## 3 Discussion  
The prevalence of AP continues to increase, despite technical and technological advances in endodontic procedures. It has recently been reported that more than half of the world's adult population presents at least one tooth with AP, making its resolution a critical issue from a public health point of view [28]. aPDT has been presented as a valuable, non-invasive, and promising alternative for the eradication of bacterial infectious diseases [29]. Some articles have used planktonic models to evaluate the antimicrobial activity of aPDT [30]. However, biofilms have been proven to be much more resistant to antimicrobial agents than planktonic bacteria (even up to 1000 times more resistant). Including patients as cases makes it possible to truly evaluate the efficacy of aPDT against complex and mature bacterial organizations. aPDT has been reported to be highly effective against endodontic biofilms. [PERSON] et al. [31] have reported, in their systemic review, that aPDT can reduce the bacterial load by 91.3% to 100%. Another study has demonstrated that aPDT effectively removes _E. faecalis_ biofilm from the RCS before or after reciprocating instrumentation [32]. A recent randomized controlled study has assessed the ability of PUI and Ca(OH)\({}_{2}\) dressings to reduce bacterial counts in 45 single canal anterior teeth with medium-sized periapical lesions, comparing them to aPDT. While aPDT yielded higher and more promising results than Ca(OH)\({}_{2}\) dressings, it did not show a statistically significant advantage over PUI [33]. Furthermore, in vitro studies have not revealed comparative advantages in using aPDT [34; 35]. This could be due to the short pre-irradiation time used and the low production of ROS species. In the cases presented here, the PS used was MB, due to its proven ability to inactivate Gram-negative and -positive bacteria [11], including _E. faecalis_[36]. The literature has reported that, in addition to eliminating bacteria, aPDT can accelerate bone formation processes in the periadicular zone and modulate bone healing [24]. Similarly, one of the main outcomes of PBMT is collaborating with bone remodeling [21]. The results reported here demonstrate that, through jointly employing aPDT and PBMT using wavelengths of 660 and 940 nm, an evident and sustained bone regeneration process was achieved after 3, 6, and 12 months of follow-up. These observations complement the results reported by [PERSON] et al. (2016) [24], who observed bone regeneration and periodontal ligament restructuring at 6 months after applying PBMT and aPDT using 660 nm DLs. On the other hand, [PERSON] et al. (2022) [22] have also reported bone repair at 6 months using dry disinfection with a 940 nm DL and PBMT. The resolution of the lesion was associated with the local application of aPDT, which has shown great potential in significantly reducing the presence of bacteria and effectively disrupting biofilms. However, longer follow-up periods are recommended to obtain more accurate conclusions [37]. While the parameters for pre-irradiation and irradiation times have been extensively discussed in the literature, a consensus has not yet been reached. Typical pre-irradiation times vary between 5 and 15 min [22; 24; 38; 39; 40], while the irradiation time ranges between 30 and 60 s [22; 27; 32; 41; 42]. These observations are consistent with the protocol used in the present study. However, it is important to note some limitations of the use of PS in aPDT. The first limitation is the potential discoloration that the PS can cause in the treated tooth, particularly in anterior teeth [43]. Several reports have indicated that 5 min of pre-irradiation with MB or TBO is enough to generate a color change at day 30 [43] and day 60 [44], compared to the baseline measurement. Some authors have reported that the combination of Endo-PTC cream with 2.5% NaOCl is effective in removing MB or TBO from the RCS [45; 46].