Marko Laaksomaa

A short look at the papers in this area of SBRT lung and liver in (DI)BH with AlignRT shows that there exist rather few number of publications about intrafractional movement. Heinzerling et al. (Use of surface-guided radiation therapy in combination with IGRT for setup and intrafraction motion during stereotactic body radiation therapy treatments of lung and abdomen, 2020) evaluated seventy‐one patients with 85 malignant thoracic or abdominal tumors treated with SBRT. All the patients were treated with abdominal compression in FB. If an intrafractional shift in any direction >2 mm for longer than 2 sec was detected by SGRT, then CBCT was repeated and the recorded deltas were compared to those detected by SGRT. 25 patients had detected intrafraction motion by SGRT during a total of 34 fractions, resulted in 25 (73.5%) additional shifts of at least 2 mm on subsequent CBCT. Continuous monitoring with SGRT during treatment was valuable in detecting potentially clinically meaningful intrafraction motion. On the other hand, there were some shifts that were greater than what was detected by SGRT.

Josipovic et al. (Geometric uncertainties in voluntary deep inspiration breath hold radiotherapy for locally advanced lung cancer, 2016) estimated geometric uncertainties in visually guided voluntary DIBH and derived the appropriate treatment margins for different image-guidance strategies. Respiratory motion was monitored with Real-time Position Management system, marker placed at the xiphoid process. Inter-fractional uncertainty in tumour position, indicating its base-line shift compared to the bony anatomy, was largest in cranio-caudal (CC) direction. As a conclusion, for single targets, visually guided voluntary DIBH radiotherapy is highly reproducible provided an image-guidance strategy with tumour registration is performed. On the other hand, authors have observed large base-line shift in a lesion close to the diaphragm.

In the study of Mueller et al (The first prospective implementation of markerless lung target tracking in an experimental quality assurance procedure on a standard linear accelerator, 2020) they noted that surface guidance techniques (Hoisak and Pawlicki 2018) or diaphragm tracking (Cerviño et al 2009, Hindley et al 2019) do not track the target directly and can suffer under unforeseen motion patterns. This study presents the first prospective implementation of markerless lung target tracking on a standard linac and provides a QA procedure for its safe clinical implementation.

Poulsen et al. (Simulated multileaf collimator tracking for stereotactic liver radiotherapy guided by kilovoltage intrafraction monitoring: Dosimetric gain and target overdose trends, 2019) investigated the potential benefit of multileaf collimator (MLC) tracking guided by kilovoltage intrafraction monitoring (KIM) during stereotactic body radiotherapy (SBRT) in the liver. Patients had 2–3 gold markers implanted close to the metastases. Authors concluded that intrafraction tumor motion can deteriorate the CTV dose of liver SBRT. The planned CTV dose coverage may be restored with KIM-guided MLC tracking.

In the paper on Nankali et al. (Geometric and dosimetric comparison of four intrafraction motion adaptation strategies for stereotactic liver radiotherapy, 2018) compared the alternatives in Calypso-guided liver SBRT, simulated alternatives were MLC tracking, baseline shift adaptation by inter-field couch corrections and no intrafraction motion adaptation. Margin reduction from no motion adaptation to baseline shift adaptation was from 9.4 mm to 4.5 mm. Largest errors were seen in CC direction. Triggered imaging resulted only slightly larger margins than with the continuous Calypso guided baseline shift adaptation. Finally they write, that It should be noted that accurate internal intrafaction motion monitoring also is warranted for breath-hold treatments to ensure optimal treatment accuracy with reproducible tumor location at each breath hold (Zhong et al 2012, Lu et al 2018).

Jaccard et al. (Clinical experience with lung-specific electromagnetic transponders for real-time tumor tracking in lung stereotactic body radiotherapy, 2019) evaluated seven patients, which were implanted with EMTs for SBRT of early stage non-small cell lung cancer and simulation was performed in FB and in DIBH. EMT positions in lungs remained stable during overall treatment and allowed real-time tracking both in FB and in DIBH SBRT. The treatment beam was gated when EMT centroid position exceeded tolerance thresholds ensuring correct delivery of radiation to the tumor.

As a conclusion of this: seems like there is a lack of important studies in the field of lung and liver SBRT showing intrafractional correlation between AlignRT data and the internal target data in both FB and particularly in (DI)BH. Modern systems (examples named in the 4 latest papers above) to reliably control the internal target during the treatment, which certainly is important, are not widely in the daily use with lung and liver SBRT. However, due to those systems reliability to control the target intrafractionally, they would offer an interesting field to investigate AlignRT together with those, at least in the publications to compare the margins.