IMRT Planning Objectives with Corvus

1. IMRT Planning Objectives with Corvus Walter Grant III, Ph.D. Baylor College of Medicine The Methodist Hospital Houston, Texas

2. Some Initial Comments • Corvus was designed by a physician, not a physicist. • His intention was that physicians would run their own plans. • So when you ask, “Why did they do it that way?”, remember the above.

3. Some Initial Comments • That physician is Dr. Mark Carol, a neurosurgeon. • As the 1st clinical user of the NOMOS system, I had the luxury and privilege of learning from “the master”.

4. What I will to do today • I will share some of the initial philosophical changes that occurred in our transition from forward planning to inverse planning. • I’ll discuss the changes from Corvus 1.0 (Peacock Plan) to Corvus 5.0 in planning philosophy. • I’ll review the intentions of the “Tissue Types” and some approaches to getting better plans, if possible.

5. Philosophy • Why am I doing IMRT at all? • To preserve some normal structure(s). • Spinal cord, optic chiasm, etc. • Maybe a previously irradiated area • Brain mets • It’s never really just to deliver a conformal dose to a target. • That’s easy if there is no critical structure close or the dose is low.

6. Philosophy • For me, that was the break through in understanding how to approach inverse planning. • The BCM mantra became, “Protect the normal structure as much as you need and then put as much dose in the target as you can.” • And it is the inverse of what I was trained to do for many years.

7. Philosophy • This was occurring in 1993 and it took only a couple of plans to identify where we were weak. • We didn’t know the biological limits of partial volume irradiation of structures. • It’s 2004 and we still don’t.

8. Philosophy • So if you are struggling with planning, try keeping these 2 principles in mind: • Protect the structure first and then dose the target. • We don’t really know partial volume tolerances, so the “best plan” will be subjective, not objective for now.

9. Philosophy • If the “best plan” is subjective, how will the physician decide that he’s got the “best plan”? • The same way he always has --- when it’s time to treat the patient. • You’ll run plans that change a few percent here, a few percent there, just to give the physician peace of mind. • We ask our physicians, “Am I treating the patient or am I treating the doctor?” and they are very honest about it.

10. Philosophy • So how can I help the physician? • I do it by considering RTP as a boundary problem. • We run an “Efficient Plan” that basically covers the target and blasts the structures. • Remember this as I use information from this plan in obtaining my clinical plan. • We run a “Conformal Plan” that protects the structures at the expense of under dosing the target.

11. Philosophy • So how can I help the physician? • This helps the physician recognize what his tradeoffs are going to be. • And it is usually when he shares his desires and fears.

12. A Quick Wrap • I have shared some planning experience and philosophy with you. • I believe these are generally applicable, not Corvus specific.

13. Peacock Plan • The original Peacock Plan (later called Corvus 1.0) had features that some of you may not have experienced, so I want to cover the significant ones. • It was not a Partial Volume Planner. • You entered a Goal Dose and controlled the “importance” with a weight from 0 to 2. • Or you lowered/raised the Goal Dose. • Tissue had a “weight” control.

14. Peacock Plan Rx Panel

15. Peacock Plan • The cost function was a simple quadratic expression whose steepness was controlled by the weight: Cost = W * (Goal Dose - Dosei)2

16. Corvus 2.0 and up • Dr. Carol believed that the only way to gather the partial volume information was to plan with it, thereby encouraging us to record the results. • So a PVP panel was introduced with Corvus 2.0 • But so was a new concept of optimization and “Goal control”.

17. Corvus 2.0 and up • Goal or Limit • % under or over • Minimum • Maximum • PV planning requires the creation of a DVH:

18. Corvus 2.0 and up • The Cost Function is no longer a quadratic on one point. • It’s control of 3 points and 4 areas. • The use of area optimization creates a planning rule that I’ll point out later.

19. Corvus 2.0 and up

20. Corvus 2.0 and up • The shape of the Cost Function is a combination of linear and exponential components that were selected by Dr. Carol. • And since he wants physicians to plan, the combinations are determined by the selection of a “Tissue Type” and not by the choice of a “weight”. • This is not the way a physicist wants to do things.

21. Corvus 2.0 and up • Since Dr. Carol is a neurosurgeon, his training is to drive dose out of the normal tissue into the target à laSRS. • Corvus controls that with the normal structure called “Tissue” which is all the non-Target tissue.

22. Corvus 2.0 and up • In Corvus 1.0, he thought Tissue should always have a weight ≈ 2, but to treat malignant tumors we wanted more homogeneous dose distributions, so I would set values close to 0.02 just to aggravate him. • One had to be careful that the maximum dose was in the Target because Tissue insures that! • He got even with me in later versions as Tissue now can’t be “turned off”. • And we found ways to take advantage of that.

23. Cleanup Time • When you approach Corvus planning, you do not want to think like a physicist. • Don’t worry about what is “under the hood”, just learn to enjoy driving the car. • But do recognize, as a physicist, that there are some traps that you should avoid. • I’ll try to point them out next.

