Session Item

How should IGRT practices be revised to reflect delivery of large doses per fraction?

Purpose/Objectives: Purpose of this work is 1) To describe the preparation for the implementation of moderate-hypofractionated treatment regimen for prostate cancer; 2) To outline practice changes after implementation of hypofractionation; 3) To define benefits and limitations and outline difficulties and opportunities of intrafraction imaging.

Materials and methods: This presentation is based on the experience of the North Estonia Medical Centre Radiation Therapy Department on implementation of moderate-hypofractionated regimen (3Gy x 20 fr) for intermediate- and high-risk prostate cancer patients using Varian TrueBeam Auto Beam Hold (ABH) technique also known as triggered imaging. During prostate cancer radiotherapy on Varian TruBeam linear accelerator, 2D X-ray images by Varian OBI system are triggered after discrete intervals (for example after every fixed gantry angle) chosen by the user. This image is then matched with DRR and if within tolerance treatment continues, otherwise treatment is stopped, and action needs to be taken. Major preparation necessities are outlined in this publication. Advantages and disadvantages, benefits and limitations of this procedure are presented.

Results: Intrafractional prostate movement monitoring using ABH requires fiducial markers which are used to track the target in treatment. In case of prostate, 3 gold fiducial markers are implanted before treatment. This requires cooperation with a urologist or radiologist as they perform the implantation. Moving from conventional fractionation (2 Gy x 35 fr) to moderate-hypo (3 Gy x 20 fr) ensures almost 100% of our patients are equipped with fiducials with minor exception for those who are administered with blood-thinners. These patients are still scheduled for conventional fractionation regimen. At the very start of the procedure implementation process the team of physicists performed home-developed QA on the marker detection accuracy. It determined the weakest point of the ABH technique – movement of the prostate in the direction to/from the OBI detector is not identified by the system. In other directions shifts of 1,5 mm and larger are detected correctly. Sample of 5 patients demonstrated around 20% of Beam Off of all intrafractional images taken. This confirms the necessity of prostate motion monitoring. PTV margins were reduced from 7 to 5 mm, which in the case of 38,54 cc prostate plus seminal vesicles results in 20% of PTV reduction (from 114,5 cc to 93,83 cc). VMAT plan containing 2 arcs requires 12 intrafractional kV images taken from every 60 ᵒ of gantry rotation (user can choose frequency of imaging by gantry angle, monitor units or seconds).  It results in additional 3-7,2 mGy per fraction, which are still on-target. The field of view however needs to be collimated properly to reduce risk of normal tissue irradiation. Procedure time is around 7 minutes longer due to intrafractional imaging. But overall treatment time is two times shorter and is better tolerated by patients. Shorter fractionation scheme is also advantageous for department logistics, especially during Covid-19 pandemic. Waiting time for the procedure and the impact of moderate-hypo cannot be assessed specifically because the number of the treatment units in the department was doubled during this time and implantation of fiducials requires 1 extra visit to the hospital at least 1 week prior simulation. The limitation of the procedure that should be mentioned is insufficient reliability of 2D/3D reconstruction, while position of the markers is represented by a single image not a pair of them. Triggered imaging also does not allow monitoring of prostate movements constantly. Thus, we still do not know whether the prostate remains on target in-between images or not.

Conclusion: Larger dose per fraction requires more accurate treatment delivery, properly designed procedure protocol and verification of accuracy. Intrafractional imaging helps to detect shifts during fraction, mitigate uncertainties of intrafractional movements and therefore makes irradiation more precise. Staying ``on target`` allows to reduce margins and decrease treated volume and reduce normal tissue damage. Nevertheless, all changes should be considered with caution and respect for data we are still missing.