Minimally-invasive alternatives for prostate cancer treatment are an unmet clinical need. We are currently conducting clinical trials using interstitial photothermal therapy (PTT) in focal (intermediate-risk) prostate cancer, targeting the largest (index) lesion, utilizing near-infrared (NIR) laser light that is delivered via one or more laser fibers placed interstitially to cover the target volume. This procedure is done using an interventional MRI suite where real-time MR thermometry is used to monitor treatment progression as a surrogate for tissue coagulation. We investigate here if photoacoustic imaging (PAI) could be used instead of MR thermometry to provide direct and higher specificity/sensitivity monitoring of the coagulation-front, particularly for the purpose of avoiding rectal wall damage. For this, we developed an in vivo canine PTT model and experiments were performed in 6 beagles with intact normal prostates, using similar approaches to those used in ongoing clinical trials. PTT also performed in vivo as well as in ex vivo porcine muscle. Initial results demonstrated the feasibility of both the PTT technique as well as an optimized monitoring platform. In ex vivo porcine muscle PAI demonstrated correlation with temperature (R2 = 0.66) that provided the impetus to move in vivo. However, due to noise and the relatively small changes in the PAI signal with coagulation this did not provide as much imaging depth or resolution as MR thermometry, the current gold standard. With newer PAI probes and deployment of the PAI light sources, it may be possible to increase the sensitivity of PAI for in vivo treatment monitoring. This work was supported by the Terry Fox Research Institute (Grant #1075).