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2.1 - Fundamentals of Treatment Planning Cheat Sheet by

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Your role as a Radiation Therapist

RT's are group of profes­sionals with direct respon­sib­ility for the admini­str­ation of radiation therapy to cancer patien­ts.
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.
The multid­isc­ipl­inary team (MDT): Radiation Oncologist or Clinician, Medical Oncology Radiation Physicist (ROMP), Nursing staff and RT’s.0

Beam’s eye view (BEV)

Prevention of Accidental Exposures

“Radiation therapy involves many steps between prescr­iption and dose delivery. Each step may involve a large number of parameters that must be selected, adjusted, recorded and commun­icated between different profes­sio­nals.”
Failure can result in failure to control the disease

Anatomical Terms

Anat­omical positi­on: This is a standard point or frame of reference that describes the human body when it is standing erect, facing forward, feet together flat on the floor, the arms slightly raised from the sides with the palms facing forward.
Ante­rior: Also known as vent­ral and refers to being in front of an organ or at the front of the body
Post­eri­or: Also known as dorsal and refers to behind an organ or at the back of the body.
Supe­rfi­cial: Refers to on or close to the surface of the body.
Prox­imal: Refers to locations that are close to the point of origin of a structure or attachment to the body.
Dist­al: Refers to locations that are further away from the point of origin of a structure or attachment to the body.
Infe­rior: Refers to organs or structures that are below another.
Supe­rior: Refers to organs or structures that are above another.
Medi­al: Refers to organs or structures closer to the midline of the body.
Late­ral: Refers to organs or structures that are further away from the midline of the body.
Supi­ne: Refers to a person lying face up.
Prone: Refers to a person lying face down.

Oncology termin­ology cont.

Primary tumour refers to the original tumour in the body– the site where the cancer first started.
Meta­static tumour or metast­asis is a tumour in a site other than its site of origin
Metastatic cancer has the same morpho­logical name and the same histol­ogical compos­ition as the primary tumour. For example, prostate cancer that spreads to the bones and forms a metastatic tumour is metastatic prostate cancer, not bone cancer.

THE PLANNING PROCESS

The goal of radiation therapy planning is to evaluate the possible treatment approaches and choose one that:
1. Gives the best dose distri­bution to the tumour whilst minimising the radiation dose delivered to surrou­nding healthy tissue
2. Is reprod­ucible
3. Is verifiable
The steps in the radiation therapy planning process include:
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)
2. Cont­ouring target volume(s) and organs at risk using the volumetric planning image data set
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
4. Eval­uating the treatment plan, and if needed, modifying the plan (e.g., beam orient­ations, apertures, beam weights, etc.) until an acceptable plan is approved by the radiation oncologist
5. Approved plan must then be impl­emented on the treatment machine and the patient’s treatment veri­fied using approp­riate QA proced­ures.

Volumes

Tolerance doses

Tolerance doses are often repres­ented as TD5/5 or TD 50/5.
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.
These measures are useful for serial organs such as spinal cord, where exceeding the dose threshold at any point can compromise the whole organ function
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).

Effect of body shape on isodose distri­bution

The isodose curve shape is influenced by the patients shape (ext­ernal body contour) and the difference in the densities of the tissues (inh­omo­gen­eit­ies) within the area of interest.
A hot spot is an area of high dose and a cold spot refers to areas of under dosing
A hot spot becomes clinically meaningful when the minimum diameter of the dose region is greater than 15mm.
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
Bone and air however have different tissue densities than soft tissue or water which needs to be accounted for in the planning process.
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.

Methods of treatment planning

Forward planning The ability to orient beams in 3D allows Radiation Therapists to develop treatment plans that use non-co­planar beams.
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.
Volu­metric Modulated Arc Therapy (VMAT) is a type of intens­ity­-mo­dulated radiation therapy (IMRT) treatment technique that uses the same hardware (i.e. a digital linear accele­rator) as used for IMRT or conformal treatment, but delivers the radiot­herapy treatment using a rotational or arc geometry rather than several static beams.
 

The Process

The process of radiation therapy treatment planning (or often referred to just as planning) calls for the integr­ation of the physical findings and diagnostic imaging inform­ation with knowledge of the pertinent anatomy, pathology, and natural history of the patients particular tumour type

BEV

Beams eye view (BEV) shows a reco­nst­ruction of the patients images (CT images) to create a digi­tally re-con­str­uctured radiogragh (DRR) with an overlay of the radiation fields as viewed from the radiation beam itself i.e the beams eye view. The example above shows 6 different angles of the BEV for a pelvic treatment.
Blue is the volume to be treated with radiation
Yellow the bladder
Orange­/brown the rectum.
Green square is the field size, that is set on the linear accele­rator, and the jaggered teeth like shapes in green are the mult­i-leaf collim­ators (MLCs)
The area within the MLCs is where the radiation is directed.

Anatomical Planes

Frontal or Coronal: is a vertical line that divides the body or structure into anterior and posterior sections. It runs lengthwise through the body.
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.
Tran­sve­rse: Also known as the axial or cros­s-s­ect­ional plane. It is a plane that runs horizo­ntally through the body or structure and divides it into superior and inferior sections.

