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Intensity-modulated Radiotherapy
This document provides an overview of Intensity Modulated Radiotherapy (IMRT). It discusses the shift from conventional to conformal radiotherapy using improved imaging and planning techniques. IMRT allows customization of radiation dose distributions through non-uniform beam intensities achieved using dynamic multileaf collimators or compensators. The clinical implementation of IMRT requires treatment planning and delivery systems. IMRT offers advantages over conventional radiotherapy for many cancer types and its use has increased substantially in recent decades.
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Introduction to IMRT by Dr. Vijay P. Raturi, a key advancement in radiotherapy.
Shift from Conventional RT to Conformal RT emphasizing the IMRT evolution and planning advancements.
IMRT's historical achievements from proposals in the 1960s to widespread adoption by 2004.
Intensity modulation techniques include compensators, wedges, custom dose distributions, and non-uniform fluence.
IMRT requires advanced planning and delivery systems for effective treatment of cancers.
Forward and inverse planning methods for defining beams, optimizing treatment plans.
Ensures accurate target volume definition using imaging modalities and segmentation protocols.
Key parameters in treatment planning including dose constraints, optimization, and DVH evaluation.
Details on multileaf collimators, their design, advantages, and the effects of leaf transmission.
Generation of leaf motion files for dynamic vs. static MLC delivery advantages, disadvantages.
IMRT's application in various cancer sites with notable effectiveness and improvement in dosimetry.
Integration of IGRT with IMRT for targeting moving tumors and patient respiratory management.
Utilization of marker systems for tracking respiratory patterns during treatment.
Algorithms for dose calculations, quality assurance procedures to ensure treatment safety and effectiveness.Specifics of IMRT delivery, parotid dose management, and improvement in treatment outcomes.Summary of IMRT benefits in reducing toxicity and enhancing treatment precision, concluding remarks.
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Intensity-modulated Radiotherapy
- 1. Intensity Modulated Radiotherapy PRESENTOR:- DR.VIJAY.P.RATURI
- 2. • To achieveGoal of Radiotherapy. • During past decades, with advances in radiological imaging and computer technology. Shift from Conventional to conformal radiotherapy
- 3. • Conventional RT:-Irradiation of pt with few beam direction, all beam are aimed at single point denoted as isocenter. • Radiation shape from each beam are manually drawn on the projected 2D images taken from an Xray Simulator. • Difference in conventional and conformal is planning CT is used to to define tumor volume and to design treatment portal accordingly • Conformal RT: 3DCRT & IMRT • Transition from 3DCRT to 4DRT.
- 4. • IMRT wasProposed over 50 years ago by Takahashi in Japan (Takahashi et al). • 1st attempted in the late 1960s by Hellman and colleagues . • In mid-1990s, IMRT began to be used in different academic centers. • Not till the late 1990s with the availability of commercial treatment planning systems did IMRT start to become widely available
- 5. • In the2002 survey, 32%of radiation oncologists were using IMRT.“(Mell LK, Roeske JC, Mundt AJ. Cancer2003;204-211) • In the 2004 survey, this percentage increased to 74%. (Mell LK, MehrotraAK, Mundt AJ. Cancer2005;104:1296) • While commonly available, it is being used to treat only a subset of patients at most centers.
- 6. • Process ofchanging beam intensity profile to achieve composite plan is called intensity modulation. INTENSITY MODULATORS - Compensators - Wedges - Dynamic Multileaf collimators
- 7. Intensity modulated radiotherapy:- •It’s a technique in which non uniform fluence is delivered to the patient from any given position of the treatment beam to optimize the composite dose distribution. • It assigns non uniform intensity/weights to tiny subdivision of beams ( a.k.a rays or beamlets) • Permits greatly increased control over the radiation fluence ,enabling custom design of optimum dose distributions.
- 8. The clinical implementationof IMRT requires at least two systems: • Treatment planning computer system that can calculate non uniform fluence maps for multiple beams directed from different directions to maximize dose to the target volume while minimizing dose to the critical normal structures. • System of delivering the non uniform fluences as planned.
- 9. IMRT system consistof :- • Optimization • Dynamic mlc conversion program • Dosimetry verification tool.
- 10. FORWARD PLANNING Planner manuallyselects no of beams, direction & shape inclusion and exclusion of wedges , weighing of each beam (manual optimization) Computer calculates resultant dose distribution from beam parameters Planner can make adjustment to improve the plan
- 11. INVERSE PLANNING (PROPOSEDBY BRAHME IN 1988) Define orientation & energies of all the beams Planner specify desired dose limits(or constraint ) for PTV & all region of interest computer optimization algorithm first divides each beam into many small “beamlets” (or “rays” or “pencil beams”) Alteration of beamlets until the composite 3-D dose distribution best conforms to the specified dose objectives. Computer then calculate the sequence of MLC leaf motions
- 12. IMRT PLANING Positioning & immobilization Image acquisition (CT,MRI) Structure segmentation IMRT t/t & planing File transfer & management Plan validation Position verificationIMRT t/t & delivery
- 13. POSITIONING AND IMMOBLIZATION •Highly conformal nature of the IMRT. • Techniques to reduce or follow the internal organ movement. • Patient comfort.
