Dose-free monitoring of radiotherapy treatments with scattered photons: Concept and simulation study

Modern radiotherapy (RT) techniques are providing increasingly higher conformality, a potential invaluable clinical benefit to the patient. Consequently, in both single and multi-fractionated RT, patient misalignments and changing internal anatomy are also becoming more critical since higher conformality may equally represent a higher risk of target underdosage or organ-at-risk overdosage. Even with rigid fixation devices, maximum positioning errors higher than 1 cm are observable. In addition, beam range uncertainties and anatomical morphological variations induced by cardiorespiratory or bowel motion, or RT-related biological responses, have been reported. The latter include tissue swelling, edema, inflammation, tumor shrinkage/growth, or filling of body cavities with unaccounted mucus or edematous tissue. State-of-the-art image-guided radiotherapy (IGRT) aims at providing feedback to the radiation oncologist in regard to these matters, some times at the cost of increased dosage (e.g. kilo and megavoltage IGRT), other times providing insufficient clinical information. We investigate a novel imaging system specially designed for monitoring both conventional and intensity/volumetric modulated photon radiotherapy (IMRT/VMRT, static and dynamic). The proof-of-principle and feasibility of such system indicate its potential for monitoring each field (and segment, for IMRT/VMRT) during all treatment fractions without whatsoever additional dose. We present a clear 2D correlation between the dose delivered in a heterogenous phantom and the number of scattered photons detected perpendicularly to the photon beam. Simulations of a high-energy, multi-hole collimator show real-time, high-detectability of abnormal (though possible) irradiation scenarios with pertinent target morphological alterations, such as tumor dislocation or formation of edematous tissue. Finally, we show unprecedented, dose-correlated profiles first obtained with a clinical linac beam.