2.1 - Fundamentals of Treatment Planning Cheat Sheet by Molly

Respon­sib­ili­ties: technical planning and delivery of the radiation dose
clinical care
psychosocial care of the patient on a daily basis
preparation, treatment and immediate post treatment phases.

Failure can result in failure to control the disease

Anterior: Also known as ventral and refers to being in front of an organ or at the front of the body

Superf­icial: Refers to on or close to the surface of the body.

Distal: Refers to locations that are further away from the point of origin of a structure or attachment to the body.

Superior: Refers to organs or structures that are above another.

Lateral: Refers to organs or structures that are further away from the midline of the body.

Prone: Refers to a person lying face down.

Metastatic tumour or metastasis is a tumour in a site other than its site of origin

1. Gives the best dose distri­bution to the tumour whilst minimising the radiation dose delivered to surrou­nding healthy tissue

3. Is verifiable

1. Establ­ishing the patient’s treatment position, constr­ucting a patient reposi­tioning immobi­lis­ation device (when needed), obtaining a volumetric image data set of the patient in treatment position (usually CT, and often also MRI and/ or PET imaging)

3. Specifying a prescr­iption dose for the Planning target volume (PTV) and dose-v­olume constr­aints for any OARs, which includes:
a. Forward planning—deter­mining beam orient­ation and designing beam apertures, and computing a 3D dose distri­bution according to the dose prescr­iption
b. IMRT inverse planning—set up initial beam orient­ations and enter optimi­sation parameters (i.e., dose-v­olume constr­aints for PTV(s) and all regions of interest) and initiate TPS optimi­sation process which generates beam fluences, resulting dose distri­bution, monitor units, and leaf motion files

5. Approved plan must then be implem­ented on the treatment machine and the patient’s treatment verified using approp­riate QA proced­ures.

TD5/5 is an estimate of the dose that will give a 5% probab­ility of a given late effect 5 years after treatment. Similarly TD 50/5 is the dose giving a 50% risk of a particular effect at 5 years.

A graphical plot of the dose of radiation and the percentage of volume of a anatomical structure can be produced by the radiation therapy treatment planning systems and is know as a dose volume histogram (DVH).

A hot spot is an area of high dose and a cold spot refers to areas of under dosing

The effect of changing tissue density on the isodose distri­bution isodose curves are created from dose output data measured in water or a water equivalent phantom. The density of water is 1.0gm/cc and is similar to that of soft tissues such as muscle and fat in humans

When using CT data to plan, radiation therapy treatment planning computer systems perform this correction by allocating a CT number to tissue within the scan based on tissue density. This ability to quickly and effici­ently account for tissue inhomo­geneity has greatly enhanced radiation therapy dose calcul­ations.

Inverse Planning The major differ­ences between forward planning and IMRT inverse planning is the use of a computer optimi­sation program that requires a formal descri­ption of the requir­ements using a mathem­atical objective function and constr­aints that are used by the program to find the solution.

Blue is the volume to be treated with radiation

Orange­/brown the rectum.

The area within the MLCs is where the radiation is directed.

Sagittal: Also known as the lateral plane and is a vertical line that divides the body or structure into left and right sides. It runs lengthwise through the body.

Adjuvant therapy prevents cancer from coming back

Chemot­herapy acts as a radio-­sen­sitiser, a drug that makes the cancer cells more sensitive to radiation

If a type of cancer is known to spread to a certain area, doctors often assume that a few cancer cells might already have spread there, even though imaging scans (such as CT or MRI) show no tumour­s.That area may be treated to keep these cells from growing into tumours.

Sometimes cancer spreads too far to be cured. Some of these tumours can still be treated to help relieve symptoms

Specific attention should be paid to organs that, although not immedi­ately adjacent to the CTV, have a very low tolerance dose (e.g. the eye lens during nasoph­ary­ngeal or brain tumour treatm­ents).

Dose criteria for OARs typically depend on the organ’s biological archit­ecture

Parallel organs (such as lung) are frequently planned using more complex dose-v­olume constr­aints

Single field isodose distri­butions are of limited use in the treatment of deep seated tumours, since they give a higher dose near the entrance at the depth of dose maximum than at depth.

• A reasonably uniform dose to the target (±5%);
• A low maximum dose outside the target (<1­10%);
• No organs exceeding their tolerance dose.

