SGRT Forum

Hi all

I have 6 pelvis patients on treatment with AlignRT but I am not happy with its accuracy. there still seems too much residual error on XVI.

Yesterday we had a patient who we have no tattoos for due to skin colour and the error was over 1.cm in two planes!!!

I am using a standard ROI for pelvis and paper towel for modesty and no gown with a good frame rate and stable RTDs.

Two of the other patients have nodal volumes and I am still getting 3+ degrees of pitch on XVI with 0.2 on AlignRT.

Can anyone tell me what I am doing wrong because if it doesn't get better I will stop using AlignRT for pelvis as I am not confident with its results

Vicky

Hi Vicky, If you can attached an image with the ROI used we can possibly held and provide more feedback.

It is worth noting that depending on internal anatomy to external anatomy you may in some cases have a difference in CBCT to XVI. With a difference so large I would put it down to the ROI initially. Also, is the difference when the Gantry is at 0, or are any of the Cameras blocked?

Hi Victoria

I understand your concern.

I think you use ROI which includes anterior portion of hips and lateral portion of hips to the midcoronal plane and not much moving skin of abdomen (to eliminate the errors caused by breathing motion and daily fat displacement), which is demonstrated in the pocketbook called "Reference guide, drawing an isocenter region of interest, VisionRT". With that ROI I have done isocenter comparison between AlignRT- and tattoo setup in pelvic RT (n=15+15). AlignRT improved isocenter accuracy in LAT direction in both systematic and random errors (p=0.02), but not in LNG direction(p=0.98). Vertical direction was based on mean couch value, taken from first three day image matches, leading to small isocenter variation (random error 0.17-0.20cm). I do not have own data of rotations between CBCT and AlignRT. That kind of fresh data could be very useful and interesting if someone acquires daily CBCTs and perform 6D corrections based on automatch in daily practice with the ROI VisionRT suggests.

(Comparison of surface matching and target matching for image‐guided pelvic radiation therapy for both supine and prone patient positions, Zhao et al., 2016.).

Are the errors systematical? If the are, can you use such VRT surface in the next day setup, which includes corrected isocenter errors and corrected residual structure errors based on IGRT. This may need retrospective analysis of the images, so that you see that the VRT surface includes such corrections that have been typical with that patient earlier days. If the named large errors are not systematical, it is a bit risky to use AlignRT for setup, if you cannot correct the errors in 6D. This is possible with the obese patients, but on the other hand this is possible also with the tattoo setup (Effect of Body Mass Index on Magnitude of Setup Errors in Patients Treated With Adjuvant Radiotherapy for Endometrial Cancer With Daily Image Guidance, Lin et al 2012.). Therefore also such study comparison could be interesting where we compare the residual structure errors in the CBCT with setup based on 1) AlignRT  2) tattoos 3) absolute couch values with indexed fixation without AlignRT or tattoos, just with checking the approximate straightness before IGRT (Eliminating Daily Shifts, Tattoos, and Skin Marks: Streamlining Isocenter Localization With Treatment Plan Embedded Couch Values for External Beam Radiation Therapy, 2019).

We use AlignRT in pelvis for both setup and intrafactional monitoring. The main focus is in intrafractional control and we acquire new VRT surface daily after image guidance. (Three-dimensional surface imaging for detection of intra-fraction setup variations during radiotherapy of pelvic tumors, Apicella et al. 2016).

After all this, I would not look behind, instead try to improve the existing, one patient at the time.

Marko

Hey Victoria,

We use for pelvis patients and have great results. It took some tweaking initially on our ROI's but this is an example of what works for us. We have noticed if you include too much of the stomach then it can skew results. Note how we only used that stable part below stomach for this one and extended further down both of the legs.

We use a vac-loc under the legs on only so the pelvis is actually flat on the table. We also use a pillow case folded in 1/3's and placed long ways to cover the patient. This seems to work well. We to do this in CT and explain why and have not encountered any patient complaints about it.

I would try playing with your ROI a little more to see if you can get better results. Don't give up on it yet!

Hi again

I think it is clever to use low vac-loc under the legs, since the more steep the knee angle is, the more AlignRT pitch can cheat you in the bony pelvis area with ROI that includes more legs. To me it seems that there can be even negative correlation in pitch between the 1) legs and the bony 2) symphysis and l5, of which 2) is typically the matching area of interest in the kV/kV images in the pelvis) , since those 1) and 2) do not necessarily move as a one rigid object. For that reason I see that if we include more ROI to the legs with the typical commercial knee/feet fixation device, we have to concentrate that the patient/knees are fixated daily properly on the couch, which may need also indexed fixation under head. These things may have influence on LNG errors we have found and pitch- related errors Victoria has found.

