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Discuss SGRT for stereotactic body radiation therapy, including breath hold techniques.
By jason
Could anyone share some experience in using SGRT with SBRT delivery? Does anything have any "good-catch" or "near-miss" examples in which the addition of surface monitoring was of assistance? To me, it makes sense for continuous monitoring as the time from CBCT to the end of SBRT delivery can be long. I would appreciate anyone's experience and input in implementing SGRT with SBRT.
By serpil_wilson_ms_dabr
Hi Jason,

We do SGRT-SBRT with abdominal compression-with respiratory plate and inflatable cuff on a Varian clinac. Most of our plans are Eclipse-VMAT 3 half Arcs and 2 of them with table kicks.  We use Align RT for setup and for patient monitoring before and during treatment.

We had a few cases where ART (Align RT) helped us greatly for SBRT patient setup and localization of very large patients, and for patients with shoulder issues where they had to be scanned with arms down. It helped us monitoring the SBRT patient during MD checking the CBCT for tumor localization as much as during the treatment.


By michael_tallhamer_msc_dabr

We have Vision RT's AlignRT and GateCT at 4 of our hospitals and use the AlignRT product as our SGRT solution for SBRT and SRS.

We use a number of techniques for SBRT including DIBH for lung and liver in addition to the more common 4DCT GTV-ITV-PTV expansion using either all 10 phases or the MIP/MinIP/AveIP composite scans. We find AlignRT excellent for the previous and GateCT works well for acquiring the latter on our Philips CTs. The nice thing is we do lots of DIBH Lt Breast so the DIBH workflow is well understood in our clinics with AlignRT so transitioning it to DIBH for SBRT required only minor tweaking to the process with the operation of the software being identical.

SGRT is great for monitoring during CBCT and physician alignment as Serpil mentioned above. We have differing skill sets with some of our doctors and the ability to monitor the patient while the process takes place has been great.

Some things to think about going to DIBH for SBRT of lung an liver

Many times the target is off axis enough to require couch centering (on a Varian linac) so care must be taken in getting a good reference capture at the CBCT position with the DIBH for monitoring during imaging.
The workflow from couch center monitoring back to original position monitoring to applying the shifts must be done carefully as not to reference out a misalignment.

Some "good-catch" or "near-miss" scenarios we have seen are below

DIBH Liver SBRT - Patient was setup with AlignRT, positioned for CBCT, monitored during and post imaging and then shifts applied. After first arc was delivered pt was instructed to take in a deep breath and hold it in preparation for second arc when AlignRT indicated that the patient had moved suddenly in the vertical and lateral directions while holding his breath (both directions >1.0cm on a 1.5cm target). Physics went into the room to find two of the three body mask pins on the patients left side (away from linac camera) had sheared off when the patient had taken his deep breath leaving the body mask still visually secured on the patient but without the ability to maintain the mobilization. Pins were replaced and patient re-setup for final 2 arcs without issue. Had we not had Align RT we could have potentially missed the issue until entering the room to get the patient up post treatment since the mask failure was not visually noticeable from the linac cameras.
Failing VacLoc bag during SBRT delivery. Patient in vacloc bag and body mask for lung SBRT. Patient exhibits strange vertical offset under body mask post CBCT shifts being applied and new reference being taken. Therapists enter room to find bag has taken on air due to small rip in bag caused by another immobilization device. Offset was <1.0cm but outside of 0.3cm tolerance.

Hope that helps

By adam_shulman
Hello Mike,

Thank you for this. I have some experience in breast DIBH, but none yet with lung SBRT. Would you please elaborate on the process for CBCT couch shift if the ISO is too far off axis? My thoughts (without any experience) to ensure we don’t VRT any errors back to zero is as follows: set up to FB as close to all zeros as possible. Then set up to BH all green (minor shifts allowed, too large of shifts require re-set up). In order to shift to CBCT ISO and not lose valuable positioning data, would we BH to all green then shift to CBCT ISO while still in that BH and acquire a new VRT? Monitor new green on BH VRT at CBCT ISO during imaging and image analysis. Then repeat the process to go back to treatment ISO: BH VRT to all green at CBCT ISO then shift to treatment ISO with applied shifts from CBCT marching while still in BH and record a new VRT. Then this newest VRT is used for treatment. Does this sound reasonable for necessary shifter CBCT ISO?

