week 9 simulation stuff

Planning overview- combine machine parameters w…

• individual patient data to customize and optimize treatment

• Requires machine data, input of patient data, calculation algorithm
  Produces output of data in a form which can be used for treatment (the 'treatment plan')

• Radiotherapy is a localized treatment of cancer

• know not only the dose but also the accurate volume where it has been delivered to.

• applies to tumor as well as normal structures - the irradiation of the latter can cause intolerable complications. Again, both volume and dose are important.

What You Need to Know for Planning

• Where the tumor is (location)

• How big and what shape it is (volume & shape)

• If it’s spreading nearby (secondary targets)

• Where important organs are (critical structures)

• Size & shape of those organs

• How sensitive those areas are to radiation (radiobiology)

To find the tumor (target), we use diagnostic tools, like:

• Basic tools:

• 
  

  • Palpation (feeling the lump)

  • X-ray

  • Ultrasound

• Advanced scans:

• 
  

  • MRI

  • PET

  • SPECT

• Planning scans:

• 
  

  • CT scan

  • Simulator radiograph (used to help line things up for treatment)

Minimum Patient Data Needed

• Target location: Where the tumor is

• Patient outline: Shape and size of the body (for beam planning)

• Simulator tools:

• 
  

  • Laser system (for marking/positioning)

  • Optical Distance Indicator (ODI) — measures distance from machine to patient

Note: Some of these functions can now be done directly on the treatment machine too!

Role of Simulation

Used 2 times:

1. Plan it – Get patient info (tumor, body outline)

2. Check it – Make sure the plan works (verification)

Also for:

• Reference images (to check setup later)

• Can be replaced by virtual sim or other scans

Virtual Simulation

Treatment fields for

• Uses CT scans + machine info to plan treatment

• Done with software on a computer (not a real machine)

Virtual sim helps to

• A CT scan of the patient is taken.

  1. The scan is loaded into special software.

  2. The team uses the software to:
     

     • Outline (contour) the tumor and organs.

     • Place the radiation beams in the best directions.

     • Add blocks/shields to protect healthy areas.

     • Plan where the skin marks (tattoos or stickers) go.

Simulator Film

• Uses wires to outline the treatment field

• Shows anatomy + field location + size

• Shows shielding areas

• Used as a reference image to check setup later

Patient Marking

• Marks are placed to line up the patient with the beam

• Connects the patient’s position to the machine’s coordinates

• Helps make sure the beam hits the same spot every time

Combining Modalities – Easy Version

• Look at scans side-by-side

• Each scan shows different details
  

  • MR angiogram: shows blood vessels

  • MR image: shows soft tissues

Image Fusion – Easy Version

• Combines images from scans like MRI or PET with CT

• Why?
  

  • MRI/PET = better detail

  • CT = better location accuracy

• Lets you outline anatomy on any scan and see it on the CT

To make a treatment plan, we need info about the beam and how it’s used:

• Beam description: QUALITY and energy

• Beam geometry: Machine angles (gantry, table, isocenter)

• Field definition: How the beam is shaped (collimators, blocks, MLC)

• Physical modifiers: Wedges, compensators

• Dynamic modifiers: Moving parts like dynamic wedge, arcs, IMRT

• Dose normalization: Sets the reference point for dose calculation

Machine Data for Planning – Easy Version

• What’s needed depends on:

• How complex the treatment is

  • What tools/resources you have to get the data

• Data can come from:
  

  • Published sources

  • Your own measurements

• Important: All data must be verified before using in planning!

Machine Data Acquisition – Easy Version

• Data comes from:

• 
  

  • The vendor or

  • Publications (like BJR 17 & 25)

  • But it must be verified first!

• Done by a physicist using:
  

  • Dosimetry tools like water phantom, ion chambers, film, etc.

• Documentation is a must — everything has to be recorded

Start of treatment planning is

Where beam is directed

Size beam

Shape

Beam’s Eye View (BEV):

A simulated image based on CT data that shows what the radiation beam “sees” as it enters the patient.

• It represents the patient’s anatomy and defined treatment volumes from the beam’s perspective.

• Helps with accurate beam alignment and planning.