Marko

Hi
I would like to give fresh updated findings based on larger image evaluation concerning pelvis setup with AlignRT. I have done offline review evaluation based on bony structures on 2D/2D kV-images from 90 patients (45 AlignRT and 45 laser setup). I investigated the residual errors in the isocenter and rotations (rot, pitch) from setup to midway compromise of symphysis and sacrum/L5 before couch corrections and errors between symphysis and sacrum, without daily IGRT as well. So, in practice the values of the 1) daily online couch corrections (VRT, LNG, LAT) were added to 2) residual errors for the named bony structure in the offline match, to get a residual error to the bony structure without image guidance. In the online match pitch couch correction was not done and if in the online match rotation was performed, it was taken into account in the results, to achieve results of residual errors without online rotation corrections. For example if the couch correction in LNG in the online match was +5 mm, I matched the image offline to compromise location of symphysis and sacrum/L5 and achieved the offline correction of - 5 mm there, finally the LNG error from setup to midway location of symphysis and sacrum/L5 was 0 mm. I am not giving results here now, but I would like to share some main findings. In this data, with AlignRT, LNG, LAT, VRT and ROT were corrected with send to couch- feature (to zero). In laser group, delta couch shift- function was in use after setup to reference tattoos. In the study, for each patients we used such ROI that VisionRT suggests in the ROI guidelines.
With AlignRT setup, residual LNG and pitch errors (in the images in the acquisition position) are the things that we need to improve in the future, because there were no difference between the groups. Specially, in LNG isocenter there were both rather large systematic and random errors, discrepancy between AlignRT deltas and the acquisition images. To improve this, maybe we should additionally use postural setup, pay more attention to that clothes are not too close to ROI and try to improve the pitch error in bony pelvis in general, beginning from CT and fixation. We have not setup the patients to zero pitch based on AlignRT, instead 3° tolerance was in use. To setup the patient to zero pitch error with AlignRT is rather difficult and time consuming, but it would be interesting to see how much it improves noticed LNG- and pitch displacements in the images. It is possible that noticed pitch- and LNG errors are related with each other.
LAT- and VRT- errors are small in the online match after AlignRT setup, remarkably smaller than with laser setup, in both systematic and random errors (1.0-1.9 mm). Residual rotation errors before online couch corrections (pelvis bony rotation and pitch evaluated in this study, not roll) are small enough to correct with daily IGRT without re-setups, in other words typically always smaller than 3° in AlignRT group. Daily IGRT is needed in both groups and this improves the accuracy on the bony structures remarkably. Margin needed for compromise location of symphysis and sacrum/L5 in LNG is around 9 mm in both groups without IGRT. The time used for setup is small (1 min) in both laser and AlignRT groups. Setup workflow is practical in both groups.
Lately, after this study, we have not delineated the ROI as caudally as suggested by VisionRT guidelines. This seems to be comfortable practice for the patient and for the user and the accuracy on the bony pelvis seems to be good as well. With this workflow, we sometimes notice displacement of LNG delta at time images are acquired and cameras are blocked. The reason may be this narrow ROI in LNG direction. Deltas comes back to zero at the time cameras are not blocked any more.
Small amount of papers exists in the literature concerning pelvis setup with AlignRT. May be the reasons are that typically daily CBCT is acquired, internal anatomy plays a big role in the image evaluation and daily 3D/6D couch corrections are performed to efficiently reduce daily isocenter and rotation errors. For those reasons, it may be not that important to know how much there is needs to do online couch corrections for isocenter or even for rotations due to daily bony structure displacement after setup, as was the question in this study. On the other hand, 6D couches are not necessarily available in all hospitals to correct the posture errors of the (bony) pelvis and additionally bony structures are still used for matching locations in several cases in pelvis. Pelvis in general is a big group of patients in RT. To leave this group outside the AlignRT setup workflow can be a big loss for example to the learning curve of the users when implementing the system into daily use. Based on the data and experience that I have, it is practical and accurate to setup pelvis patients with AlignRT.
Best regards Marko

Hi there , does anyone have any new tips based on your experience so far with alignRT and pelvis treatment with? Want to get the best ROI and seems like reducing pitch might be an issue on CBCT . Many thanks

Hi Margaret

With the pelvic ROI demonstrated here with Ellen (but including less legs from caudal parts, in fact cutting the additional legs extensions outside of the ROI, leading to rather narrow ROI in CC direction), pitch in the kV-LAT images were evaluated from 45 AlignRT (A) patients. For comparison, 45 laser setup (L) patients were evaluated. In total, 1850 images were offline evaluated (969 L, 881 A). Errors between symphysis and L5 in AP direction (VRT), demonstrating pitch, in LAT images exceeded 5 mm (A) in 7% and (L) in 11% of the fractions. Corresponding exceeding between L5 and symphysis were in LAT direction (A) 9% and 10% (L) in AP images, demonstrating the rotation. These all values given, show the errors in the images between the structures after setup, before IGRT and online match corrections. We think that pelvis has been one of the easiest site to setup with AlignRT, and accurate enough. Often posture of the patient is, based on AlignRT, good in the first place and there is no need to touch the patient. So, title of this paper https://doi.org/10.1016/j.tipsro.2021.11.005 based on my experience seems to fit with pelvis setups as well. Can AlignRT be significantly more accurate positioning method than laser setup in pelvis in general from rotations part? May be, but have been satisfied with our accuracy with simple and quick workflow and decided not to fine-tune it in general, allowing AlignRT deltas at the positioning to be within 3° thresholds from pitch part, for example.

Unfortunately I do not have data from problematic cases concerning pitch, and how those were solved with the aid of AlignRT. Offline image evaluation has been in big role here to correct systematic pitch errors. We teach and ask patient to move a pelvis according to found systematic pitch error in the offline images, to lift a back a little, to press it towards couch... Usually this helps and new reference capture is taken after patient reaches a good posture in the images.

Regards Marko