55 yr old male, a case of post op Ca buccal mucosa. Just before starting RT pt had spontaneous # mandible while eating. The # was fixed with a metallic plate n pt is now due for RT with completely healed skin wound.
What would be important points to be considered before RT planning?
Hi Suruchi,
A very interesting problem that is often seen clinically not in the setting of a spontaneous # per se but as a part of reconstructive surgery. The issues that come to my mind are:
What does the literature say about this?
In this the authors set up an experimental system using a cadaver jaw and a metal plate screwed onto the mentum and treated the setup with parallel opposed beams. TLD were used to measure dose perturbations at four locations:
The plate was primarily placed in the mental surface so thats where the greatest perturbations in the dose should occur.. The results were not very significant. The mean difference in the doses at each of these locations with the plate and without the plate was 1 - 4% only.
This is an older article that did a simple experiment. A phantom in front of which a plate of various metals (of varying z) was placed. A Co60 beam was then used to irradiate the phantom and dose recorded. The experimental system allowed calculation of doses at the surfaces of the plate and 1 mm in front and behind the plate.
The results were as follows:
| Material | 1mm in Front | At Plate Surface | Behind Plate | 1mm deep to plate |
|---|---|---|---|---|
| Vitallium | + 10% | + 40% | - 29% | - 10% |
| Titanium | + 5% | + 25% | - 20% | -5% |
| Aluminium Plate | NA | + 6% | - 8% | NA |
| Iron Plate | NA | + 35% | - 16% | NA |
| Tin Plate | NA | + 60% | - 24% | NA |
| Lead Plate | NA | + 85% | - 13% | NA |
For a parallel opposed beam geometry however the magnitude of dose perturbations was much less.
So what would be the important consequences:
But given the modern world I suspect you would be doing IMRT and the shadowing and backscatter phenomenon would not necessarily cancel each other out. The ideal solution would be to contour the plate and the screw and assign them a HU according to the Z number and let the TPS take over. Here we typically contour the areas of artifacts in the oral cavity and assign them a tissue equivalent number as most people have dental implants or fillings.
Oh one more thing for you as a radiation oncologist the biggest problem from the plate will be two fold:
A caveat is of course that the entire post has focussed on EBRT .. with brachytherapy the problems can be minimized with a good implant and adequate internal shielding)
Interesting case, can you tell us about the full path staging? Implanting a BM post op is tricky,what to implant is an issue.
Hi,
Thanks Santam for the information.
Nikhilesh, Its a case of Ca BM, pT3pNoMo, deeper layer of Ms involved, with thickness of 1.8 cms, bone and margins were uninvolved, brachy would not be possible b'coz of the scars n the last one being so fresh. Also it wd be difficult to define target volumes for brachy, ,, RT can also not be delayed as it is approaching almost 7 wks post 1st Sx.
Would be planning him with plate drawn as the volume n as Santam said let TPS do the rest of job.
Healing of bone post RT wd be major concern now, n wd probably limit the dose to entire mandible n plate to minimum, hope that helps!!!
Hi Suruchi ,
Interesting and often faced problem .
Would be very interested to know :
a) issues you faced during contouring the implant , esp in terms of the artifacts etc
b) what was the material of the implant and what z you assumed
c) what HU you assigned to the contoured implant and is there any reference as to what HU can be assigned for implants of different z?
Will get back to u Ayan with details… :-)
Hi,
I guess not much to add after the detailed elaboration already done, but some more thoughts…
1. Spontaneous fracture of mandible is not common. Was mandibulotomy part of the first surgery? It could mean that the vascular supply to mandible is already compromised as a result of surgical procedure. Unfortunately, if does develop ORN the blame will come on radiation as the culprit.
I agree with Santam.. it would be important to keep in mind about possibility of mandibular involvement (either direct or infiltration along inferior alveolar nerve). Was there any perineural invasion noted on pathology? If there is suspicion, I think MRI may be a useful modality to assess enhancement along the nerve, though I understand the titanium plate will result in some artifact even with MRI.
2. You could try using a customised wax mouth-guard/ dental tray during the treatments to cover the gingivo-labial sulcus and tongue/ floor of mouth area, if you think it is in approximation to the site of the plate. This can reduce mucositis and make the treatment more tolerable. I am sure you will take care that the mucosa which is part of target volume is not being blocked by doing this.
3. About assigning a HU number to the plate, I have some doubts. From what I understand the HU number is part of the image data which is generated right at the time of planning CT. So the TPS should already have the information on HU number for the metal plate. I guess we will need some physics input here.
Assigning a HU number is important when we need to override a particular area of the CT.. like for example as Santam pointed, the region of the scatter artifact is contoured and alloted a tissue equivalent density for calculation of isodose distribution. In this case as well, in addition to contouring the metal plate, one would need to contour the area of scatter and then use the override function to allot it a tissue density.
4. I assume, you have already consented this patient for a higher risk of ORN and have already done a pre-RT dental prophylaxis and post-RT dental care.
5. One final thought, this is a situation where we need to ensure no hot spots within the mandible contour beyond what is expected in the region directly around the metal plate.
PS: You may also want to start looking around for facilities with HBOT facility for this gentleman… there is a good chance he may need it :-)
The reason why i told to assign a HU as HU relation with electron density is not linear like in case of high Z material. The famous pic from FMK does show this relationship so a TPS may not assign an accurate value .. however yes a physicists input is most recommended.
The pic below is taken from Faiz Khans book Chapter 12 and you can certainly find some useful references there
I was thinking the same way as Pranshu mentioned in point no3. That the system will automatically assign the HU for the plate, All u need is to contour whole artifact including plate then contour the metal plate then minus the plate from the total artifact and assign it tissue equivalent HU. But santam has also raised a valid point from physics aspect.
