The mechanism of 5-ALA selectively accumulated in tumour tissues is not very clear, but some researchers believe the blood-brain barrier breakdown is a key factor . Now 5-ALA can be used as a mature tool for the surgery, but it still has some limitation. Some literatures reported that the absence of fluorescence can't indicate the absence of tumour, and conversely, some fluorescent tissue was detected to be non-tumour at all after histological assessment of corresponding tissue biopsies . In 2016, a prospective phase II clinical trial was raised to assess the correlation of 5-ALA intraoperative fluorescence intensity and degree of histologic cellularity during resection of high-grade gliomas. From the results of 211 biopsies with 59 patients, only fluorescence intensity 3 (robust brightness-lavalike orange, evaluated by surgeons) correlated 97.4% with cellularity Grade 3 (highly cellular with abundant tumor cells, evaluated by pathologists), but the absence of 5-ALA fluorescence correlated with the absence of tumor only 37.7%, and 35.4% non-tumour biopsies demonstrated 5-ALA fluorescence. But the authors believed the absence of macroscopic fluorescence doesn't definitely indicate the absence of 5-ALA or porphyrins. One possibility they inferred of non-fluorescent tumour tissues was most likely to the inability of microscope, which failed to capture the adequate fluorescence. And non-tumour presenting fluorescence might be due to the inflammation or other reactive changes which may exhibit fluorescence as well .