The role of Radiation Therapy

Radical Intent: To cure or shrink early stage cancer
Adjuvant therapy prevents cancer from coming back
For cancers that can be cured either by radiation or by surgery, radiation may be preferred because it can sometimes preserve the organ’s function
Chem­oth­erapy acts as a radi­o-s­ens­iti­ser, a drug that makes the cancer cells more sensitive to radiation
The drawback of giving chemot­herapy and radiation together is that side effects tend to be worse. It’s often better to use radiation before or after chemo.
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.
Pall­iative Intent: To treat symptoms caused by advanced cancer
Sometimes cancer spreads too far to be cured. Some of these tumours can still be treated to help relieve symptoms
Radiation might help relieve symptoms such as pain, trouble swallowing or breathing, or bowel blockages that can be caused by advanced cancer.

Radiation therapy CT-sim­ulator

A radiation therapy CT-sim­ulator consists of a diagnostic quality CT scanner, laser patient positi­oni­ng/­marking system, virtual simulation 3D treatment planning software, and various digital display systems for viewing the digital recons­tructed radiog­raphs (DRRs)
The CT scanner is used to acquire a volumetric planning CT scan of a patient in treatment positi­on.

Organs at risk (OAR)

The organ at risk is an organ whose sensit­ivity to radiation is such that the dose received from a treatment plan may be signif­icant compared with its tolerance, possibly requiring a change in the beam arrang­ement or a change in the dose.
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).
Planning Organ at Risk Volume (PRV) a margin is added around the OAR to compensate for that organ’s spatial uncert­ainties
Dose criteria for OARs typically depend on the organ’s biological archit­ecture
Serial organs (such as the spinal cord) often have a maximum dose constraint
Parallel organs (such as lung) are frequently planned using more complex dose-v­olume constr­aints

Medical termin­ology

Dosimetry

Isodose lines or curves give a visual, as opposed to a tabular, repres­ent­ation of the dose at various positions across the radiation field. The data for each isodose curve is obtained from measur­ements acquired in a homoge­nous, usually water filled phantom, where all the points within a radiation field have the same percentage depth dose
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.
Guidelines for single photon beams:
• A reasonably uniform dose to the target (±5%);
• A low maximum dose outside the target (<1­10%);
• No organs exceeding their tolerance dose.
Single fields are often used for palliative treatments or for relatively superf­icial lesions (depth < 5–10 cm, depending on the beam energy)
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.
Mult­i-field isodose distri­but­ions When multiple beams are utilised for a patient treatment, isodose distri­butions provide an effective means of visual­ising the resultant combined beam dose.
Inserting a beam modifier such as a wedge or tissue compen­sator may modify isodose distri­but­ions.
Wedge pair. Two beams with wedges (often orthog­onal) are used to achieve a trapezoid shaped high dose region. This technique is useful in relatively low lying lesions.
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.
Three field box. A technique similar to a four field box for lesions that are closer to the surface (e.g. rectum). Wedges are used in the two opposed beams to compensate for the dose gradient in the third beam.

Beam modifi­cation devices

Wedges
Wedge Factor (WF) is the ratio of doses at a reference depth with and without wedge for identical field size under similar experi­mental condit­ions.
Bolus is a tissue equivalent material placed in contact with the skin to achieve one orboth of the following: increase the surface dose and/or compensate for missing tissue.
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.
To compensate for missing tissue or a sloping surface, a custom made bolus can be built that conforms to the patient’s skin on one side and yields a flat perpen­dicular incidence to the beam.

Magnif­ication Factors

Images such as digitally recons­tructed radiog­raphs (DRR’s) used in radiation therapy simula­tion, treatment and planning all display an enlarged or magnified image of the object of repres­ent­ation. This occurs as the imaging device is always placed a greater distance from the object being imaged. The degree of magnif­ication is dependent on the geometric arrang­ement of the source (or x-ray target), the patient or object being imaged, and the imaging device
The divergence or spread of the radiation beam is directly propor­tional to the distance from the source.
To determine the magnif­ication factor of an image we need to know the target to image distance and the target to object being imaged distance, or the size of the object being measured.
This geometric relati­onship is used to determine the magn­ifi­cation factor (MF).
 

Planning of radiation fields

Contem­porary treatm­ent­-pl­anning computers allow the incorp­oration of 3- dimens­ional anatomic data into the planning of radiation fields
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

Planning of radiation fields cont.

Radiation therapy is a clinical discipline that is strongly influenced by constant change in techno­logy.
Recent develo­pme­nts:
• Advances in comput­erised treatment planning;
• Develo­pments in electr­onics;
• Research in biological effects of radiation;
• Advances in radiation protec­tion;
• New techno­logies in the areas of cancer diagnosis;
• Refined visual­isation of tumours /human anatomy;
• Research and unders­tanding of genetics;
• Unders­tanding individual responses to treatment regimes
The aim is to deliver an adequate dose of radiation to the tumour, whilst minimising dose to the surrou­nding normal tissues.

