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TRANSCRIPT
ABHIJIT DAS
2ND YEAR PG
DEPT. OF RADIOTHERAPY
AHRCC
ARC THERAPY
SHIFT OF MACHINES AND CONCEPT
• The dawn of the 20th century, arc therapy Involving dynamic field shaping using a multileaf collimator was first described by Takahashi in 1965.
• 1982,Brahme et al solved an integral equation for a hypothetical target wrapped around a critical structure and treated with arc therapy.
• In 1993, another form of IMRT using rotational fan beams, called Tomotherapy, was Proposed by Mackie et al.
• Intensity modulated arc therapy (IMAT) was introduced by Cedric X.Yu in 1995
THE BASIC CONCEPT OF ARC THERAPY :
• The delivery of radiation from a continuous rotation of the radiation source and allows the patient to be treated from a full 360 DEGREE beam angle.
• Arc therapies have the ability to achieve highly conformal dose distributions and are essentially an alternative form of IMRT.
A major advantage over fixed gantry IMRT is the improvement in treatment delivery efficiency due to –
• The Reduction In Treatment Delivery Time .
• The Reduction In MU Usage(the amount of radiation output per unit of time is referred as monitor unit).
• Subsequent Reduction Of Integral Radiation Dose To The Rest Of The Body.
• The Availability Of Extra Time Within A Set To Employ IGRT.
ARC-BASED THERAPIES:
Tomotherapy
IMAT
volumetric modulated arc therapy (VMAT)- single arc forms of IMAT
Tomotherapy techniques can be subdivided –
1. axial or serial tomotherapy (where the radiation is delivered
slice by slice)
2. helical tomotherapy (HT) (where the radiation is delivered in a
continuous spiral).
HT has been evaluated in a variety of tumor sites
and it can generally achieve either similar or
improved dose distributions compared with fixed
field IMRT, with variable results on treatment time
comparisons.
The TomoHelical delivery mode provides IMRT and 3D CRT treatment delivery in a continuous (360°) helical pattern, using thousands of narrow beamlets, which are individually optimized to target the tumor.
• The TomoDirect delivery mode is a discrete angle, non-rotational delivery mode. TomoDirect allows creation of treatment plans that include between 2 and 12 target-specific gantry angles. During treatment delivery, all beams for each target are delivered sequentiallywith the couch passing through the bore of the system at an appropriate speed for each gantry angle.
HIART SYSTEM
• Tomotherapy - a combination of a CT scanner and a linear accelerator that can
deliver the radiation in a fan-shaped distribution, similar to CT imaging with a
continuously rotating radiation source, while the patient is moved through the
machine.
PARTS DESCRIPTIONS
LINAC 6 MV S-band (3 GHz) linear accelerator
DIRECTION OF ROTATION CLOCKWISE FROM FOOT END /speed varies according to plan.
ENERGY FOR TREATMENT 6 MV photon beam
POWRED BY MAGNETRON
SAD 85 CM
MAXIMUM RADIATION FIELD
LENGTH
150 CM WITH COUCH AT HEIGHT OF ISOCENTER PLANE
TREATMENT VOLUME - Tomohelical 80 Cm (Transverse Diameter) X 135 Cm (Longitudinal)
For Typical Patient Set-up.
Tomodirect 40 Cm (Transverse Diameter) X 135 Cm (Longitudinal)
For Typical Patient Set-up.
PARTS DESCRIPTION
ENERGY FOR IMAGING 3.5 MV photon for imaging.