- 17. IMAGE ACQUISITION • Goalof treatment should be discussed. • For Inverse planning a CT scan is required . • Slice spacing not more than 5 mm. • Different Imaging modality may required to identify the correct target volume and OAR (CECT,MRI,PET, etc) called as image augmentation • Should be compatible with the available treatment planning system.
- 18. STRUCTURE SEGMENTATION orImage segmentation • Most important and critical step • The success of IMRT is closely tied to the accuracy of the target and OAR delineation • Department should develop a protocol. • Should follow the guidelines (RTOG, relevant atlas ). • ICRU 50,62 should be used for the nomenclature, Prescribing, Recording, and Reporting.
- 20. TREATMENT PLANNING • Beamenergy , No of beams and orientation • Isocenter placement (ICRU Reference point) • Min Dose, Max Dose, Dose volume ( Biological Constrains) • Optimization – Fluence Modulation • Deliverability ( Reasonable MU, Treatment time) • Final dose calculation( Dose grid size, algorithm)
- 21. PLAN EVALUATION • IMRT/VMATplans needs to evaluated very carefully • Inspecting and Comparing DVH is useful. • Planner needs to be inspect the Isodose in each slice. • Is the dose uniformity in the target acceptable? • Are the stated plan goals for the hot spots and target coverage satisfied? • Are the stated plan goals for normal tissue sparing satisfied? • Is the organs contoured entirely or not ?
- 22. Characteristic of MLCs Thedesign system of MLC differ in many ways with respect to their manufacturer:- a> Location of MLC relative to the conventional Jaws b> Single focused or double focused MLCs( Elekta & Varian are single foccused) c> Physical characteristic or shape of the leaves d> Leaf movement restrictions e> Maximum achievable field size
- 23. Advantage : Smallerand lighter MLC design with physical clearance between gantry head and patient
- 26. • Most MLCshave cutting tongue and groove pattern on the side of leaves • It reduces the radiation leakage
- 27. Leaf end transmission Convexleaf edge maintain a geometric pneumbra over all field size
- 29. GENERATION OF LEAFMOTION FILES :- • “Leaf sequencer” is an algorithm that translates the beam intensity pattern produced by the IP system into an instruction set describing how to move the MLC leaves during beam delivery. • Result of optimization may be either continuous 2-D intensity profiles, or discrete 2-D intensity matrices with discrete resolution. • DMLC delivery (referred to as a sliding window) delivers a continuous 2-D intensity profile, whereas STATIC delivery (referred to as step & shoot) results in “discretized” intensity patterns.
- 30. ADVANTAGE OF STATICMLC :- • Advantages of static, or step-and-shoot, IMRT are that it is conceptually simple, there are no requirements to control the individual leaf speeds (thus simplifying the MLC control system) or delivery dose rate. • Interrupted treatment is easy to resume, easy to verify an intensity pattern for each field, and fewer MUs are required in comparison with DMLC delivery. DISADVANTAGE OF STATIC MLC :- • Require prolonged treatment time, particularly for complex intensity patterns
- 31. ADVANTAGE OF DYNAMICMLC :- • The advantage of DMLC delivery is that it can deliver a smoothly varying intensity profile. DISADVANTAGE OF DYNAMIC MLC :- • Delivery mechanism is rather complicated, involving leaf-speed and dose-rate modulation • Requires the precise control of individual leaf speeds; and small errors in the calibration of leaf position could (for very highly modulated fields) result in significant errors in delivered doses
- 32. • IMRT isbecoming a mature technology, and is widely applied to many cancer sites. • Many treatment-planning comparison studies have demonstrated the clear dosimetric advantages of IMRT. • Clinical results from the past decade have shown the improvement of local tumor control and reduction of treatment toxicities for prostate cancer, head and neck cancer, and other cancers. IMRT Utilization
- 33. Site % Prostate 85% Headand neck 80% CNS 64% Gynecology 35% Breast 28% GI 26% Sarcoma 20% Lung 22% Pediatrics 16% Lymphoma 12% IMRT PRACTICE SURVEY (2004) TOP TREATED SITES Mell LK, Mundt AJ. Survey of IMRT Use in the USA-2004 Cancer 2005;104:1296
- 34. Volume modulated arctherapy • Radiation dose is delivered to the tumor while simultaneously moving the MLCs leaves and the gantry. • Both the dose rate and gantry speed vary in this system. • Advantage: a> excellent dose distribution in deep seated tumors b> faster delivery c> Reduction of MUs ( decreasing the risk of secondary cancer)
- 35. Image guided radiotherapy •IGRT = IMRT with image guidance • Goal of IGRT is to eliminate or reduce the uncertanities associated with target volume defination and it intrafractional and interfractional organ motion • 4DCT, 4DPETCT , 4D-CBCT are applied to R.T guidance and verification • Respiratory gated IMRT
- 36. RESPIRATORY GATING WITHIMRT •Cancers that are subject to tumor motion during respiration benefit from respiratory gating. •Internal structures can move a significant amount- The diaphragm and liver can move up to 3 cm and the pancreas, kidney and thorax can move up to 2 cm •Respiratory gating is used to minimize the increased margins of the treatment volume that are directly related to respiratory movements
- 37. •Cancers that aresubject to tumor motion during respiration benefits from respiratory gating. •These include lung, breast, pancreatic, stomach, liver, and prostate . •Breast cancers are usually treated at the point of maximum inhalation because this increases the lung volume, decreasing the amount of lung tissue in the field.