Would it make more sense to plan the king sbrt with an isocenter not inside the GTV so that no shift is necessary with CBCT? That way the alignrt wirkflow is less prone to missed and hidden errors with VRT!

last question: lung sbrt patient with hands over head. Patient DIBH is predominately from stomach (chest doesn’t move much—this if your tracking region is on the chest wall and ribs, BH is not much different from FB. What region of interest in alignrt would you use for a patient like this, ribs + belly

thanks for your help! If you know of any good papers or videos on starting an sbrt program with alignrt, I would love to know! I just changed hospitals last week. I previously used only breast but will soon need to use alignrt for other sites with my new position. Thanks!
By parkp1
Hello, Adam or anyone else, have you found any answer to your above questions?   I have similar concerns and questions.

Concern 1: For deep inspiration or expiration breath-holding patient, majority of the patient use belly to control their breathing.  And for generally speaking, for the comfortable breath-holding (not deep inspiration) cases, upper torso/chest above sternum exhibits small motion.   Given that the AlignRT practice guide generally ask user to contour the "stable" area and avoid "belly" area, how do we know if the magnitude number being monitored through the system is accurately reflecting the true tumor position during breath-holding?  For example, if we assume the tumor position moves 1cm between the regular breathing to deep inspiration breathing, but the reference ROI that is mostly above sternum area only moves 3-5mm because the patient is belly breather?

Concern 2: Without Active Breath Control system, how do we ensure the patient is holding the exact same breath depth during CBCT acquisition and during treatment beam delivery?  Imagine a workflow, where you setup patient initially based on FB ROI (or BB), then you ask patient to hold breath while acquiring CBCT.  Now based on the acquired breath hold CBCT, you calculate the required shift by comparing the CBCT to planning breathold CT.  After you move the couch, you now have to acquire another reference surface image due to large shift in couch.  At this point, how do we ensure, the new reference surface is going to be acquired at the right depth of breath hold as was done for the breath hold CBCT that was used to allow couch shift?

I would greatly appreciate if anyone can share some information/documentation/research papers. Thank you!

Peter Park

By daniel_vetterli
Hi Peter

I try to comment your concern 2. We take two CTs one in free breeze (FB) and one in breath hold (BH). The FB CT is taken with a low dose protocol, as we only need to extract the body structure for FB initial setup using AlignRT. Our ROI is on the breast. Once this is done, we switch to the BH body contour. And yes it is true, depending on patient the difference between FB and BH can be small. To confirm BH position we don't use CBCT but an AP or PA kV-image and an MV-image in the direction of one treatment field. Before the imaging we let the patient hold the breath and check with AlignRT, mainly vrt. With the AP/PA image we correct only lat and rtn (based on lung/rip interface). With the MV-image from beam direction we correct vrt and lng. Immediately before applying the shift, we let the patient again hold ist breath and check with AlignRT. If vrt is ok, we apply the shift (still under BH) and take immediatly after the shift an new reference surface (still under BH). Like this we can be pretty sure that the new reference surface was captured with the same BH Level as during CT. Before applying the treat field, we check in BH (and new reference surface) with an MV-"before" image if the thorax wall matches the DRR structure. Then we Beam On and take an MV-movie. So you can check offline the level of BH. Most of the time this works quite well.
I guess similar/adapted procedures can be applied to other regions.

Best regards
By marko_laaksomaa Finland flag
Hi Peter

Concern 1: I think we need to do intrafractional imaging in addition to AlignRT to be sure that the target is in its correct location. Can we in addition to AlignRT use for example triggered imaging to see the markers in the liver and online/offline control/evaluate the marker position (The Use of Triggered Imaging for Intrafraction Target Verification in Liver SBRT Breathhold, Walb 2019)? Internal diaphragm is reported to correlate better with liver than the bony structures in the CBCT (Availability of applying diaphragm matching with the breath-holding technique in stereotactic body radiation therapy for liver tumors Daisuke Kawahara et al, 2016).  Which one is better for external ROI with AlignRT: bony chest wall or diaphragm to correlate with the liver movement/location during the treatment? VisionRT suggests in reference guide to include bony chest wall for liver SBRT, as you mentioned. I think we need more data and papers of correlations of the ROIs to the liver/markers before we can say which ROI is better and can we overall rely on AlignRT alone during the beam delivery, if we are treating the internal moving target.