DOSE FOR IMAGING 0.5-3 CGY
DETECTOR SYSTEM 528 channels, single-row xenon ion chamber array
used
for image acquisition
IMAGE RESOLUTION 512X512(0.78 PIXELS)
SCAN TIME TYPICALLY 2 MINUTES PER 10 CM LENGTH AT 4
MM SLICE SPACING.( 2,4,6 mm slicing available)
FIELD OF VIEW (FOV) 40
CM DIAMETER
FIELD OF VIEW (FOV) 40 CM DIAMETER
SOURCE TO DETECTOR
DISTANCE
145 CM
IMAGING
• The beam from the accelerator is collimated by a multileaf collimator
consisting of 64 leaves each of which project a shadow of 6.25 mm at the
patient generating a total fan beam width of 40 cm. (pneumatically driven)
• By using a separate collimation ("jaws") system above the multileaf
collimators, the "slice thickness" can range between 0.5 to 5 cm. it is a
specially designed machine for helical, fan beam delivery.
• multileaf collimation system is specifically designed to minimize leaf
transmission and interleaf leakage - important considerations for narrow
beam, multislice delivery procedures. Average MLC leakage - 0.25% (typical)
• Axis of travel is in one direction.(IEC-y axis)
Radiation Characteristics
One of the key differences is the lack of flattening filter, which makes the dose more uniform at greater depths. As a result of this, the photon fluence profile is shaped differently when compared to a traditional radiotherapy system. the conical shape of the profile implies that there will be an increased average dose rate - thus reducing the imaging time & No scatter outside the field
Procedures descriptions
3-D Imaging. standard diagnostic imaging equipment or CT-simulators
Definition of Target Volume and Organs at Risk
contour the target volume as well as the organs at risk. Thiscould be done at the CT-simulator or on a conventional 3-D treatment planning computer---> after the image data set has been transferred to the treatment planning system.
Tomotherapy Co-
registration
a rigid-body adjustment that will only provide translational, rotational, pitch and yaw calculations. The Registration process allows the user to define structures for co-registration including the Whole Image (Mutual information with no thresholding), Bone and Tissue Technique (pixel threshold > 0.3 g/cm3), or a Bone Technique (pixel threshold > 1.1 g/cm3) as the focus for registration. N.B- The simplest thresholding methods replace each pixel in an image with a black pixel if the image intensity is less than some fixed constant T (that is, ), or a white pixel if the image intensity is greater than that constant
Data Transfer to PlanningComputer
planning computer which will perform the delivery optimization calculations
PLANNING AND DOSE DELIVERY
the full gantry rotation is divided into 51 projections. Each projection is characterised by its own leaf opening pattern and covers an arc segment of approximately 7°. The available rotation period may be between 15 and 60 s (typically around 20 s). As such, each projection takes between 0.2 and 1 s with all leaves shut for a short time between projections. The delivery assumes constant dose rate of the linac.
Optimized Planning.
The tomotherapy treatment planning system provides "inverse planning" capabilities.Three factors are predefined before starting the calculations:
1.Pitch : 2.Modulation Factor: 3.Field width:
The pitch factor is defined as couch movement per rotation in units of the FBT(fan beam thickness).
The MF represents the ratio of maximum leaf opening time to the mean leaf opening time of all MLC leaves, which open in a projection
The FBT is achieved a compromise between fast treatment times and dose modulation in thesuperior/inferior direction
• The smaller the pitch factor, the longer the treatment time.
• a small pitch improves the capability of dose modulation and the ability to deliver high doses per fraction.
• A potential problem with large FBT and large pitch is the dose distribution away from the central axis. The beam divergence will cause variations in overlap between adjacent rotations, which increase with distance from the axis of rotation. -‘THREAD EFFECT
• MF is proportional to the overall treatment time, and with typical physical constraints for the tomotherapy delivery,
• can be selected between 1 and approximately 6.
• A small MF results in short treatment times and is adequate for relatively symmetrical targets close to the central axis of the patient.
. A large FBT results in larger volumes covered in any projection and a higher central axis dose output while it reduces the scope for conformality and detailed dose modulation in cranio/caudal direction of the patient.
Kissick et al showed a pitch factor of 0.86/integer number (e.g., 0.43, 0.287, 0.215, etc.) minimisesthe thread effect).