Digital Reconstructed Radiographs (DRRs):

Images created from CT data that simulate an X-ray view of the patient.

Field placement and arrangement relative to patient

• Based on Beam’s Eye View (BEV).

• Include divergence-corrected anatomy (accounting for how the beam spreads).

• Help guide patient setup and treatment accuracy.

Tools for Optimization:

These tools help shape the dose to match the tumor while protecting normal tissue:

Optimize treatment by bettering it by knowing

•    Radiation quality – energy/type affects depth

    •    Entry point – angle to avoid critical structures

    •    # of beams – more beams = better dose shaping

    •    Field size – fits tumor, limits normal tissue

    •    Blocks – block beam from healthy tissue

    •    Wedges – tilt dose for even coverage

    •    Compensators – adjust dose for body shape

Beam Placement & Shaping – Approach

Many factors come into play w beam shaping

Entry point – where beam enters the body

    •    Field size – outline of the beam on the surface

    •    Blocks – shapes beam in to avoid normal tissue

    •    Wedges & Compensators – modify dose

Modern 3D mplanning incorporates additional strategies to enhance dose conformity and protect healthy tissues

Beam placement is a 3d approach need to know if

Multiple beams: Utilizing several beam angles to distribute the dose more evenly.

    •    Dynamic delivery: Adjusting beam intensity during treatment, as seen in Intensity-Modulated Radiation Therapy (IMRT).

    •    Non-coplanar beams: Delivering beams from different planes to better target complex tumor shapes.

    •    Dose compensation: Using advanced algorithms to modulate dose distribution, not just compensating for missing tissue.

    •    Biological planning: Considering tumor biology and tissue response to optimize treatment.

Dose-Volume Histogram (DVH

Key Features of an Optimal Target DVH:

    •    High Coverage at Prescribed Dose: Most of the target volume receives the intended radiation dose.

    •    Steep Drop-Off Beyond Prescribed Dose: The volume receiving doses higher than prescribed decreases rapidly, minimizing exposure to surrounding healthy tissues.

In radiation therapy, an ideal DVH helps visualize how radiation dose is distributed across different tissues.

Tumor (Target Volume)

High Dose to All: The entire tumor should receive the prescribed dose to ensure effective treatment.

    •    Homogeneous Dose: The dose should be evenly distributed within the tumor to avoid underdosing or overdosing specific areas

Critical Organ (Organ at Risk)

Low Dose to Most of the Structure: The majority of the critical organ should receive minimal radiation to reduce the risk of damage.

These DVH characteristics aim to maximize tumor control while minimizing exposure to surrounding healthy tissue

2D Conventional Radiotherapy

•    Imaging: Utilizes orthogonal X-rays (anterior-posterior and lateral views).

    •    Planning: Based on anatomical landmarks; does not require CT scans.

    •    Beam Arrangement: Simple setups like the 4-field box technique, often used in emergencies (e.g., spinal cord compression).

    •    Limitations: Less precise targeting; higher exposure to surrounding healthy tissues.

3D Conformal Radiation Therapy (3DCRT)

•    Imaging: Relies on CT scans to create a three-dimensional representation of the tumor and surrounding anatomy.

    •    Planning: Forward planning—clinicians manually design beam arrangements to conform to the tumor shape.

    •    Beam Arrangement: Multiple fixed beams shaped to match the tumor’s geometry.

    •    Advantages: Improved targeting compared to 2D; better sparing of normal tissues

Intensity-Modulated Radiation Therapy (IMRT)

Imaging: Advanced imaging (CT, MRI) for precise tumor and organ delineation.

    •    Planning: Inverse planning—computer algorithms optimize beam intensities to achieve desired dose distributions.

    •    Beam Arrangement: Multiple beams with varying intensities; can include dynamic delivery methods.

    •    Advantages: Highly conformal dose distributions; excellent sparing of critical structures; suitable for complex tumor shapes.

IMRT Delivery Technique

•    Step-and-Shoot: Delivers radiation in multiple static segments; the beam is turned off while the multileaf collimator (MLC) adjusts between segments.

    •    Sliding Window (Dynamic MLC): Continuously moves MLC leaves during radiation delivery, allowing for more refined dose modulation.