24. Corvus 5.0 Planning • We have a large database of clinical outcome for mainly 3 sites. • Cranial, H&N and Prostate • The goal is to try to match what has given clinically acceptable outcome: • Doses • MU’s

25. Corvus 5.0 Planning • As information for the non-Corvus users, there are 4 issues to address in planning: • Complete the PVP • Pick an optimizer • Apply uncertainties for setup error and organ motion. • Place your beams.

26. Corvus 5.0 Rx Panel

27. Picking Types

28. Prescription Instructions

29. Corvus 5.0 Optimizer

30. Corvus 5.0 Optimizer

31. Corvus 5.0 Optimizers • It is Simulated Annealing • Optimizes both pencil beam intensities and monitor units (MU). • Uses all intensity levels and constrains to discrete levels at end. • Applies a Gradient Descent optimization at end. • Will always give the best plan. • Will always require the most MU. • Use the Efficiency Slider Bar • Continuous

32. Corvus 5.0 Optimizers • It is Simulated Annealing • Optimizes discrete pencil beam intensities. • Since MU settings are not optimized, all MU settings are equal. • Applies a Gradient Descent optimization at end. • Usually as good as Continuous for most plans. • Will always require less MU. • As much as 1/3 has been seen compared to Continuous. • We use it as “default” in 5.0 for MIMiC delivery. • Discrete

33. Corvus 5.0 Optimizers • It is Simulated Annealing • Optimizes MU settings only. • Basically no modulation so just a conformal plan. • Continuous (MU Only)

34. Corvus 5.0 Optimizers • Begins by optimizing MU Only • Then optimizes beamlets using a gradient descent algorithm. • Probably will produce more homogeneous results requiring less MU. • Good for simple plans (prostate & cranial), but poor for complex plans (H&N). • We use it as the default for our MLC delivery. • Smooth Downhill Search

35. Be careful about rules of growth because you can wipe out structures with targets and you’ll wonder why you can’t control the dose they receive. Corvus 5.0 Immobilization

36. Corvus 5.0 Beams

37. Now Comes the Hard Part • At least it’s hard for me because I have to share 11 years of clinical inverse planning experience in about 11 more minutes. • I remind you that there are only clinically acceptable plans, not correct plans.

38. Now Comes the Hard Part • My goal is not to get the best plan first. • Remember, I don’t believe the physician knows that plan or shares that with me initially. • My goal is to know what to do on the 2nd iteration in case the physician isn’t satisfied.

39. How I Approach 5.0 Planning • I am going to cover the “art” of putting numbers in the Corvus Rx Panel. • I’m going to discuss how I prepare myself to get the next iteration. • I’m going to remind you now that I’ll “think like a doctor” because that’s the way Dr. Carol designed the system.

40. Corvus 5.0 Rx Panel

41. How I Approach 5.0 Planning • Fill in the panel with true clinical numbers. • This will help you build a foundation for expansion to other disease sites. • Run an “Efficient Plan” • Set all Structures as Reference Tissue Type. • If you fail to dose the Target, then something is wrong. • Check Anatomy • Check entries in Rx Panel. • For MIMiC it could be a required isocenter shift.

42. How I Approach 5.0 Planning • If you do dose the target, then see what structures meet their goals. • Most distal structures can meet goals because “Tissue” is designed to drive the dose out of normal tissue into the target. • I leave those as Reference Structures so that the optimizer doesn’t spend time minimizing the Cost Function on non-important structures. • Gotcha Mark!

43. How I Approach 5.0 Planning • You can run a “Conformal” plan by loosening the limits for the targets. • Lower minimum dose. • 10~20% volume under. • Reference Target is useful only if there is more than one target. • Doesn’t help much except to see how many MU’s are required to treat.

44. How I Approach 5.0 Planning • When entering the numbers, follow the rules in the Prescription Information window. • Structure Limits should be set independent of Structure Type. • The dose limits should be clinical values. • If 50% of the parotid gland can exceed 22 Gy, then put those numbers in.

45. How I Approach 5.0 Planning • If the target is more important than the structure, set the Structure Maximum dose to Target Goal dose. • If the structure is more important than the target, set the Target Minimum dose to the Structure Maximum dose. • If the structure is critical, set the Target Minimum dose to the Structure Limit dose. • Help the system understand where you want it to put the steep dose gradient.

46. How I Approach 5.0 Planning • Never set Structure Minimum to “0”. • Remember that Corvus is optimizing areas as well as control points. • If I don’t know any better, I set the minimum about 50% of the Limit and adjust when I see the Statistics Panel. And one that I still have to fight with my staff,

47. So Let’s Get an Acceptable Plan • If we can put all these steps together, then we should be able to get an “acceptable plan”. • Here are the iterations we teach at Baylor.

48. So Let’s Get an Acceptable Plan • Step 1 • Insure that the anatomy is correct and that there are no “stray voxels”.

49. Where did these come from? • Anatomy review

50. So Let’s Get an Acceptable Plan • Step 2 • Enter the desired target dose into the Rx panel. • Follow the rules at the bottom of the screen