Creation of Verification Data
consists of the expected beam intensity at the detector array for each gantry angle and couchposition. This intensity pattern is referred to as a "sinogram" because each point irradiated in the patient maps a sine wave pattern at the CT detector as the gantry revolves. Sinograms can actually be obtained for various processes including a CT sinogram as described above, an MLC sinogram, a registration sinogram, a verification sinogram and a planned detector sinogram. each is implemented in a very specialized manner to address a specific task.
A sinogram is an array of pencil beam intensity values as a function of gantry angle (horizontal axis in this Figure). Each vertical row corresponds to one angular view. A point object that is straight and parallel to the z-axis will appear as one cycle of a sinusoidal curve when the gantry revolves by 360 degrees.
Procedures descriptions
Pre-Treatment Megavoltage CT
Tomotherapy Delivery.
Delivery Verification.
While the patient is being treated, the detector array isactively measuring the radiation transmitted through the patient (for each pulse of the linac). This is used to determine actual radiation incident on the patient and can be used to verify dose delivery during or after treatment
Dose Reconstruction.
Using the incident radiation fluence delivered to the patient and the CT information that was obtained before the treatment, the dose actuallydeposited in the patient can be computed and compared to the planned dose. If necessary, corrections can be made to subsequent fractions
pros cons
Total Body Irradiation (TBI)
greater control over the dose distribution and to spare organs that may be at risk.HT provided excellent conformal lung sparing with mean doses not exceeding 10 Gy.
Helical Tomography (HT) for TBI isthat it can’t be used when the body length of the patient exceeds 145 cms.
Whole brain helical Tomotherapy
Whole brain helical tomotherapy is a possible treatment option for patients suffering from malignant melanoma with four or more brain metastases.-hippocampal sparing
All HT plans had a higher dose brainExposure.
Inoperable Lung Cancer
Tomotherapy could produce acceptable coverage for the target while decreasing the mean dose to sensitive surrounding areas such
USE
• Intensity Modulated Arc therapy where the following three parameters are modulated simultaneously:
1. Gantry rotation 2. Dose rate 3. Leaf speed
• Like tomotherapy, IMAT is delivered in an arc manner.
• Instead of using rotating fan beams as in tomotherapy, IMAT uses
rotational cone beams of varying shape to achieve intensity
modulation.
• Cedric x.Yu concept:
1. with the increase in the number of gantry angles, the number of intensity
levels at each gantry angle can be Reduced without degrading plan quality.
2. plan quality is a function of the total number of quanta defined as the product of the number of beam angles and the number of intensity levels. ( it is
the total number of aperture shape variations that determines the plan quality).
• Based on this fact a single arc with sufficient number of apertures variations would
be able to create the optimal treatment plans.
• Because linear accelerator at the time can not vary the dose rate dynamically
during gantry rotation. Most previous works on single arc IMAT was under the
assumption that the machine dose rate has to be constant during arc rotation.
Beamlet-Based Inverse Planning:
• Two-step approach to treatment planning:
1. Fluence map optimization–Delivery constraints ignored2. Leaf sequencing –Accounts for delivery constraints
Aperture-Based Optimization:• A One-step process:
1. Contour-based planning:
• Anatomy contour-based• Isodose contour-based
2.Direct Aperture Optimization
DAO Optimization :
• Introduction of an automated planning system in which the traditional intensity optimization is bypassed, and instead directly optimize the shapes and the weights of the apertures. -------‘‘direct aperture optimization.
• All of the MLC delivery constraints are included in the optimization.
• The leaf settings and the aperture intensities are optimized simultaneously using a simulated annealing algorithm.
• The DAO algorithm takes as input 1.beam angles, 2.beam energies, and 3. number of apertures per beam angle.
• The algorithm then cycles through all of the variables, which are to be optimized. These variables are 1.the leaf positions for each aperture and 2. the weight assigned to each aperture.
1.Highly conformal IMRT plans with only 3 to 5 apertures per beam.
2.Delivery in traditional 15 minute time slots.
3.The user has complete control over the complexity.
4.Provides optimal aperture shapes and weights.
