Observation of tumor morphological changes in lung irradiation with orthogonal ray imaging: RTmonitoring - A simulation study

Orthogonal ray imaging is a new potential technique that consists in detecting radiation dispersed in the patient and emitted perpendicularly to the beam axis. RTmonitoring aims at real time radiotherapy (RT) monitoring. OrthoCT addresses low-dose patient morphologic imaging, mainly on-board for assisting RT (not studied here). We report Geant4 simulations analyzing the capability of RTmonitoring to detect (1) a lung tumor deviation of 9.36mm, (2) a tumor shrinkage of 9.36mm, and (3) a tumor growth of 30mm. The original spherical tumor has the density of water and a diameter of ca 30mm. In RT treatment planning security margins are added around the tumor to be irradiated in order to account for these and other RT disturbing mechanisms. Here, despite not addressing a real treatment plan, we simulate the aforementioned tumor morphological changes to investigate whether an RTmonitoring system can be useful in detecting such changes. Indeed, tumor dislocation/modification in the lung represents a topic of strong research and development worldwide for all forms of RT. A beam missing the dislocated tumor will represent an underdosage that may be strongly correlated with tumor relapse. On the other hand, the detection of tumor shrinkage in lung RT may allow for a reduction of the dose field, which represents an important relief of the dose burden to the surrounding healthy lung. It is known that healthy lung tissue subjected to RT doses result at times in fibrosis which impairs respiratory function. Finally, detection of tumor growth represents equally an important asset in RT due to the evident tumor underdosage if such scenario is missed. We show that the counts distributions obtained with RTmonitoring result in a very high visual correlation both with the simulated, original dose distribution, and with the tumor location. Therefore, this technique is likely to represent a high potential asset for image-guided RT, adaptive RT, and real-time RT dose verification.