Stereotactic Prostate Radiotherapy With Dose Escalation Focused on the "Dominant Intra-prostatic Lesion" (DIPL) Delineated by Multi-parametric MRI and 68Ga-PSMA PET (Prostate-SIB-PSMA)

Overview

In localized intermediate- and high-risk prostate cancers (according to the NCCN classification), external radiotherapy delivering a "high" dose (dose equivalent 78-80Gy EQD2, α/ß=1.5) to the entire prostate volume combined with hormonal treatment, if necessary, has shown its benefit in terms of recurrence-free survival and is considered a standard treatment for this indication. Two fractionation modalities (number of sessions) are considered as therapeutic standards, conventional fractionation (39 to 40 sessions of 2 Gy in 8 weeks) and moderate hypo-fractionation (20 sessions of 3 Gy). More recently, phase II and two phase III studies have shown equivalence in terms of safety and efficacy of "extreme hypofractionation" (5 or 6 sessions) for these localized cancers, using stereotactic-type techniques. In view of the current data, this fractionation is considered a therapeutic standard in some countries (notably the USA) and an option in France. Delivering higher doses, beyond 80 GyEQD2 would improve tumor control, as demonstrated by randomized studies using brachytherapy, but at the cost of an increased risk of urinary toxicity. As an alternative to this combination of external radiotherapy and brachytherapy, an innovative approach of external radiotherapy has been developed to increase the therapeutic ratio of patients with localized prostate cancer, based on an escalation of the radiation dose (\> 95 GyEQD2) focused on the macroscopic tumor or "dominant intra-prostatic lesion" (DIPL), the area most at risk of local recurrence after conventional dose radiotherapy (3). This external radiotherapy technique consists in performing a conventional dose irradiation on the whole prostate, with at the same time (at each session) a higher dose ("Boost") on the DIPL. This is a modality known as "simultaneous integrated boost" (SIB). The feasibility of simultaneous integrated boost (SIB) on the DIPL has been proven in external radiotherapy using conventional fractionation in the phase III FLAME study and the results in terms of long-term tumor control of this study showed a benefit in terms of biological recurrence-free survival. Feasibility in terms of tolerance has also been established for very hypofractionated regimens (5 sessions), in particular in the HypoFLAME study that followed the above-mentioned study . Multiparametric MRI (mpMRI) is used to identify and delineate the "dominant intra-prostatic lesion" (DIPL), and is the most commonly used modality in clinical studies that have evaluated SIB techniques. However, several studies show that PET imaging, particularly 68Ga-PSMA PET, significantly improves the correlation between the image-defined DIPL and histological data and may improve the likelihood of tumor control. A dosimetric simulation study also showed that dose escalation based on 68Ga-PSMA PET could improve local tumor control with an acceptable level of toxicity . Moreover, 68Ga-PSMA PET could be used to select the patients who could benefit most from this dose escalation, by excluding patients with lymph node or distant metastases.