5.No leaf sequencing.
6.Can be used for IMAT treatment planning.
Direct Aperture OptimizationBenefits
Small number of apertures can produce large number of intensity levels
N= Number of intensity levelsn= Number of apertures
For 3 apertures, 7 intensitiesFor 4 apertures, 15 intensitiesFor 5 apertures, 31 intensitiesFor 6 apertures, 63 intensities
3 SeparateRotations withDifferent IntensitiesPer Rotation
Yields 7 Unique Intensity Levels
Otto developed a single arc IMAT algorithm that he referred to as Volumetric Modulated Arc Therapy (VMAT), using this DAO algorithm.
IMAT to VMAT New Developments
Delivery Control Systems
Elekta and Varian have introduced new linac control systems that will
systems that will be able to change the MLC leaf positions and dose
rate while the gantry is rotating.
Elekta - PreciseBeam Infinity® [can be delivered using single or
multiple arcs]
• Varian -RapidArc® [RapidArc always uses single arc to deliver
treatment]
• Philips Medical Systems, Inc - SmartArc
Both are using the term Volumetric Modulated Arc Therapy
VMAT Plan Design:• Single arc vs. Multi-arc delivery• Coplanar vs. Noncoplanar
Single vs. Multi Arc:• Increasing the number of arcs provides additional flexibility in shaping the dose distribution. • The key questions are which cases benefit from the use of multiple arcs and what number of arcs should be used
ADVANTAGES:
(Over Tomotherapy)
(1) IMAT does not need to move the patient during treatment and avoid
abutment issues seen with serial tomotherapy;
(2) IMAT retains the ability of using Non-coplanar beams and arcs, which has
value for brain and head / neck tumors;
(3) IMAT uses a conventional linac, thus complex rotational IMRT treatments
and simple palliative treatments can be delivered with the same treatment
unit.
• Varian: Eclipse RapidArc
• Philips: Pinnacle SmartArc
• Elekta : ERGO++
• Elekta: Monaco VMAT
• Nucletron : Oncentra MasterPlan VMAT
Inverse Planning
basic arc parameters, e.garc length and couch angle
coarse segments around the arc are generated
The fluence maps are converted to MLC segments, two per angle.
MLC segments are filtered, evenly redistributed around the arc, and interpolated segments are added to reach a final fine arc spacing
resulting segments are optimized using machine parameter optimization to satisfy dose volume objectives and leaf motion, dose rate, and gantry speed constraints.
To limit computation time, initially optimized intensity maps with a coarse angular resolution, then convert to MLC segments, and redistribute the segments around the arc at a finer resolution.
Optimized fluence maps are converted to MLC leaf and jaw segments using a conversion algorithm that produces segments with leaf motion that travels from one side of the target to the other, also known as sliding window.
Arc Sequencing
• The algorithm uses a simulated annealing based optimization and minimizes the discrepancy between the optimized and sequenced intensity maps. (fluence map)
• The algorithm iteratively changes the leaf positions and aperture weights and rejects any change that and aperture weights and rejects any change that violates an VMAT delivery constraint.
Arc Sequencing
• Gantry speed and dose rate must constant throughout each arc.
• All beam weights within an arc must be equal.
IMAT Constraints:
Interconnectedness of Adjacent Beam Shapes:
Leaf motion between adjacent angles is limited by leaf Leafmotion between adjacent angles is limited by leaf travel speed and gantry rotation speed.
Electron arc therapy
a special radio-therapeutic technique in which a rotational electron beam is used to treat superficial tumour volumes that follow curved surfaces.the technique is well known and accepted as clinically useful in the treatment of certain tumours, it is not widely used because it is relatively complicated and its physical characteristics are poorly understoodThe dose distribution in the target volume depends in a complicated fashion on the electron beam energy, field width, depth of the isocentre, source to axis distance (SAD), patient curvature, tertiary collimation and field shape as defined by the secondary collimator.
