Accumulating evidence has suggested that photodynamic diagnosis (PDD) using new optical imaging agents (5-aminolevlinic acid [5-ALA] or hexaminolevulinate [Hexvix]) has an advantage detecting the flat tumor of the bladder, especially carcinoma in situ, or tiny papillary tumors, which might be overlooked by conventional white-light cystoscopy.1 Recent randomized controlled trials have shown that PDD-assisted transurethral resection of bladder tumors (TURBT) yields better clinical outcomes in terms of progression-free survival than conventional TURBT.2 Most readers might have simple questions about PDD. How does is the red fluorescence imaging generated from cancer tissue? How is the optical imaging agent administered, and accumulated specifically, if true, in the tumor? Are there any adverse effects (AE) including photosensitivity? Intravesically or orally administered 5-ALA is uptaken to cells by active transport and incorporated into mitochondria. As ferrochelatase, a key enzyme of final process of heme synthesis, is inherently absent in tumor cells, an intermediary metabolite (protoporphirin IX [PPIX]) is specifically accumulated in tumor cells. Dysfunction of the transporter gene, which acts as a efflux pump of PPIX in tumor cells, might accelerate the specific accumulation of the agent.3 Regarding AE of 5-ALA, no severe photosensitivity, which is a common and severe AE of the conventional optical imaging photosensitizer, have been reported. As 5-ALA is not approved by Japan, a multi-institutional prospective phase I/II study is in progress as the investigator initiated clinical trial.
One of the drawbacks of PDD, however, is a relatively higher false positive rate. Although red fluorescence is theoretically generated only from tumor cells, false positive fluorescence is often encountered in the clinical setting, leading to lowering specificity to 50–60% in the literature. Why does false red fluorescence take place? One of the possible reasons is an optical artifact as a result of tangential observation. Artifact is often observed at the bladder neck and prostatic urethra. Other possibilities have been reported to result from cystitis, squamous metaplasia, mild to moderate dysplasia, or hyperplasia. Can all false positivity be ignored as a non-cancerous phenomenon?
It is well known that urothelial tumor is chronologically or spatially multifocal as a result of harboring either a field defect, or dissemination of the clonally-selected cancer cell. Non-random gene alterations have been reported to occur frequently at chromosomes 7, 9 and 17, and those alterations strongly affect patient outcome in bladder cancer. We hypothesized that false positive bladder mucosa might harbor premalignant lesion (Fig. 1).4 Lengauer et al. postulated the concept of chromosomal instability, which is evaluated by a higher aberrant fraction of the copy number, a non-modal fraction of the copy number distribution in each chromosome.5 The aberrant fraction of chromosome 9 by fluorescence in situ hybridization analysis was significantly higher in false positive fluorescent mucosa than that in the non-fluorescent counterpart. Array comparative genomic hybridization using established bladder cancer cell lines showed 9p24.1 to be a commonly deleted region for fluorescent positive cell lines.4 Hartmann et al. reported that the deletion of p53 was highly associated with chromosome 9 alterations in the flat urothelial lesions detected by fluorescent cystoscopy.6 Other investigators reported a close association between chromosome 9 alterations and fluorescent mucosa. A phase III randomized controlled trial proved that PDD-guided transurethral resection, with the resection of all visible fluorescent mucosae, yielded a statistically higher progression-free survival than did conventional transurethral resection.2 The study suggests that an incomplete resection, or ignoring the fluorescent mucosa with PDD except for optical artifacts, might thus leave a potential source for bladder tumor recurrence.
Could basic research shed light on false positivity of PDD? Yes, such false positive mucosa by PDD might be a potential reason for exploring the mechanism of oncogenesis in urothelial cancer.