In radiation therapy, once the target volume, beam orientation, and shape are determined, the next step is

calculate the appropriate beam-on time or monitor units (MUs) to deliver the prescribed dose to the target.

Forward Planning

Application: Commonly used in 3D Conformal Radiation Therapy (3DCRT).

    •    Process:

    •    The planner manually selects beam parameters, including angles, shapes, and modifiers.

    •    After calculating the resulting dose distribution, adjustments are made iteratively to achieve the desired coverage of the target while sparing healthy tissues.

    •    Characteristics:

    •    Relies heavily on the planner’s expertise and experience.

Inverse Planning

Utilized in IMRT to enhance dose conformity to complex tumor shapes.

    •    Process:

    •    The planner defines the desired radiation dose for the tumor and sets dose limits for nearby healthy structures.

    •    The planning system calculates the optimal intensity patterns for the radiation beams, adjusting the multileaf collimator (MLC) positions accordingly.

    •    This iterative process continues until the best possible plan is achieved, balancing effective tumor targeting with the protection of normal tissues.

dose displsu options

ose display options

Dose interacting with patient cooor wash or dose painting

Diff colors diff percentages of dosesinilar to isodose lines

Photon beams

diverge and electron beams bulge at depth.

    •    This makes it hard to line up adjacent fields perfectly.

    •    If not careful, it can cause underdose or overdose at the junction

Used when: adjacent fields

•    Fields are large or irregular

    •    Different beam energies are needed

    •    Treating previously radiated areas

Example: 10 MeV electrons used for deeper targets.

Abutting Fields – Simple & Easy

•    Happen at the skin surface or deeper

    •    If they touch at the skin, beams overlap more at depth (due to divergence)

    •    Can cause hot spots (too much dose where they meet)

    •    Overlap can start just below the surface

    •    Always check tissue tolerances to avoid damage

Gaps – Simple & Easy

•    Gap = space between two fields at the skin

    •    Fields meet (abut) at depth

    •    Used to avoid overdose where beams would overlap

    •    Lower dose is okay near the skin

    •    Helps spare tissue at depth by reducing hot spots

Feathering – Simple & Easy

•    Feathering moves the junction during treatment to spread out dose

    •    Helps avoid hot spots (overdose) and cold spots (underdose) at field edges

    •    Prevents damage where dose exceeds normal tissue tolerance

    •    Gap area alone may not treat the tumor well

To avoid divergence:

    •    Use different jaws (e.g., close Y jaw in supraclav field)

    •    Adjust gantry, collimator, or couch angle to match field edges correctly

Abutting Fields: examples

•    Bilateral Head & Neck with supraclavicular fossa field

    •    Tangential breast fields with supraclav field

gaps examples

•    Craniospinal axis treatments (brain + spine)

    •    Mantle & para-aortic fields for lymphoma

Craniospinal Irradiation – Simple & Easy

What it treats:

    •    Whole brain and spinal cord (craniospinal axis)

Technique:

•    2 lateral brain fields

    •    1 or 2 posterior spine fields

Positioning:

    •    Usually prone

    •    May need 3 spine fields if patient is long

    •    Use gaps at skin to avoid overdose

    •    May need feathering to blend junctions

Blocks:

    •    Help reduce divergence and protect normal tissue

Why Magnification Factor Matters:

•    Helps adjust field size when no gradicule is used

    •    Tracks tumor changes (like shrinking or weight loss)

Key Points:

    •    Magnification Factor = correction used to get real size

    •    Depends on:

    •    Distance from object to source

    •    Distance from object to film

    •    It’s a measured ratio (like 1.05 or 1.10) to adjust for enlargement

Beam Divergence – Simple & Easy

    •    As the beam leaves the linear accelerator, it spreads out (diverges)

•    More SSD = more divergence (due to inverse square law)

    •    Bigger field size = more divergence

Why We Account for Divergence:

    •    To know where the beam will go

    •    To protect nearby critical structures

    •    To adjust angles (collimator, couch, gantry) for better alignment

    •    Can use half-beam block (set one Y jaw to 0) to stop divergence on one side

Note:

    •    There is no divergence at the central axis (center of the beam)