Two approaches electron pseudo arc (series of overlapping stationary electron fields)
continuous rotating electron beam.
angle ß concept
• offers a semiempirical technique for treatment planning for electron arc therapy. The characteristic angle ß for an arbitrary point A on the patient’s surface & is measured between the central axes of two rotational electron beams positioned in such a way that at point A the frontal edge of one beam crosses the trailing edge of the other beam.
• The angle ß is uniquely determined by three treatment parameters: f,(SAD) , d, (the depth of the isocentre) and w, the field width.
• Electron beams with combinations of d and w that given the same characteristic angle ß actually exhibit very similar radial PDDs, even though they may differ considerably in individual d and w Thus the PDDs for rotational electron beams depend only on the electron beam energy and on the characteristic angle ß.
DIFFICULTY:
• Photon contamination:
the photon contribution from all beams is added at the isocentre and the isocentre might be placed on a critical structure.
• field shape of the moving electron beam defined by secondary collimators. (homogeneity of dose)
1.cylindrical geometry (e.g. the chest wall), the field width can be defined by rectangular photon collimators.2.a spherical geometry (e.g. the scalp), a custom built secondary collimator defining a nonrectangular field of appropriate shape has to be used to provide a homogeneous dose in the target volume
Prostate cancer is one of the most common tumour sites treated with IMRT worldwide, VMAT is new alternative.
IMRT
VMAT
STUDY PATIENTS OUTCOME
Palma et al
10 1.The lowest doses to the OARs were achieved in the VMAT plans, which required42% fewer MU compared with the fixed field IMRT plans2. improved OAR sparing (improved rectal wall sparing)
MSKCC 11 1. improved rectal wall sparing with a resultant improved Normal Tissue Complication Probability (NTCP) of rectal wall by 1.5%, 2. lower doses to the bladder wall (not statistically significant) and femoral heads
Ost et al 12 reducing the dose to the rectum.(V50 Gy was 45% in the 7-field IMRTvs32% in VMAT,p=0.001)
Kopp et alRapidArc
292 1. VMAT and IMRT similar PTV coverage (VMAT less homogeneous). VMAT –slightly higher D2%
2. VMAT better than IMRT (sparing of rectum at high doses, bladder, femoral heads, penile bulb)
Yoo et alRapidArc
10 1. Primary plans – IMRT better than VMAT (PTV coverage, conformity). Boost plans – similar PTV coverage, homogeneity; IMRT had worse conformity compared to VMA
2. OAR: Boost plans – IMRT and DA VMAT better than SA VMAT. Higher integraldoses to body with VMA
This variation regarding target volume homogeneity and conformity could be due to a number of factors:
• The number of arcs used in the VMAT plans (in general, double arc plans can achieve higher conformity and homogeneity compared with single arc plans).
• The type of VMAT optimization approach and the number of fields used in the fixed field IMRT plans.
• PTV coverage. (prostate/prostate and seminal vesicles/prostate +nodal burden)
PROS AND CONS:1. VMAT plans generally use fewer MU (up to 65% fewer) compared with fixed field IMRT.
2. improved efficiency of VMAT delivery with a reduction in treatment delivery times.
3. intrafraction motion may be of particular relevance in prostate radiotherapy as there may be significant changes in rectal and bladder volumes within the time period required to deliver an IMRT fraction.
4. Prostate has lower α/ß ratio, thus hypofractionated schedule may be better, so faster VMAT delivery may be attractive solution.
5. It is worth noting that the optimisation and dose calculation times for VMAT planning are longer compared with fixed field IMRT (up to x 4)
study N site outcome
Verbak-elet al
12 Nasopharynxoropharynx hypopharynx
Both has similar PTV coverage. DA VMAT better than SA VMAT and IMRT for homogeneity.Parotid dose lower with DA VMAT (by average 2Gy) compared with SA VMAT and IMRT.
