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Three-Dimensional Conformal Radiation Therapy Chapter 8 Perez 2008 Presented by :Dr .M .Dorchin

Modern anatomic imaging technologies, such as x-ray computed tomography (CT) and magnetic resonance imaging (MRI) provide a fully three-dimensional model of the cancer patient's anatomy, which is often complemented with functional imaging, such as positron emission tomography (PET) or magnetic resonance spectroscopy.

Such advanced imaging now allows the radiation oncologist to more accurately identify tumor volumes and their relationship with other critical normal organs. Powerful x-ray CT-simulation and three-dimensional treatment planning systems (3DTPS) have been commercially available since the early 1990's and three-dimensional conformal radiation therapy (3DCRT) is now firmly in place as the standard of practice.

In addition, advances in radiation treatment-delivery technology continue and medical linear accelerators now come accurately with sophisticated computer-controlled multileaf collimator systems (MLCs) and integrated volumetric imaging systems that provide beam aperture and/or beam-intensity modulation capabilities that allow precise shaping and positioning of the patient's dose distributions .

Conformal treatment plans generally use an increased number of radiation beams that are shaped to conform to the target volume. To improve the conformality of the dose distribution, conventional beam modifiers (e.g., wedges, partial transmission blocks, and/or compensating filters) are sometimes used.

This chapter will review the critical components that make up the conformal therapy planning and delivery process, focusing mainly on the forward-planned 3DCRT process.

Historical Development of Conformal Therapy and 3-D Treatment-Planning Systems Conformational treatment methods were pioneered in the 1950s and 1960s by several groups, including Takahashi in Japan, Proimos , Wright et al., and Trump et al. in the United States; and Green et al. in Great Britain.

This work continued into the 1970s, when several groups actually implemented computer-controlled radiation therapy, including the Joint Center in Boston project led by Bjarngard et al. and Kijewski et al. , and the Tracking Cobalt Project led by Davy et al. at the Royal Free Hospital in London.

Sterling et al. are credited with the first 3D approach (dose calculation and display) to treatment planning.

1- Evaluation of Treatment Planning for Heavy Particles (1982–1986) University of Pennsylvania School of Medicine and Fox Chase Cancer Center Lawrence Berkeley Laboratory and University of California Massachusetts General Hospital M.D. Anderson Cancer Center - University of Texas 2- Evaluation of Treatment Planning for External-Beam Photons (1984–1987) University of Pennsylvania School of Medicine and Fox Chase Cancer Center Memorial Sloan-Kettering Cancer Center 3- Washington University in St. Louis Evaluation of Treatment Planning for External-Beam Electrons (1986–1989) University of Michigan Washington University in St. Louis 4- Development of Radiation Therapy Treatment Planning Software Tools (1989–1994) University of North Carolina University of Washington National Cancer Institute Research Contracts in Support of Three-Dimensional Radiation Therapy Treatment Planning

In the 1990s, the commercial availability of 3DTPSs led to widespread adoption of 3D planning and conformal therapy as the standard of practice. One of the keys to this development was a series of research contracts funded by the National Cancer Institute (NCI) in the 1980s and 1990s to evaluate the potential of 3D planning and to make recommendations to the NCI for future research in this area.

Three-Dimensional Treatment Planning

Forward-based 3D planning for conformal therapy typically involves a series of procedures summarized in Table 8.2

These include establishing the patient's treatment position (including constructing a patient repositioning immobilization device when needed), obtaining a volumetric image dataset of the patient in treatment position, contouring target volume(s) and critical normal organs using the volumetric planning image dataset, determining beam orientation and designing beam-block apertures or MLC leaf settings, computing a 3D dose distribution according to the dose prescription, evaluating the treatment plan, and, if needed, modifying the plan (e.g., beam orientations, apertures, weights, modifiers) until an acceptable plan is approved by the radiation oncologist.

Three-Dimensional Treatment Planning Process Table 8.2 Three-Dimensional Treatment Planning Process Step 1:Patient positioning and immobilization *Construct patient repositioning/immobilization device *Establish patient reference marks/patient coordinate system

Step 2: Image acquisition and input *Acquire/input CT (MR or other imaging data) into three-dimensional radiation therapy treatment planning system.

Step 3: Anatomy definition *Geometrically register all input data (such as CT, MR) *Define and display contours and surfaces for organs at risk *Define and display contours and surfaces for target volumes *Generate electron density representation from CT or from assigned bulk density information

Step 4: Dose prescription *Specify dose prescription for planning target volume(s) *Specify dose tolerances for organs at risk

Step 5: Beam technique *Determine beam arrangements (beam's-eye-view and room's-eye-view displays) ? *Design field shape (blocks, multileaf collimator leaf settings) *Determine beam modifiers (compensators, wedges, partial transmission blocks) *Determine beam weighting

Step 6: Dose calculations *Select dose-calculation algorithm and calculation grid *Input dose prescription *Perform dose calculations *Set relative and absolute dose normalizations

Step 7: Plan evaluation/improvement *Generate two- and three-dimensional isodose displays *Generate dose & volume histograms *Perform visual DVH and isodose comparisons *Use automated optimization tools if available *Modify plan based on evaluation of the dose distribution

Step 8: Plan review and documentation *Perform overall review of all aspects of plan and obtain physician approval *Generate hard copy output including digitally reconstructed radiographs

Step 9: Plan implementation and verification *Transfer plan parameters into treatment machine record-and-verify system *Set up (register) the real patient according to plan (verification simulation optional) *Perform patient treatment quality assurance checks including independent check of monitor units.

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