physics treatment planning systems (lecture 3)

Gross Tumor Volume (GTV)
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Organ filling changes (bladder, rectum), changes throughout treatment, breathing.ITV accounts for tumor motion due to:2D simulationPlanar films are used for alignment and field setting. No heterogeneity correction used. "How thick is the patient" for alignment purposes. No info about tissue.3D simulationCT Scan in simulated treatment position. Heterogeneity correction and good localization of target volume4D simulationTargets in the thorax and abdomen are in motion during treatment. Phases of the breathing cycle can be scanned to assess target. ITVs or gating can be used to compensate on treatment.Bins10 slices images for 4D in the same spot of the body to monitor breathing cycle.2D techniquesGenerally for emergencies only or Total Body Irradiation. More common for regular patient care in 3rd world countries3D techniquesStationary/fixed beam. Uniform intensity delivered from each field. Includes wedging, even though wedges technically deliver a non-uniform intensity.shape of the target volume, the type of tissues adjacent to that volume, the patient's size, and the beam energies available.The choice of beam combinations is dependent on theConformal Arc TherapyContinuously rotating gantry that does not use the MLC to vary the intensity of the delivered beam. Static conformal arc, Dynamic conformal arcIntensity Modulated Radiation Therapy (IMRT)high-dosage radiation delivered via a beam that changes its dosage and shapeIMRTStationary fields delivered at different gantry angles with varying apertures (MLC)Parallel opposed fieldsa uniform dose is delivered (±5-10%) to a very large volume from d/max at the AP field to d/max at the PA field.VMATcontinuously rotating gantry with varying apertures (MLC)Parallel opposed fields which tumorsThis distribution is good for large lesions or central tumors that also include superficial nodal areas.When parallel opposed fields is not optimumDose distribution is not optimum when target is close to sensitive organs at risk.Three field plansreduce the dose to superficial tissues while maintaining a high central dose. Whenever another beam is added, the isodose lines are pulled in the direction of that new beamFour field boxConsists of two sets of parallel-opposed fields at right angles to each other.Four-field diamond techniqueVariation that can be used to avoid treating through the femoral heads. The rectal doses can also be reduced using this method, as compared to the four-field boxConformal ArcLinac rotates around isocenter to create an extremely high dose gradient around a very small region. SRS of brain. Can be used with cones. High-resolution MLCs.Non-uniform fluencedelivered in IMRT from each stationary beam angleIMRTwhere the radiation beam fluence is non-uniform allows for greater shaping of dose distributions within the body and hence increased sparing of normal tissues. MLC's moving to do this.FluenceThe number of radiant-energy particles emitted from or incident on a surface in a given period of time, divided by the area of the surfaceVMAT leavesthe leaves are in continuous motion as the gantry rotates around isocenter."Control Point"Each delivered segmentDifferences of IMRT/VMATsteep dose gradients (if you miss then you miss big), portions of the target are completely covered.Tissue inhomogeneitiesvolumes within the patient that have non-uniform tissue densities.Heterogeneity correctionscorrections that account for the inhomogeneities. CT scan data is required to apply for these correctionspixelsThe patient contour is filled with a matrix of calculation points. Dose is calculated at each of these pointsvoxelsIf images are stacked in slices, the pixels take on a 3rd dimension and become•CT based planning systems (Model/Simulation algorithms)•Elekta (XiO, Monaco) •Varian (Eclipse) •RaySearch (RayStation) •Philips (Pinnacle) •BrainLAB (iPlan)Second Check Systems•Second Check Systems (Correction-based algorithms) •MU check •RadCalcLeast accurate dose calc algorithmcorrection based methodsclinical standard dose calc algorithmmodel based methodmost accurate (gold standard) algorithmmonte-carlo simulationCorrection Based Methodsthe dose is calibrated at a single point under known conditions and all other conditions are "corrected" using tables normalized to this single calibration point.Generates for correction factors in one single MU calcmachine settings (field size, wedges) and patient parameter (SSD, depth)1% transmissionTransmission between MLC leaves. So if it was at 0 degrees you would be getting extra dose.The linac and the patientThe problem of dose calculation is broken up into two zones:The linacThe radiation exiting the linac head can be computed as beamlets. The model of the linac head may or may not be based on monte carlo simulations, but it is not re-simulated for patient planningThe patientThis is where use of Models vs. Monte Carlo differMonte Carlo Methodmethod was named after a famous casino in Monaco because it relies on interaction probabilitiesMonte CarloPre-calculated histories of Linac machine settings are stored in tabular form and accessed for beam calculation. •Particle histories are simulated from the entrance of the patient. The direction and energy distribution of compton scattering events are traced until all of the energy is absorbed (down to a threshold value). Both photons and scattering electrons need to be traced"Random Walk"the path of a variable whose changes are impossible to predict. Where electrons go.TG-53Going over rad therapist responsibilities. provides an extensive amount of tests that check all aspects of the TPS. •need to be "Accepted" from the vendor and then "Commissioned" before treatment.Image storage/transfer, contouring, planning, dose calcs, plan export.Acceptance and Commissioning tests of softwareDICOM transfer integrity and software functionalityAcceptance tests verifyCommissioning tests•PDDs •Profiles •Output factors •MLC verification •Asymmetric field verification •Contour & DVH verification •Point measurements •Special tests •Oblique incidence •Surface irregularity •Tangential geometry •IMRT/VMAT Test cases •TG-119 (phantom tests) •Clinical cases (patient imaging used)Linac Commissioning dataverify the dose calculated for all of these various fields configurationsRadiation therapist rolesQA is very important. •Checking setups for safety •Prescription accuracy according to physician orders •Monitor Unit verification •Field parameters •Imaging orders •Patient Specific QA performed. Have to be able to judge when errors in treatment planning may have occurred.Safety notices are also released by the ______when "bugs" are identifiedvendorWhich factors influence the construction of a PTV from a CTV?Patient set-up uncertainty Organ motion Proximity of a critical structureWhen photon beams are wedged, the "wedge angle" ishe angle through which the isodose curve at d = 10 cm is turned by the wedgeVariations in size, shape, and position of the target due to a patient's breathing and internal organ motion are accounted for in creating theITVWhich type of dose calculation algorithm is considered the most accurate?Monte Carlo Method