Plan evaluation

Breast Cancer : Radiation therapy Plan evaluation & Recent advances in RT 4DCT Respiratory GatingDr. Lokesh Viswanath M.DProfessor, Dept of Radiation OncologyKidwai Memorial Institute of Oncology

AROI KC July 2016 : Lecture no 4 (Rad Onc)

The goal of radiotherapy is to deliver a prescribed dose of radiation to the Target while sparing surrounding healthy tissues to the largest extent possible

RT Planning for Breast CancerAs treatment techniques become more complex - - - > Plan evaluation >> >>> cumbersomeInteraction with the Radn Physcist at various steps There is loss of clinical feeling of the adequacy of treatment plan - Virtual

Need for Plan Evaluation : to find optimal radiotherapy machine settings to deliver desired dose distribution, these settings are patient specificTo assess the target coverage and normal tissue sparing

Plan Evaluation ProcessAfter contouring The Physics team works on the RT planRad Onc Plan evaluation and supervision at various steps

Iterative & interactive ProcessInitial beam arrangementPrimarily based on clinical experience:CW: MT:LT CW + Ax&SC:MT:LT + APBCRT: MT:LTBCRT+ Ax&CS: MT:LT + APPost AB: MT:LT + AP + PAUse of BEV displaysBeam orientation feasibilityMLC settingsBlock apertures : Cardia + electrons for under-dosed zonesReview of dose distributions generatedMultilevel 2D display (isodose lines superimposed on CT images)Color wash (superimposed)DVHModification of beam arrangements based on above parameters

2F 3FFIF>5F (Tangential / every 300)non CoplanarRotational (complete/limited)

Rooms eye view REVTo Display dose cloud along the rendered PTVs & OARHot & cold spotsSkin view -> beam aperture projection on the skin of virtual patient. Suggest : autocontour bone 3D skeletal View with skinPlan approvalPlanned dose distribution approve (Uniform dose delivery to TV)+7% to 5% of the prescribed dose with dose to critical structure below tolerance levelConstrains of absolute Max dose, Median dose & volumes specifiedEvidence based Dose volume constrains : Target & OAR (document reason for non compliance)

Dose reporting and dose prescription:

ICRU reference pointLocationClinically relevant / unambiguouswhere it can be accurately determinedIn a region where there is no steep gradientGenerally - Centre of PTV / intersection pointSingle spatial reporting : dose volume reporting

TVPoint dose ( Grid assigned to single voxel)PastMin DoseMax DoseICRU 83Near Minimum Dose (D98%)Near maximum Dose (D2%)Median Dose (D50%)For serial like organ / structuresMaximum Dose > single calculation point Dmax or D 0%D2% to be reported

Dose homogenity

Dose coverage of PTV to be kept within specific limit +7% & -5% of the prescribed doseIf the degree of desired homogeneity cannot be achieved : Radiation Oncologistto decide weather the dose heterogenity is acceptablePart of PTV with high risk CTV /GTV higher dose here might be advantageousSlight under dose of PTV is acceptable particularly if it is in close proximity to OARCheck single Fraction coverage of PTV in absolute dose mode > evaluate hot & cold zones> change dose per fraction if necessaryFreedom to prescribe parameters in his or her own way or current practice

IMRT plan evaluationComplex UnconventionalDose distribution > highly conformalDoseDose volume parameters Min DoseMax DoseMin Dose to specified fractional volumeVolume structure receiving a specified dose or higherD98% or D near Minimum dose to at least 98% of PTVCorresponding D2% - dose received by the most heavily irradiated 2% of PTV

DVH : Differential /cumulative

Do not provide any spatial informationDVH complements > spatial dose distribution tool (isodose)

Designing beam

Beam orientation > is it possible to setup clinical judgment , test for clearance between gantry/patient/couch

hard copy:: Evaluate for geometric accuracy of plan output . (note - issues with CT simulation artifacts)

Note grid size (effect on dose distribution) Bin size (effect on DVH)

QA - supervisionPlan review> approve > sign &dateBeam normalization isocentre (shift to suitable location in case of non tissue medium/under or near MLC)MU to realize the dose prescription (independent check/hand calculation/independent computer calculation). IMRT additional check review optimization parameters, min gap size, min MU/seg, max dose in /out of targetPhantom measurements, Machine point dose and spatial distribution

Observe patient setup

MLC setting Block fabrication / mountingReview portal imagesWedge / compensator alignments

Verification SimulationEPIDu/s Video surfacingStatic KV imagingKV CBCTMV helical CBCTTMV CBCT

Issues with Respiratory motion Respiratory Gating: Introducing Systematic error in our favour

RT for Breast CancerChallengesLarge difference in tissue thickness in RT field : IMRTClose proximity to Lung / Apex & HeartTarget motion during breathingRT field skin boundary tissue / airSignificant inhomogenityMost planning system (inverse) cannot handle skin flash appropriately

Setup uncertanities

Breathing motionBreast tissue mobile (portion of breast tissue may move out of skin line)