Vanettiet al
29 Oropharynx,hypo pharynxLarynx
VMAT better than IMRT at sparing spinal cord (D2%, mean dose),brainstem (D2%, mean dose) and parotid glands (mean dose).DA VMAT better than SA VMAT.VMAT – lower integral doses to body.
Cleme-nteet al
8 Oropharynx HT better than VMAT and IMRT for coverage of electivePTV/ homogeneity. VMAT better than IMRT in conformity (no difference in homogeneity).HT – lower doses to brain, parotid, oral mucosa, oesophagus.VMAT and IMRT lower doses to mandible. VMAT – slightly lower mean dose to ipsilateral parotid glandcompared with IMRT.
HEAD AND NECK CANCERRADIOTHERAPY
PROS AND CONS:
In the post-operative pharyngeal patients, PTV coverage was inferior in the single arc VMAT plan compared with the IMRT plan. The double arc plan was equivalent to IMRT and triple arc was superior in terms of PTV coverage and homogeneity.(Guckenberger et al)
In primary pharyngeal patients, both single arc and double arc VMAT plans were inferior to the IMRT plan, while the triple arc plan was equivalent.
In the paranasal sinus group, all VMAT plans were inferior to the IMRT plan for dose coverage, particularly in the region between the orbits.(Guckenberger et al)
The mean dose to the lenses in this group was also higher in the VMAT plans compared with the IMRT plans.
Central nervous system tumours
Radiotherapy for intracranial tumours can be challenging owing to the proximity of these tumours to numerous critical structures.Benign lesions:cranial tumours where long life expectancies are predicted raises the need for highly conformal techniques to reduce radiation dose to the surrounding normal tissue.
LESIONS TRIALS TARGET OAR
Benign lesions
Acoustic neuroma,meningioma,pituitaryadenom
Fogliataet al RapidArc
VMAT and HT slightly betterthan IMRT for PTV coverage(D99%, D98%)
VMAT and IMRT better than HTat sparing OARs (brainstem,optic structures
Acoustic neuroma(radiosurgerysingle 12.5Gy
Lagerwaardet al RapidArc
Similar PTV coverage. VMAT better than DCA forconformity
VMAT and 5DCA – similarmaximum doses for cochlea,brainstem, trigeminal nerve
MALIGNANT LESION
Wagneret al Rapidarc
High grade glioma
VMAT slightlybetter than IMRT forconformity (both VMATand IMRT better than CRT)
VMAT slightly better than IMRTand 3D-CRT for OAR sparing(chiasm, brainstem)VMAT –highest mean dose to normalbrain and V5Gy of healthytissue
shafferet al RapidArc
High grade glioma
Similar PTV coverage,conformity and homogeneity
VMAT better than IMRT at sparinglateralised OARs (retina, lens,optic nerves). No significant differences in sparing ofcentralised OARs (brainstem, chiasm). VMAT – higher mean dose to normal brain (by 12%
Lagerwaardet al RapidArc
Brain metastases
integrated plans (VMAT)significantly better thansummated plans forconformity
Smaller volume of normal brain receivingLow dose radiationIntegrated plans (VMAT) – higher maximumdose to lenses
Lung cancer
• EARLY STAGE LUNG CANCERS : Stage I non-small cell lung cancer (NSCLC) SBRT has emerged as
an alternative treatment option to surgical resection for patients who are medically inoperable, giving excellent local control rates (up to 95%).
• usually delivered with hypofractionated radiotherapy schedules and using multiple non-coplanar fixed beams occasionally combined with dynamic arcs.
• IMRT and HT have also been evaluated using this approach. These techniques can improve dose conformity compared with conventional radiotherapy, but at the expense of prolonged delivery time.
McGrathet al
Stage Ia NSCLC(SBRT 48 Gy in12 fraction)
VMAT better than 3D-CRT for conformity at 80% and 50%isodose levels. No differencein homogeneity
IMAT better than 3D-CRT at sparing lung (V20 Gy, V12.5 Gy, V10 Gy, V5 Gy). Nosignificant difference in mean dose to other OAR
Onget al
Stage I NSCLCSBRT 54Gy in 3fractions 55Gyin 5 fractions/60Gy in 8 #
VMAT better than 3D-CRT,DCA and IMRT forconformity at 80% and60% isodose level.
