ESTRO 2024 - Abstract Book

S4040

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

Purpose/Objective:

Changing anatomy of locally advanced prostate cancer patients receiving radiation therapy for prostate and involved lymph node regions presents challenges in accurate radiation delivery. Daily variations in bladder and rectal filling induce differential changes in position and orientation of the prostate and involved lymph nodes, requiring generous margins even when using daily IGRT. Online adaptive radiotherapy (OART) can address these anatomical changes and ensure accurate target coverage while using small margins. However, the OART process of recontouring targets and OARS is time consuming and challenging with current CBCT image quality. This study aimed to leverage standard image guided radiation therapy (IGRT) methods, using dual rigid registration to develop a swift online adaptive workflow, thereby reducing the time and resources needed at the treatment machine.

Material/Methods:

To eliminate the need for a radiation oncologist’s (RO) constant presence at the treatment machine, our novel method relies on IGRT driven recontouring. The approach capitalizes on the observation that both prostate [1, 2] and lymph node regions [2] exhibit locally near rigid daily variations. The initial step entails rigid registrations of prostate and bony anatomy (translations and rotations) from which a Deformation Vector Field (DVF) with two distinct locally rigid regions is created. The first region encompasses the prostate plus a margin. The second region includes the remaining part of the patient up to the external of the patient, minus a margin, keeping the external of the patient invariant from the planning CT. Between the rigidly defined regions, intermediate areas are interpolated towards each other to ensure a smooth transition between the regions. With the locally rigid DVF, a synthetic CT (sCT) is generated along with associated structures from the original planning data. As the prostate rotation is correlated and determined by the bladder and rectal filling, this sCT effectively corrects for daily variations in position/shape of the prostate CTV, and parts of rectum and bladder adjacent to the prostate, while only necessitating prostate and bony anatomy registrations, thereby significantly streamlining the process. The algorithm was geometrically validated by registering the bony anatomy and prostate in sCTs to their corresponding CBCTs, which should yield minimal translational and rotational errors. The Hounsfield units (HU) were validated by comparing the HUs between the original planning CT and the sCTs within the two rigid regions of interest. Finally, the rigidity of the transformation of the prostate CTV was tested by validating the invariance of the volume of the prostate CTV. The remaining part of the OART workflow was built around standard Elekta equipment and software. CBCT acquisition and registration are performed in XVI (5.0.6) and the adaptive plans are created in Monaco TPS (6.1.1), by creating a reference plan before treatment, and using this as a template for the adaptive plan. Most of the steps needed for this were scripted. After export, the adapted plans are promoted into Mosaiq (2.8.3) by overwriting the original beams of the reference plan. To validate the process, the adapted plan is imported in XVI as well, after which a second CBCT is acquired and registered. After a secondary couch shift, the plan is delivered. A trial was designed to monitor the feasibility of this OART approach.

Results:

One patient has been included in the trial thus far. Median treatment time was 40 minutes. Patient setup, localization, image registration and creating the new sCT and structures took 10 minutes, importing into Monaco, re-optimizing, and plan export 16 minutes, import in Mosaiq and XVI, and acquiring the validation scan 10 minutes, and delivering the plan 4 minutes.