  Dose painting has been developed to modulate the required dose in the target
area without increasing the toxicity in healthy areas. Apart from determining
the accurate location and size of tumors, quantitative functional imaging can
be used to implement the dose painting. Functional imaging, such as
multi-parameter MRI and PET CT, allows us to achieve biological dose escalation
by increasing the dose in certain areas or voxels that are therapy-resistant in
the gross tumor volume while reducing the dose in the less aggressive area or
voxels. Functional imaging can serve as an indicator of therapeutic
intervention in radiotherapy due to microscopic tissue properties. With such
biological indicators, the personalized radiation dose can be tailored to a
specific contour or a voxel using dose painting. In this review, we firstly
discuss several quantitative functional imaging techniques including PET-CT and
multi-parameter MRI. Furthermore, theoretical and experimental comparisons for
dose painting by contours (DPBC) and dose painting by numbers (DPBN), along
with outcome analysis after dose painting are provided. Finally, we conclude
major challenges and future directions in this field through which we hope to
inspire exciting developments and fruitful research avenues.
