry/mass spectrometry method. An admixture of 10−2 M sodium dodecyl sulfate and 70% isopropanol (SDS/IPA 8:2) was selected as the decontamination solution. High amounts of cyclophosphamide were retrieved. The initial contamination on the floor was over 20 ng/cm2. Three decontaminations with SDS/IPA were carried out at Day 61, Day 68, and Day 71. The amount of cyclophosphamide decreased to 0.45 ng/cm2 at D134. However, high values were still measured on the caster despite successive decontaminations, with a maximal value of 19.78 ng/cm2 observed at Day 106. Continuous monitoring in our unit led us to highlight the inefficiency of our emergency procedure to eliminate high cyclophosphamide contamination. The procedure involving the SDS/IPA admixture was more efficient on the floor compared to the caster, which is a different surface type and porosity. This work highlights the importance of improving the procedures of incident management using contamination monitoring and repeated decontamination procedures adapted to different contaminants and surfaces.