Here is my approach to the early phase of IMRT to get an idea of what kinds of doses are achievable.
- outline your organs at risk (e.g., PAROTID_L)
- draw your GTVs (e.g., GTVp, GTVn)
- draw your CTVs (e.g., CTVp, CTVn, CTVn0)
- expand the CTVs to your PTVs (e.g., CTVp + CTVn + movement margin = PTV7000, CTVp + CTVn + CTVn0 + movement margin = PTV6000)
- expand the lower dose PTV by 0.7cm and make a ROI called penumbra (e.g., PTV6000 + 0.7cm = penumbra)
- take each organ at risk and expand to Planning Restriction Volume (e.g., PAROTID_L + movement margin - penumbra = PAROTID_L_PRV), this ROI is the onl;y part where you can control and reduce dose to the organ. The parts of the organ not included are actually inside the PTV (which means you have decided that area's dose already), and also inside the physical penumbra around the PTV which, unless you have some way of bending photons and altering tissue interaction worked out, you can't change!
- now ask your planners to produce a plan where the PAROTID_L_PRV constraint is 10Gy. This of course is impossible but the next iteration will relax this dose to 11-12 Gy and so on until the PTV coverage is acceptable.
- Then you know that you have what is probably the best plan with PTV coverage and minimal dose on the PAROTID_L.
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