In the current radiotherapy machines which deliver VMAT, IMRT, SRS or SBRT, what is the role of field light.
In a conventional 3D-CRT it is understood the requirement of field light, where you get a reference of the field getting irradiated from the light.
But in the current machines is it really required to have a field light ?
Are there any machines which doesn't have field light? Tomtherapy doesn't have a field light I guess. Does IEC states the requirement of Field light is mandatory ?
Really important is to hit the treatment volume. If you have methods to control the target volume by 3D imaging (CT, other imaging techiques) during the treatment, you don´t need field light. But of course you need help to adjuste the patient in the correct position eg by laser markers etc.
For the vast majority of IMRT/VMAT cases the light field has no utility, as stated above imaging techniques have superseded this. However, we still find the light field an important tool in breast IMRT to assess that any swelling is contained within the flash, as this is also a superficial site it can also be useful to make reference to skin markers using the light field. Additionally, the light field can also be a useful tool in QA.
Typically, IMRT/VMAT is image-guided. So you should lining up using CBCT weekly and kV daily. The field light can be used for initial alignment (for either patients or QA devices).
Most conventional linear accelerators have field lights because they are used for 3D cases. Dedicated SRS systems such as Cyber Knife and Gamma Knife do not have field lights - with CK relying upon image-guidance and GK relying on a fixation device. BrainLab SRS is also a great example of image guidance trumping the field light alignment.
I don't believe there are any regulations from the IEC requiring field lights. But if you don't have image guidance, then field lights (and lasers) are the best way to reproducibly set up patients and check flash in 3D cases.
Does this help? If so, please upvote this answer. If not, let me know so I can clarify! Good luck, CT.
Thank you gentleman for your valuable points.
When I was going through IEC 60977, clause 8.1.2 clearly states the difference in light field and radiation field to be with in some limit...etc. Does it mean a light field is mandatory ?, If so CK and Tomotherapy also should comply to this, and I don't find a light field in them.
Dr. Kuppa -
I'm not sure, but I believe that the IEC regulation only applies if you have a light field to begin with - most likely because the IEC regulation was written with traditional linear accelerators in mind.
In short, the light-field's intended purpose was to allow a clinical setup (non image-guided). But with integrated image guidance in CK and Tomo, it is certainly acceptable to forgo the light field.
If you need to establish some sort of precedent, AAPM recommendations (my clinic is in the US) in AAPM TG 135 (CK) and AAPM TG 148 (Tomo) discuss the inherent image-guidance which excludes them from requiring a light-field.
Does this help? If so, please upvote this answer. If not, let me know so I can clarify! Good luck, CT.
Thank you Dr Tien,
AAPM recommendations are very useful.
Some how I feel that IEC standards has to be properly revised.
asking for mechanical scales like gantry rotation specially for machines which have ring gantry is also not possible.
For example view ray which has an MRI along with radiation... the patient bore looks like a tunnel and there is no point in having a light field or to provide a mechanical scale for gantry rotation.
I am facing same issue with range light which is conventionally used for measuring TSD, target to skin distance.. with IR cameras you can accurately place the patient and reverify.
When IEC standards were made C shape gantry which VARIAN made was considered I guess. With recent ring gantry's coming into market and as a medical equipment developer its getting difficult to comply all the old standards which may not be relevant to the current technology.
Hi every one.
We had contacted our IEC representative and as we discussed above they have confirmed field light is not mandatory but all the functionalities which field light does has to be addressed by other means and has to be clearly addressed in the project risk management document and has to be demonstrated practically during the validation.
Thank You all for your valuable contribution and time.
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