VMAT better sparing of chest wall(V45Gy, V30Gy, V20Gy) compared to 3D-CRTVMAT – higher lung doses (V20Gy,V5Gy) compared with 3D-CRT
Lung cancer
• EARLY STAGE LUNG CANCERS : Stage I non-small cell lung cancer (NSCLC) SBRT has emerged as an alternative treatment option to surgical resection for patients who are medically inoperable, giving excellent local control rates (up to 95%).
• usually delivered with hypofractionated radiotherapy schedules and using multiple non-coplanarfixed beams occasionally combined with dynamic arcs.• IMRT and HT have also been evaluated using this approach. These techniques can improve dose
conformity compared with conventional radiotherapy, but at the expense of prolonged delivery time.
McGrathet al
Stage Ia NSCLC(SBRT 48 Gy in12 fraction)
VMAT better than 3D-CRT for conformity at 80% and 50%isodose levels. No differencein homogeneity
IMAT better than 3D-CRT at sparing lung (V20 Gy, V12.5 Gy, V10 Gy, V5 Gy). Nosignificant difference in mean dose to other OAR
Onget al
Stage I NSCLCSBRT 54Gy in 3fractions 55Gyin 5 fractions/60Gy in 8 #
VMAT better than 3D-CRT,DCA and IMRT forconformity at 80% and60% isodose level.
VMAT better sparing of chest wall(V45Gy, V30Gy, V20Gy) compared to 3D-CRTVMAT – higher lung doses (V20Gy,V5Gy) compared with 3D-CRT
doses to the chest wall in this study were significantlylower in the VMAT plans, but at the cost of increaseddose to the lungs.
Brocket al
Stage I NSCLCSBRT 60Gy in8 fraction
1. non-coplanar and VMAT better than co-planarfor PTV coverage.2. Non-coplanar CRT better than coplanar CRT for lung V11Gy (no significant difference for V20Gy).
A RECENT STUDY BY Holt A, van Vliet-Vroegindeweij C, Mans A, Belderbos JS, Damen EM.:The VMAT plans used double partial arcs avoiding the contralateral lung. While PTV coverage was similar between all three techniques, both non-coplanar IMRT and coplanar VMAT performed better then coplanar IMRT in reducing dose to healthy lung tissue.
Pelvic malignancies (lower gastrointestinal tract)
Site Inference Studies problems
Anal cancer
DA VMAT slightly better thanSA VMAT and IMRT in PTV coverage & OAR like penile bulb, external geniatalia.A significant reduction in MU (of up to 70%) and treatment time.
Clivio et alVieillot et alStieler et al
the paucity of data for conventional IMRT in anal cancer
Rectal cancer
No significant difference in bladder sparing.VMAT significantly better than 3D-CRT for conformityNo difference PTV coverage
Richetti et al
• IMAT was one of the first arc techniques evaluated for whole abdomino-pelvic radiotherapy (WAPRT) in the treatment of relapsed ovarian cancer.
GYNECOLOGICAL CANCERS
STUDY RESULTS AND INFERENCE
Wong et al two anterior arcs were sufficient in treating the target volume adequately with acceptable sparing of OARs.
Cozzi et alVMAT with five-field conventional fixed field IMRT
• show similar target volume coverage with improved homogeneity& conformity with VMAT
• E.g: rectum, mean dose andV40 Gy in the VMAT plans were 36.3 Gy & 51.5%, respectively, compared with 42.5 Gyand 78.7% in the IMRT plans
• The increase in the number of fields can improve the quality of the IMRT plans leading to less differences with the VMAT plans, but it takes higher MU and longer treatment time.