Concern 2: I share your concern. There is always a risk of posture changes concerning the BH and chest wall posture, once the patient travels several centimeters in BH from CBCT_ISO to treatment_ISO and at the end of that trip we acquire VRT surface for treatment. At least patients needs to be well trained what will happen. I agree with Daniel that succeeding of the surface used for the treatment and additionally in this case succeeding of the couch shift action after CBCT and thereby target accuracy should be verified somehow before treatment. 1) kV/kV images? for approximate verification: 2) use separate AlignRT fields for setup and treatment. Ask patient to BH to treatment VRT (just acquired based on CBCT), pause, change field and monitor setup BH surface for a while to see are the RTDs that much out of the zero values as you shifted the couch based on CBCT match and let patient breathe again. Go back to treatment VRT. 3) Once you monitor patient in FB at BH field, deltas should be different from BH all the time as much IF the patient can relax her/his ROI area back to original setup position after all the BHs (which they do not exactly usually do) but these 2 and 3 are only for approximate evaluation that the BH amplitude remains the same all the time.

Hope there was something useful

Regards Marko
By marko_laaksomaa Finland flag
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.
By marko_laaksomaa Finland flag

Some time ago here was a discussion on the forum about diaphragm level displacement in DIBH. I think these results fits well under this topic also. After that question I did IGRT study, where I compared interfractional displacement 1) between the vertebra th6 and the sternum in VRT and LNG and displacement 2) between vertebra th 6 and the diaphragm level in LNG, all evaluated from the LAT kV-images. Patients (n=25) were treated with RPM quidance, marker box placed on the sternum with window level +-2mm. Displacement between vertebra and sternum (BHL) in the group were (Σ, 0.22 cm, σ 0.16 cm in VRT and Σ 0.37 cm, σ 0.26 cm in LNG) demonstrating typical BHL values in DIBH studies with RPM. Displacement between vertebra and diaphragm in that group in LNG was (Σ 0.64cm, σ 0.42 cm). Errors between vertebra vs. diaphragm were typically much larger than the errors between vertebra vs. sternum in LNG. There were even 2 cm residual displacement error in the diaphragm level, even though the BHL (vertebra vs. sternum VRT and LNG) was ok. This means that what happens on the bony chest wall in DIBH does not necessarily correlate with the internal anatomy interfractionally. The theoretical reasons are related to diaphragms anatomical day to day variations and differences in the breath holds which are not necessarily visible in the bony structures. This knowledge has not much to do with the variation of the diaphragm compared to sternum/chest wall between the BHs intrafractionally, since this was not investigated.

Some words about breast DIBHs concerning belly breathing. With breast DIBHs we use RPM at the CT, since there is five other linacs in the hospital where AlignRT is not installed and with those all DIBHs are still treated. At the CT, RPM box is placed on the rigid sternum, since it is investigated and published that such box location leads to improved accuracy in the realized BHL evaluated from LAT images (vertebra vs. sternum). Sometimes box needs to be placed more caudally on top of the diaphragm, because 1) movement of the box is that small on the rigid area or 2) external diaphragm hides the visibility of the marker. Still, on average only 2% of all patients are such that the chest wall movement is less than 5 mm in vertical direction when they come to linac with AlignRT. On average, chest wall ROI movement in vertical direction from FB to DIBH is 1.1cm. On the first fraction with AlignRT, 2% of the patients are after vertebra match in BH DICOM inside chest wall ROI thresholds, although they are breathing free, mostly indicating that they are completely belly breather without significant movement on the chest wall. Then re-planning is needed and once those patients are instructed to take a breath slightly differently at the CT, BHs and treatment has gone fine.

BR Marko
By marko_laaksomaa Finland flag
Feasibility of Optical Surface-Guidance for Position Verification and Monitoring of Stereotactic Body Radiotherapy in Deep-Inspiration Breath-Hold
Naumann et al., Front. Oncol., 25 September 2020.