Unknown Primary with Neck Metastasis

Unknown Primary with Neck Metastasis

Harsha Dod 27 May 2010 07:38

Is any one treating Unknown Primary with neck metastasis with IMRT.

I would like to know the dose prescription for different areas and the principle behind the same.

Re: Unknown Primary with Neck Metastasis

AAM 27 May 2010 12:21

Yes.

Dose levels are identical to other H&N. As a matter of routine I do not cover potential sites of primary because toxicity is excessive. My only failure with a primary site resulted in an overlap of perhaps 1-2cm of 70Gy/35Fx on a previous dose of 60Gy/35Fx.

1. whole ipsilateral neck (CTVn + CTVn0 = 60Gy/35Fx)
2. all gross disease and visible nodes (down to 4mm size) (CTVn = 70Gy/35Fx)
   1. gross disease (CTVn) determined by PET
3. I use:
   1. a RT-determined isocentre on the midline bones of the mid neck
   2. EPID each day to keep within 3mm of isocentre
   3. 3mm expansion on PTV
      1. (CTVn + 3mm) + (CTVn0 + 3mm) = PTV6000
      2. (CTVn + 3mm) = PTV7000
   4. mucosa OAR
      1. recognize the air-tissue boundary with autocontour tool (volume #1)
      2. expand this volume by 2mm (volume #2)
      3. subtract volume #1 from volume #2 to leave a 2mm annulus which matches the mucosa
      4. draw 4mm wide volume where mucosal surfaces are apposed
   5. bilateral parotid/submandibular OAR
   6. produce PRVs
      1. make a "PTV_PRV" by expanding the PTV6000 by 7mm (this corresponds to the minimal penumbra id conformality is 100%)
      2. make the OAR_PRVs
         1. expand the OARs by movement margin
         2. exclude the portion that falls inside the PTV and PTV_PRV
         3. Organ_PRV = (Organ + 3mm) - PTV_PRV
      3. planning constraints
         1. start with PRV constraints of 10Gy and then relax in 1-2Gy steps until PTV coverage becomes acceptable
         2. contralateral parotid and submandibular glands should be able to kept below 10-14Gy max
         3. mucosa PRV should be able to be kept to <25-30Gy

Re: Unknown Primary with Neck Metastasis

Rohit Malde 27 May 2010 12:32

Andrew, do you have any fascinating pictures for demonstration purposes, esp mucosa OAR.
Last time when u uploaded a few , they were great.

Re: Unknown Primary with Neck Metastasis

Harsha Dod 27 May 2010 12:40

Thanks .

Further questions.
1. Nasopharynx is not treated electively.
2. Contralateral neck, do i treat as microscopic to 50 Gy.
3. I have no clue regarding ( this corresponds to the minimal penumbra id conformality is 100% )

Re: Unknown Primary with Neck Metastasis

AAM 27 May 2010 13:40

Further questions.

1. Nasopharynx is not treated electively.

No. The only occasion where I might think about doing this is when there is a Level 5 LN. In the case where I believed that NP was the culprit, I would use bilateral neck RT with nasopharynx BUT I would curtails doses to oral/pharyngeal mucosa and parotid, i.e., accept less PTV coverage in node negative areas to keep parotid doses low (mean parotid doses ~16-18Gy)

2. Contralateral neck, do i treat as microscopic to 50 Gy.

This is a judgment call. I treat ipsilateral neck alone. If you treat contralateral neck, even with IMRT, you will be hard pressed to deliver high doses if you see a primary appear later. As I see it, the choice is everything (nasopharynx, tonsil, hypopharynx and bilateral necks) or what is at high risk (ipsilateral neck alone)

3. I have no clue regarding ( this corresponds to the minimal penumbra id conformality is 100% )

• What I mean is this - when you define a high dose PTV, you are actually defining the 100% conformality line. That is, the PERFECT plan would have the 95% isodose lying directly over the PTV line.
  • Is this assertion clear?
• When you have defined this PTV=95% isodose line concept, the physics reality is that the PERFECT penumbra around that line will be about 5mm to 50% (corresponds to the old light field edge), and by 7mm the dose will be down to ~25-30%. You can't change this, it's a matter of physics. You can visualize this by expanding the PTV by 7mm (the name I give it is PTV_PRV) - its a theoretical volume and only used to build PRVs.
  • Is this second assertion clear?
• So now you have the OAR - it moves by the same amount as the CTV movement (in my case 3mm). That expanded (OAR + movement) becomes a slightly larger volume and has three parts:
  1. the part that overlaps the PTV - you can't alter the dose here because in assigning a high dose PTV, you have decided to deliver target dose there and sacrifice the organ.
  2. the part that overlaps the penumbral margin (that is, the part of the expanded OAR inside the PTV_PRV) - you can't alter the dose here either because having decided on a high dose region, you HAVE TO HAVE A PENUMBRA AROUND IT.
  3. the part that is outside 1 and 2, to which you have some chance of altering dose, called "Organ_PRV".
• The IMRT constraint is then set to this Organ_PRV volume and the dose constraint is set ridiculous low ("unattainable" is a good place to start!) and then gradually increased until an acceptable plan is obtained. There are two things to say about this [(OAR + 3mm - PTV_PRV) = Organ_PRV] approach:
  • • this approach allows the algorithm to concentrate on what is important, that is, spare dose to areas where dose can be spared.
    • this approach uses an ALARA approach to dose restriction to critical structures. Restriction the parotid mean dose to 26Gy is not an acceptable aim. If the ALARA dose is a mean of 16Gy, then why deliver 26Gy? If the ALARA dose is 32Gy when ensuring PTV coverage, then why deliver 26Gy and underdose the PTV?

Last edited on 27 May 2010 13:53 by AAM Show more

Re: Unknown Primary with Neck Metastasis

AAM 27 May 2010 13:42

I'll do some work on a pictorial essay … when I have time!

Re: Unknown Primary with Neck Metastasis

Nikhilesh Patil 27 May 2010 17:31

Excellent stuff Andrew,
Harsha did you blind biopsies (NPX,Base tongue, tonsils) for your patient? We doing a PET study for Unknown primary, where the PET results are blinded and till now invariably there is a small primary sitting somewhere in Oropharynx.

Re: Unknown Primary with Neck Metastasis

Harsha Dod 28 May 2010 12:46

Thanks a lot Andrew. Your explanation has stimulated the grey cells.

This patient has level V node, and it is Squamous cell carcinoma !
I have gone ahead and contoured it as NPC.

Nikhilesh, no, blind biopsies were not taken. But PET-CT had already been done before the patient was referred to us.