• Brachytherapy has advantage of organ immobilization with very steep dose gradients and highly conformal dose distributions, which are not currently matched by IMRT techniques - therefore, the general consensus is that IMRT or VMAT will not replace the role of brachytherapy in gynaecological cancers
Breast cancer mortality is decreasing owing to a combination of factors including earlier diagnosis via screening and improvements in therapy.
Johansenet al
Breast (chest wall and nodes including internalmammary nodes
VMAT and IMRT better than CRT for conformity.& homogeneity.VMAT and IMRT better than CRT at sparing ipsilateral lung. CRT – lowest doses to contralateral lung. VMAT – lowest doses to contralateral breast
Nicoliniet al
Breast (Bilateral, SIB to tumour bed)
VMAT better than IMRT at sparing heart and lungs (medium-high dose level).for lungs, IMRT better at sparing at low dose levels.VMAT – higher mean and integral dose to healthy tissue
Qiu et al Breast (partial breastradiotherapy
VMAT better than 3D-CRT at sparing ipsilateral normalbreast tissue, ipsilateral lung.
Popescuet al
breast (+regional nodes includingInternal mammarynode
VMAT – lower mean dose to healthy tissue but higher V5Gy compared with 3D-CRT and IMRT.
Breast cancer
• The increased risk of secondary malignancy secondary to low dose radiation is currently not accurately quantifiable but should be borne in mind when deciding on the treatment strategy or radiation technique for patients.
• IMRT will still play an important role in breast radiotherapy, particularly within the setting of partial breast dose escalation for high-risk disease, which is currently being investigated in the IMPORT-HIGH trial.
• partial arcs can be used in case of use of VMAT.
• While inverse planned IMRT is necessary for complex target volumes, simpler forward planned techniques using multiple segmented tangential fields may be able to produce acceptable dose distributions for less complex cases while also minimizing low dose radiation to surrounding normal tissue.
• Arc therapy is an alternative form of IMRT in which continuous rotation of the radiation source allows the patient to be treated from a full 360 DEGREE beam angle
• Mainly three types: Tomotherapy, IMAT, volumetric modulated arc therapy (VMAT)- single arc forms of IMAT
• Tomotherapy - a combination of a CT scanner and a linear accelerator that
can deliver the radiation in a fan-shaped distribution, where patient moves
continuously through gantry. Simultaneous MVCT & KVCT imaging is
available.
• lack of flattening filter makes the dose more uniform at greater depth resulting in an increased average dose rate - thus reducing the imaging time & No scatter outside the field.
• The intensity pattern created by expected beam intensity at the detector array for each gantry angle and couch position, is referred to as a "sinogram" which is used in verification data in tomotherapy.
• Intensity Modulated Arc therapy where the following three parameters are modulated simultaneously: Gantry rotation ,Dose rate ,Leaf speed.
• Instead of using rotating fan beams as in tomotherapy, IMAT uses
rotational cone beams.
• Introduction of DAO helps to bring the concept of VMAT .
• Elekta and Varian have introduced new linac control systems:
PreciseBeam Infinity & RapidArc respectively
• It is of two types: Single arc vs. Multi-arc delivery & Coplanar vs. Noncoplanar• multi arcs gives flexibility in treatment planning.
• Advantage over tomo is : not to move the patient during treatment delivery/can use non coplanar beams/no new machine installment
• For treating superficial tumors electron arc therapy is used but it is technically difficult thus not popular.
• Prostate & head-neck cancers are highly discussed topics in VMAT and tomotherapy since critical structure avoidance is necessary along with dose escalation.
• In prostate intrafractional motion ,rectal and bladder wall sparing and hypofractionated RT is of special mention . double arc plans can achieve higher conformity and homogeneity compared with single arc plans.
• Similarly in head and neck double or even triple arc plan supersedes imrt which is superior to single arc plan.
• In Paranasal sinus all VMAT plans were inferior to the IMRT.
• partial arcs can be used in case of use of VMAT in breast RT
• general consensus is that IMRT or VMAT will not replace the role of brachytherapy in gynaecological cancers