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[Charlotte_McAlinden]

Hi,

Is anyone using CBCT imaging for breast DIBH patients? We currently CBCT all breasts and use the RGSC system for DIBH which will automatically beam hold the CBCT if they were to move out of breath hold (we spilt the CBCT into 3 manageable chunks anyway). Do you have any issues with patients moving out of breath hold thresholds and the CBCT continuing or any ways of managing this? It seems a step back for us to not have the automatic beam hold for CBCTs on these patients.

Any experiences or thoughts would be greatly appreciated.

[laurenvacker1]

Curious as to what the RGSC system is?

[schang1]

Our preferred method is the partial arc cone beam at the treatment isocenter (not centering the couch), which allows patients to complete imaging in less than 35 seconds during a single breath hold.

[Laressa Priddy]

Hi schang1

Please can you share how you are able to do partial arcs CBCT without couch centre for breast patients? We have always found that the gantry collides with patient arms/couch.

[GeorgeG]

I'd also be interested to hear how you do the partial CBCTs without centering the couch please. We use Varian Truebeam.
Thanks
George

[schang1]

We only perform DIBH CBCT for Breast/Chest Wall treatments when using the VMAT/IMRT technique. For this, we use a breastboard with a 5-degree tilting angle and a VacLok on top of it. Additionally, we use a 5-cm shifting index bar to ensure the tabletop is as central as possible. The isocenter is positioned in the lung beyond the chest wall and is less than 24 cm vertically to the tabletop. The 200-degree partial arc CBCT begins at gantry 179 degrees to enhance AlignRT visualization. I hope this helps.

[Laressa Priddy]

Thank you for sharing that's really helpful.

[marko_laaksomaa]

Hello

We are as well grateful of your answer and interested to test this in practice. This kind of possibility has not ever come to anyones mind. I would like to ask some questions, if it is ok?
1. Do you use Varian linacs?
2. Are all of your breast patients suitable for half arc in DIBH with this workflow, I mean collision risk? If they are, please give us tips how it is possible.
3. If all are not suitable due to risk of collision, how do you know beforehand which are not suitable or do you just test it in practice at the first fraction? You mentioned level of 24 in VRT, is this some kind of level to begin to implement half arc CBCT and do you have some kind of level for LAT plan value to know that with this value CBCT is not possible without center couch? Do you have elbows visible at the CT, so you can beforehand calculate that this will collide?

We tested this with RTTs, to rotate gantry and imagers with Truebeam, with 5 cm laterally shifted index bar. We tried to simulate typical isocenter location in breast RT and noticed that at least with our breast board and test patients not all the half arc CBCTs would be possible to implement due to collision.

Greetings to Varian, it would be great if the image detector at the CBCT could rotate at the distance of 160, instead of current 150.

So, if you have some additional tips, those would be useful.

Marko

[schang1]

Hi Marko,

I am pleased to share my experience with the SGRT community. In response to your question:

1. We are using Varian TrueBean v2.7 with AlignRT.
2. We have mastered this technique to avoid collisions with the contralateral elbow and the posterior of the couch. During the process, we perform a couple of specific steps. Based on our experience, we have found that the partial-arc CBCT (200 degrees) technique is well-suited for most cases.
   a) Simulation — The key is to avoid collisions with the contralateral arm/elbow during the treatment. So, an asymmetric arm setup is encouraged, especially for IMRT/VMAT cases. We set the contralateral arm lower to the posterior and extended it more straight than the ipsilateral arm. You can also set the contralateral arm to the side of the torso if you feel comfortable doing so.
   b) Treatment Planning —
   • Couch shifting: the couch top is placed 5 cm toward the treatment side.
   • Isocenter placement: For IMRT/ VMAT breast/chest wall cases, the isocenter is kept vertically within 24 cm of the tabletop. This location is optimal to avoid a kV panel collision with the couch while performing the CBCT. We only perform CBCT for VMAT/IMRT cases. The 3D cases only take film daily, so the isocenter placement is at the traditional location in the breast, not in the lung.
3. When patients have limited arm mobility, we adjust the position of the contralateral arm side to be closer to the head rather than protruding outwards. We also ensure that the isocenter is positioned lower. If the linac schedule permits, we conduct a preliminary dry run of the patient immediately after the simulation to address any potential collision concerns. In addition, our therapists always rotate the gantry with KV imagers out before performing the CBCT to ensure proper clearance and enhance patient safety.
   
   Recently, I had the opportunity to try out MapRT. The software can simulate various situations with different setups, such as the linac with an electron cone, the imager arm out, or patient shifts. If you really want to comprehend the collision issue in a quantitative way, MapRT is a great choice.
   
   I spent time experimenting with our setup on the linac and planning, so I came up with the lateral and vertical numbers. I believe you can also experiment with your setup to find the best settings for most patients.