- 38. TRACKING RESPIRATION •Respiratory gatinguses marker systems that “learn” the patient’s respiratory pattern. EXTERNAL MARKER INTERNAL MARKER • RPM by varian. • Reflective marker learns patients breathing pattern. • Camera system - sends signal - reflected off markers - respiratory wave form created • Implanted gold seeds • Use fluoroscopy to locate
- 39. •The most commonlyused marker is an external marker known as the Real-time Position Management (RPM) system by Varian. •It’s a little plastic box with two reflective markers normally placed halfway between the patient’s xyphiod and Navel in order to take advantage of the Greatest external displacement. •A camera located near the foot of the table sends a signal that is reflected off the metallic markers and translated into a waveform whose shape represents the patient’s movement with regard to the respiratory cycle. •Implanted gold seeds are internal markers that require a surgical procedure for placement. During simulation and treatments they are tracked in real-time using flouroscopy. Real time positioning system
- 40. RESPIRATORY WAVEFORMS inspiration Expiration phases Beam offBeam off Beam on Beam on Irregular respiration Beam on System automatically shuts of till normal respiration is back
- 41. •Treatment plans areformed using the respiratory waveform and the CT images •The treatment plan is designed to place the treatment field around the target volume during a specific phase of the respiratory cycle. •For most treatment plans, the beam on time correlates with exhalation because the anatomy is in relatively the same position for the greatest period of time.
- 42. RESPIRATORY GATING SYSTEMBY VARIAN
- 43. Dose calcuation algorithm •More complex the intensity modulation, more will be the M.U required • One time monte carlo calculation of pencil beam algorithm is used.
- 44. Quality assurance (American association of physicist in medicine report 82 ,2003 • Simple daily film QA • EPID • Dosimetric test
- 45. Frequency Procedures Tolerance Beforefirst t/t Individual field verificaiton and plan verification 3% Daily Dose to test point in each IMRT field 3% Weekly Static Vs sliding window field dose distribution 3% Annually All commissioning procedure: leaf stability, speed , leaf acceleration, MLCs , leaf position, static Vs sliding window field 3% IMRT Quality Assurance Program
- 46. IMRT IN HEADAND NECK CANCER • Variable numbers of co planar beams are used(5-15) • Maintain target and nodes coverage as in conventional t/t • Keep dose to cord and brainstem tolerable. • Reduce dose to parotid as much as possible. • Concomitant boost can be done in single plan ,there is no need of subsequent electron boost.
- 47. • Eisbruch etal (1999): A mean parotid dose of < 26 Gy should be planning goal. • Eisbruch et al (2007): Substantial parotid flow recovery (upto 86% of pretreatment levels) at 2 years if mean doses are between 25-30Gy. • Eisbruch et al (2010): Severe xerostomia (<25% of baseline) avoided if mean parotid dose kept to <20Gy (if one parotid is to be spared) or <25 Gy (if both are to be spared) Parotid dose and xerostomia
- 48. CONCLUSION • Avoidance ofcircumferential irradiation of rectum to 55Gy with minimal compromise of PTV coverage is achievable with IMRT ƒ. • use of IMRT reduces acute GI/GU toxicity rate when compared with the 4FB technique. IN HEAD AND NECK CANCER:- • IMRT plan show improve dose distribution especially sparing of parotids. • Plans with more beams is better than with less beams, however improvement saturates after beyond certain number of beams. plan quality deteriorates when beam number is <=5. IN PROSTATE:-
- 49.