Impact of technological advances in treatment planning, image guidance, and treatment delivery on target margin design for prostate cancer radiotherapy: an updated review

Abstract

Recent innovations in image guidance, treatment delivery, and adaptive radiotherapy (RT) have created a new paradigm for planning target volume (PTV) margin design for patients with prostate cancer. We performed a review of the recent literature on PTV margin selection and design for intact prostate RT, excluding post-operative RT, brachytherapy, and proton therapy. Our review describes the increased focus on prostate and seminal vesicles as heterogenous deforming structures with further emergence of intra-prostatic GTV boost and concurrent pelvic lymph node treatment. To capture recent innovations, we highlight the evolution in cone beam CT guidance, and increasing use of MRI for improved target delineation and image registration and supporting online adaptive RT. Moreover, we summarize new and evolving image-guidance treatment platforms as well as recent reports of novel immobilization strategies and motion tracking. Our report also captures recent implementations of artificial intelligence to support image guidance and adaptive RT. To characterize the clinical impact of PTV margin changes via model-based risk estimates and clinical trials, we highlight recent high impact reports. Our report focusses on topics in the context of PTV margins but also showcase studies attempting to move beyond the PTV margin recipes with robust optimization and probabilistic planning approaches. Although guidelines exist for target margins conventional using CT-based image guidance, further validation is required to understand the optimal margins for online adaptation either alone or combined with real-time motion compensation to minimize systematic and random uncertainties in the treatment of patients with prostate cancer.

Keywords: cone beam computed tomography; image guided radiation therapy; magnetic resonance imaging; planning target volume; prostate cancer; radiation therapy.

Figure 1.

Summary of key topics impacting PTV margins for localized prostate cancer treatment. Abbreviations: GTV = gross tumour volume, CBCT = cone beam CT, TCP = tumour control probability, NTCP = normal tissue complication probability, PTV = planning target volume.

Figure 2.

Representative patient (A) CT image and contours with the intra-prostatic GTV as the innermost contour, prostate CTV contour outlining the prostate, and 5 mm PTV in around the prostate CTV along with the corresponding CBCT image used for IGRT on a standard linear accelerator (B) and T2-weighted MR image collected on 1.5 T MR-linac (C) ∼1 h prior to the CBCT.

Figure 3.

Contrasting the target contouring and inter-fraction shape variation for both conventional and adaptive RT with the target (simulated prostate) in a transparent oval and outline-only oval representative of the target contour position on the reference and daily imaging. In conventional planning the reference plan contouring errors are systematic across all treatment fractions, whereas theoretically with online adaptive contouring uncertainties at each individual fraction represent a random error in the overall treatment delivery. Inter-fraction target differences, including rotations and target shape changes (a simple expansion in this example) cannot be accounted for with conventional RT, but with adaptive RT is able to properly account for these changes.

Figure 4.

Simplified illustration summarizing the factors and technology typically deployed to support different magnitudes of margin reduction.