Penumbra is defined as the region near the edge of the field margin where dose falls rapidly. The width of the penumbra is influenced by: the size of the radiation source, the source to collimator distance, and the SSD.

Inserting a beam modifier such as a wedge or tissue compen­sator may modify isodose distri­but­ions.

Four field box. A technique of four beams (two opposing pairs at right angles) producing a relatively high dose box shaped region. The region of highest dose now occurs in the volume portion that is irradiated by all four fields. This arrang­ement is used most often for treatments in the pelvis, where most lesions are central. Opposing pairs at angles other than 90º also result in the highest dose around the inters­ection of the four beams.

Wedge Factor (WF) is the ratio of doses at a reference depth with and without wedge for identical field size under similar experi­mental condit­ions.

To increase the surface dose, a layer of uniform thickness bolus is often used (0.5–1.5 cm), since it does not signif­icantly change the shape of the isodose curves at depth.

The divergence or spread of the radiation beam is directly propor­tional to the distance from the source.

This geometric relati­onship is used to determine the magnif­ication factor (MF).

With beam’s eye view (BEV) techno­logy, radiation delivery can be planned so as to ensure that the radiation field adequately covers the target and spares or minimises the dose to the non target healthy tissues

• Develo­pments in electr­onics;

• Advances in radiation protec­tion;

• Refined visual­isation of tumours /human anatomy;

• Unders­tanding individual responses to treatment regimes

Neoplasms can be either benign (non-c­anc­erous), in situ (pre-c­anc­erous) or malignant (cance­rous).

BCC: basal cell carcinoma

Ca: cancer or carcinoma

Gy: gray (unit of radiation equal to 100 rad)

NED: no evidence of disease

1. Patient factors – previous radiation therapy if any, relevant past medical history, perfor­mance status, age, social situation, patients wishes and likelihood of compliance

Radical treatment generally requires higher doses of radiation with the intent of disease control.

Palliative treatment is delivered with the primary intent to relieve pain, and improve quality of life. Palliative doses of radiation therapy are generally lower and delivered over a shorter duration of time.

This radiation therapy prescr­iption defines:

• Dose of radiation in Gy

• Dose per fraction

• Constr­aints to healthy organs surrou­nding the tumour (organs at risk or OAR)

Set-up Margin (SM) takes into account all uncert­ainties in patien­t-beam positi­oning.

IM uncert­ainties are due to physio­logic variations (e.g., filling of bladder or rectum, movements due to respir­ation, etc.) and are difficult or almost impossible to control from a practical viewpoint.

Internal Target Volume (ITV) is the volume formed by CTV and IM

GTV – Gross tumour volume: the palpable or visible extent of malignant tumour.

Treated volume: the actual volume enclosed by the isodose distri­bution repres­enting the prescribed dose of radiation.

Photon beam radiation therapy is carried out with a variety of beam energies and field sizes under one of two set-up conven­tions:

an isocentric set-up with a constant Source to axis distance (SAD).

Reference dose – need to determine what the dose will be at 100% isodose. This is usually isocentre with isocentric techniques or at Dmax for fixed SSD techni­ques. If a fixed SSD technique is used and prescribed at a depth other than Dmax, the dose delivered at 100% needs to be calculated and applied. For example, if a dose of 4 Gy per fraction is prescribed at a depth of 5cm when treating a lumbar spine, which has a %DD of 94%, the dose at 100% would be 4.26Gy per fraction. This value would be used on the top line of the calcul­ation.

OF/CCF/ASF – Output factor­/Cone Correction Factor/ Area Scatter Factor. These are all the same thing, but may be labelled differ­ently depending on which department you go to. All take into account the change in scatter conditions from the jaws with different field size.

ISL/IVSLF – Inverse Square Law or Inverse Square Law Factor needs to be taken into account when treating at a distance that is not the calibrated distance (i.e. 100cm) as this will alter the beam proper­ties. The formula for ISL is: (Source to Calibrated Distance ÷ Source to Ref Point Distance) 2 X100 – Linacs calibrated for centigray, but generally prescr­ibing in Gray, so need to multiply all by 100.

Late responding tissues: express injury several months to years after irradi­ation e.g., kidney, lung.

The risk of major late effects is usually dose limiting in radiation therapy.