IMRT : SolutionExpand PTV & optimize coverage of entire PTVPortion of PTV in air > add virtual tissue / manually open certain imrt segments to take care of skin flashInterest in IMRT (FIF)Left Ca Br spare myocardium from high dose regionimprove PTV coverage

PROBLEMS OF RESPIRATION MOTION DURING RADIOTHERAPYA. Image acquisition limitationsB. Treatment planning limitationsC. Radiation delivery limitations

RT delivery limitationsDelivery in the presence of Intrafraction Organ motionResults in deviation between intended dose and dose actually deliveredAveraging/smearing of RT dose over the path of motionMotion artifact > dose variation >20% single filedCare during Hypofractionated RT

Recognize the effect of Respiratory motion on CT simulation for RT planningImage artifacts : planning CT / CBCTArtifact significant & unpredictableDifficulty in Tumor visualization Uncorrected > lead to uncertainties in Target visualizationBeam placement Compromise overall effectiveness of treatmentScan speed Slow T smeared Faster - T position and shape captured in arbitary

Ways to compensate for motion: to minimize its impact on treatment integrity

>> 4 D imaging > 4D target delineation Increasing planning marginAbdominal compression (forced shallow breathing)Respiratory gatingReal time tumour tracking

Motion artifact

4D CT

Respiratory Gating

2 main approaches Internal e.g. RTRT implanted marker. (Precise, real-time localization during RT) fluroscopic imagingExternal External respiratory surrogates Markers on abdominal/Thx surfaceCompression beltSpirometer signals

The location of the infrared camera at the foot of the couch for tracking the marker block in the RPM system. 4D Imaging : 4DCT

Respiration phase Tagged image acquisition multiple data sets

Methods used in the management of respiration motionRespiratory gated techniques.Breath-hold techniques.Forced shallow breathing methods.Real-time tumor tracking methods.

ONOFF

RPM

Light weight plas tic box with 2-6 passive infrared markersPatients abd wall xipisternumMonitor charge coupled device video camera (Imaging & Treatment room)Surrogate signals of surface motion (amplitude / phase gated)RPM during CT simulation to acquire pt geometry in gating window and to setup gating windowMajor strength:Non invasiveEasy to use Well toleratedTechniqueBreath holdDeep inspiratory breath holdPatients ability for breath hold >15sec , repeatedlyBreathing coaching: any monitoring technique can be usedSurface markerSpirometerABC deviceRPMAlign RT

ABC

ABC

Patient A

Free Breathing

As the patient inspires: observe air entry anterior to cardia . Separation of cardia and chest wall > 8mm. Also not the change in shape of the mediastinum / cardia

Free breathig & DIBH: note the separation achieved between the cardia and chest wall

Note the portion and volume of chest wall that would have got irradiated during free breathing

Note the portion of cardia exposed in the tangential field during conventional 3 D CRT Plan in free breathing

Note the exclusion/sparing of cardia in the tangential field during conventional 3 D CRT - Field in Field IMRT Plan in Deep Inspiratory Breath Hold

Limited Tangential zone Rapid Arc with

FIF 3DCRT (Tangential Fields) v/s Rapid Arc

ResultsFree BreathingDIBHSD +SD +Lt Lung (V20)25.91 cc1.616.4 cc1.9Heart DoseMean8.1Gy1.52.9 Gy1.06Maximum50.6 Gy1.631.44 Gy13.3

Tumor trackingMost idealMost technologically intenseReal time tumour localizationDynamicallly / seamlessly integration : Fast processing & relay of infoCorresponding repositioning of beamMotion freezing methodsReal time 3D position informationMarker lessMarker guidedImplantable transponders

Real time video guided IMRT

Camera capture full fram 3D surface image through single snapshotPatient setup parameters determined semiautomaticallyIMRT leaf segments are modified in real timeSystem compensated for changes in surface topology by changing treatment plan rather than adjusting position

Thank you

Interactive Secession for Students

Technical issuesPatient positionArm abducted >90 0 - Ax Ly overlap humeral head< 90 0Large pendulous breast SupineLateralProne

MT : LT

Separation:> 22cms > dose in-homogenity > less cosmetic resultUse 10-18Mv (50%)Maintain in-homogenity between 93 % - 105%Use Degrader to modify buildup in beams Use simple IMRT (FIF) or DMLC Alignment of TangentialCW contour / slope : Pt positioned - SlopeCollimator rotationBeam splitterMLCSC field : Tangent Superior edge remains true vertical

RT CW+RNTechnically challengingField matching difficultiesAnatomic variations between patientsLack of clear evidence superiority of any single approachField matchingSC & CW : just below clavicular headSingle isocentre techniqueCW & IMN match line (hot /cold)

CLDCLDIpsilateral lung1.5 cms 6%2.5 cms16%3.5cms26%

Special attention to minimizing volume of heart irradiatedCardiac sequelae : even small amount of heart in field can affect cardiac function

SolutionsField PlacementsCardiac Block3DCRT / IMRTDIBH

Thank You