Treatment planning involves defining target volumes and organs at risk on imaging scans. Virtual simulation uses CT scans to delineate structures and localize the treatment isocenter. This allows improved volume definition compared to conventional simulation. Treatment planning optimization selects parameters like beam number, weighting, and modifiers to best conform the dose distribution to planning objectives while minimizing dose to organs at risk. Beam modifiers like wedges and compensators can be used to modify the dose distribution for target coverage and organ sparing.

1. Treatment Planning: Volume Definition: Beam Selection References: Radiation Therapy Planning, Bentel Treatment Planning in Radiation Oncology, Khan and Potish ICRU 50, 62

2. ICRU definitions

3. GTV - palpable or visible extent of tumor CTV - GTV + subclinical microscopic disease PTV - geometric concept designed to cover CTV Treated Volume - volume enclosed by dose level appropriate to treat disease Irradiated Volume - volume that receives significant dose

4. Internal Margin: variations in size and shape of CTV during treatment Set-up Margin: uncertainties in patient positioning and alignment PRV: planning organ at risk volume includes margins on critical structures

5. Volume Definition: Imaging Modalities CT, US, MRI, PET, Nuc Med, Spect fMRI, Optical?, …….. Addition of margins

10. Coordinate Systems: Patient: internal reference point Imaging: simulator isocentre/none Treatment: isocentre

12. Virtual Simulation: - Immobilization CT Coordinate system Structure Delineation Isocentre localization Beam placement/definition

19. Problems: Images are static and organ motion is not evident Correlation of imager/patient/treatment coordinate systems is non-trivial - DRRs Resolution of data set is limited by slice thickness - structure definition/DRR Imaging modality - image fusion

21. Advantages: Improved volume definition Patient data collected in digital form for dose calculation Speed

22. Conventional Simulation Immobilization Diagnostic energy X-rays replace Megavoltage beams Lower patient dose, better images, real-time fluoro External coordinate system same as treatment coordinate system

23. Volume Definition External reference palpation,visual radio-opaque markers Internal reference bony landmarks, other anatomical transfer from CT contrast agents, internal markers

24. Lateral Field Nodes outlined With solder

27. Problems: External contours must be obtained for dose distribution calculation Time consuming Volume definition is difficult

29. Advantages: Organ motion can be visualized on fluoro Co-localization of simulation/treatment geometries Treatment geometry problems can be avoided

30. Treatment Planning Objectives (Goals, Desires, Constraints, etc…) Deliver a uniform dose to PTV Deliver as little dose as possible to OAR Keep integral dose low Reduce number of high dose ‘hot spots’ outside PTV KISS

31. Treatment Parameters Degrees of Freedom (with apologies to True Statisticians) Number of treatment beams Individual beam energy Relative beam weighting Shielding Primary beam profile modifiers Patient modifiers (bolus, and other?)

32. Treatment Optimization Selection of treatment parameters that best conforms to planning objectives Manual: based on experience - time consuming - artform? Automated - forward calculation - compensation Automated - inverse planning - optimization algorithms

33. Patient Modifiers: Bolus: tissue equivalent material Placed directly on patients’ surface Purpose is usually to reduce skin sparing Can be used to ‘block up’ complex surface to simplify dose distribution

34. Numbers of Beams KISS Conformal RT Ability to escalate dose Higher demands on setup accuracy

36. Wedges: modify primary beam profile so as to produce isodose lines at angle wrt to surface Open beam 45 degree wedge

37. 15 degree 30 degree 45 degree 60 degree Different wedges available for Varian 600C

38. Types of wedges Physical: a wedge shaped piece of metal (steel or lead) machined to shape the primary beam profile. Must be physically placed in head of machine. Limited selection of wedge angles. Universal: a physical wedge with very high wedge angle permanently in head of machine. Different effective wedge angles are obtained by combining open and wedged beams for different fractions of treatment. Dynamic: one field jaw sweeps across field during treatment so that integrated dose-distribution matches that of physical wedge .

39. Use of wedges I: To correct for patient contour

40. Variation at level of isocentre: 40% 5%

41. Use of wedges II: To correct for beam attenuation when using multiple fields Example: 3 field plan, variation in treated volume: 30% 5%

42. Example: wedged pair, dose variation in treated volume: 50% 5%

43. Compensators Missing tissue : corrects for patient surface to give uniform dose to a surface perpendicular to central axis of beam. Compensation Plane

44. Primary Beam Profile Modulation Physical: Attenuators, compensators. Thickness is calculated using attenuation coefficient of compensator material I p * = EXP (-  t p ) Dynamic MLC: similar to dynamic wedge, MLC leaves are moved during treatment to affect required distribution

45. Compensators Dose : corrects to give uniform dose to an arbitrary surface in patient Compensation Surface

48. Forward Algorithm: Additional Complications Dose compensators: compensation surface is not a constant SAD - will require additional ISF factor Primary beam profile is not flat (horns, penumbra). How/should one correct for beam profile? Introduction of shielding gives differential scatter loss across field - integrate scatter dose point by point

49. Compensators: Inverse Algorithm Optimization problem Need a good forward dose calculation algorithm Divide beam into many smaller ‘pencil’ beams Adjust pencil beam weights iteratively to achieve uniform dose on compensation plane Usually flat plane, solution exists

50. Example: neck: compensate to give uniform dose along midplane throughout treatment field

51. Uncompensated Compensated 15-20% <5%