Medical termin­ology

Oncology termin­ology

Ocol­ogy (Greek ‘oncos’ = tumour) is concerned with the study and treatment of neoplasms. Neoplasm means ‘new growth’.
Neoplasms can be either benign (non-c­anc­erous), in situ (pre-c­anc­erous) or mali­gnant (cance­rous).
Cancer is the general term given to a range of neoplasms, occurring when a group of cells grows and multiplies uncont­rol­lably. Approx­imately 200 different types
Abbre­via­tions:
Adj: adjuvant therapy
BCC: basal cell carcinoma
bx: biopsy
Ca: cancer or carcinoma
Chemo: chemot­herpay
Gy: gray (unit of radiation equal to 100 rad)
mets: metastases
NED: no evidence of disease
TNM: tumour node metastases (this is a staging system for tumours)

Pre-pl­anning and the planning process

Prior to commencing any Radiation Therapy treatment several factors must be consid­ered:
1. Patient factors – previous radiation therapy if any, relevant past medical history, perfor­mance status, age, social situation, patients wishes and likelihood of compliance
2. Tumour factors – type of tumour, extent of disease, natural history of disease, treatment intent, treatment options, expected toxici­ties, known clinical outcomes of treatment management approa­ches.
STAGING AND PRE- PLANNING
Stag­ing is determ­ining the extent of the patients’ disease, after which trea­tment intent is decided ie. radical (curative, adjuvant) or pallia­tive.
Radical treatm­ent generally requires higher doses of radiation with the intent of disease control.
Adjuvant radiation therapy is delivered in addition to another cancer treatment (often surgery). Adjuvant treatment is given after the primary treatment to lower the risk that the cancer will come back.
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.
Once treatment intent is determ­ined, the Radiation Oncologist provides a trea­tment prescr­ipt­ion.
This radiation therapy prescr­iption defines:
Tre­atment volume the area to be treated
Dose of radiat­ion in Gy
Frac­tions :The total number of radiation treatments
Dose per fraction
Freq­uency of treatm­ent (daily, twice a week, twice a day etc);
Cons­tra­ints to healthy organs surrou­nding the tumour (organs at risk or OAR)
• The planning techni­que or approach to delivering the intended treatment may also be specified by the Radiation Oncolo­gist, or in some situations or clinical centres this decision is determined by the Radiation Therapist, or possibly as a team decision.
These factors are all essential to establish prior to planning commen­cin­g.

Defining the treatment volume

Internal Margin (IM) takes into account the variations in size, shape, and position of the CTV
Set-up Margin (SM) takes into account all uncert­ainties in patien­t-beam positi­oning.
The IM is referenced to the patient’s coordinate system using anatomical reference points and the SM is referenced to the treatment machine coordinate system.
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.
SM uncert­ainties are related largely to tech­nical factors that can be dealt with by more accurate setup and immobi­lis­ation of the patient and improved mechanical stability of the machine.
Internal Target Volume (ITV) is the volume formed by CTV and IM
Planning target volume (PTV) is formed by the CTV, and the IM and SM combined. It is the volume required to receive the prescribed dose of radiation.
GTV – Gross tumour volume: the palpable or visible extent of malignant tumour.
CTV – Clinical tumour volume: is the GTV with a margin added to include sub clinical spread of disease. The CTV is usually stated as a fixed or variable margin around the GTV (e.g. CTV = GTV + 1 cm margin), but in some cases it is the same as the GTV (e.g. prostate boost to the gland only).
Treated volume: the actual volume enclosed by the isodose distri­bution repres­enting the prescribed dose of radiation.
Irra­diated volume: the volume that has received a signif­icant radiation dose in relation to normal tissue tolerance.
Photon beam radiation therapy is carried out with a variety of beam energies and field sizes under one of two set-up conven­tions:
a constant Source to surface distance (SSD) for all beams
an isocentric set-up with a constant Source to axis distance (SAD).
In an SSD set-up, the distance from the source to the surface of the patient is kept constant for all beams, while for an SAD set-up the centre of the target volume is placed at the machine isocentre.

Five field radiation technique.

Dose Calula­tions

Radi­ation therapy treatment planning systems (RTTPS) are utilised extens­ively to produce an approp­riate isodose plan
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.
TAR/TPR – Tissue Air Ratio or Tissue Phantom Ratio. The only difference between these labels is the depth used to calibrate the data (1.5cm vs 10cm), which is why TPR tends to be used with higher energies. If you are treating with a fixed SSD technique, the TAR will not be applied as the isocentre is on the skin surface. This factor takes into account the scatter conditions in tissue
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.
WF and PF – Wedge factor and Plate factor. Any access­ories in the path of the beam will attenuate the beam, and this needs to be taken into account when calcul­ating monitor units.
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.

Normal Tissue Tolerance

Acute responding tissues: express injury during or within 2-3 weeks of the completion of radiot­herapy e.g., skin, oral mucosa
Late responding tissues: express injury several months to years after irradi­ation e.g., kidney, lung.
In clinical radiation therapy the volume of tissue irradiated is an important factor determ­ining the clinical tolerance of an organ.
The risk of major late effects is usually dose limiting in radiation therapy.

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