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Helical Tomotherapy - a description of the machine data and some clinical experience

Helical Tomotherapy - a description of the machine data and some clinical experience

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Helical Tomotherapy Helical Tomotherapy Presentation Transcript

  • Helical Tomotherapy Sterzing, Schubert, Sroka-Perez, Kalz, Debus, Herfarth. Strahlentherapie und Onkologie. 2008 Jan 11;184(1):8-14. Presenter: Dr Santam Chakraborty Date: 21.02.08
  • Introduction
    • Tomotherapy is a method of radiation therapy using a rotating x-ray beam source.
    • Amalgam of the words: (Mackie,1993)
      • Tomography
      • Radiation Therapy
    • Tomotherapy in it's serial format preceded MLC based IMRT (1994)
    • One of basic driving rationale for Tomotherapy is it's coupling with a superior IGRT technology.
  • Need for Image Guidance
    • IMRT and other conformal techniques – prone to geographic errors
    • Significant changes in patient anatomy during treatment have been demonstrated
    • Intrafraction organ motion and deformation errors create further uncertainties
    • Advanced volumetric imaging techniques allow quantification of these errors – basis of IGRT
  • Overview of IGRT Techniques
  • IGRT – Panacea or Problem ?
    • Need for IGRT spurred the development for more advanced image guidance systems
    • However flip side is:
      • Cost of technology
      • Cost of time
      • Cost of personnel training
      • Cost of additional QA and maintainence
    • Present study attempts to answer some of these questions.
  • Patients and Methods
  • Study Design
    • Retrospective chart review
    • Study period: July 2006 and May 2007
    • Number of patients: 150
    • Reasons for the use of tomotherapy:
      • Complex tumor geometry and proximity of organs at risk
      • Need for image guidance when immobilization was problematic or interfraction variations were to be minimized
  • Site Distribution
  • Tumor Types Treated
    • Nine patients were treated with single-fraction radiosurgery (liver n = 6, lung n = 3).
    • 12 patients were treated for multiple lesions in one procedure.
  • Radiotherapy Planning
    • Done on dedicated workstation by dosimetrist and oncologist software (Siemens)
    • Inverse Planning: Tomotherapy planning station v 2.2.1.55 (TomoTherapy Inc. Madison, US)
    • 4D CT scans obtained for thoracic and abdominal tumors where respiratory motion was felt to be a problem
    • Plan verification:
      • Film and ionization chamber based.
      • Integrated verification software provided in TPS
  • Immobilization
    • Individualized devices were used
      • Cranial Leisons :
        • Scotch-cast masks = 18
        • Soft thermoplastic masks = 11
      • Other sites :
        • Combined mask and vacuum bag = 60
        • Vacuum bag only = 31
        • Pillow for legs and feet = 30
    • 3 point isocenter localization technique used.
  • MV CT Imaging
    • Pretreatment MV-CT in 98.2% in 3,026 fractions
    • Exception: Patients immobilized by scotch cast masks – deviations < 1mm – Imaged weekly (N = 18)
    • Length of CT scan: Variable
    • Image registration with KV-CT :
      • Algorithm based fusion:
        • Bony landmark based
        • Soft tissue based
      • Manual Correction:
        • 50% of fractions: Range of 1mm ( < 5% for 3mm)
  • Setup Correction
    • Dose distribution to OAR and target analyzed on MV-CT scans
    • Translations corrected by couch movement directly
    • Roll correction : Adjusting the gantry start position.
    • Vector for correction:
    • Dose distribution analysis possible after re-contouring in the MV CT scan.
  • Results
  • Time Effort
    • Fast introduction: 20 patients per day after 4 weeks
    • Average on table time 24.8 min
    • Average treatment time 10.7 min
    • Time required for image guidance remained constant
      • Mean Treatment time: 11.3 min vs 10.6 months – after 30 and 100 patients respectively
      • Mean On table time: 25 min and 24.6 min after 30 and 100 patients respectively.
  • Time Effort
  • Time Effort
  • Imaging and Position Accuracy
    • Prostate cancer patients are detailed.
    • In 7.7% fractions patients had detected rectal filling that impaired treatment delivery significantly.
    • Rotational setup variations that could have lead to increased dose to organs at risk required correction in 6 patients.
    • Mean detected correction vector 6.9 mm
      • < 5 mm in 833 fractions (27.5% of fractions)
      • ≥ 5 mm in 2193 fractions (72.5% of fractions)
  • Discussion
  • Major Advantages highlighted
    • Applicable where highly conformal dose distributions are required.
    • Also considered useful for long segment and multiple target involvement or in targets in close proximity to critical organs
    • Image guidance for precise treatment of difficult targets in difficult patients.
    • MV-CT advantage in reducing bony targets
    • Possibility of dose guided radiotherapy
  • Critiques
    • A retrospective study with a highly heterogeneous population.
    • No worthwhile statistical considerations can be derived
    • In essence a descriptive study
    • However the importance of the study is that it helps the reader to go through one of the largest published working experience with helical tomotherapy.
    • Allows us to gain a quick insight into the pitfalls and benefits of this technology
  • Design Characteristics
  • Tomotherapy
    • Tomotherapy essentially means slice therapy – radiation is delivered in a thin fan beam configuration
    • Tomotherapy was the earliest type of functional IMRT system available.
    • Two types:
      • Serial (NOMOS corp): The delivery of multiple fan beams with discrete table increments between each axial gantry arc.
      • Helical (Tomotherapy Inc.): Continuous synchronized gantry and table motion. From the patient’s point of view, the source describes a helical trajectory.
  • Design Principles
    • Designed around a ring gantry similar to a helical CT scanner.
    • Non coplanar treatments ruled out
    • Use of IMRT obviates need for non coplanar treatment.
    • Ring gantry maintains its isocenter to tens of microns as compared to a millimeter diameter on the best C-arm Linac gantries 1 .
    • Nominal SAD = 85 cm
    • 1 – 6 rotations per minutes
    Jeswani S, Mackie TR, Aoyama H. Overview of the HI-ART TM Helical Tomotherapy System.
  • Machine Interior Fenwick JD, Tome WA, Soisson ET, Mehta MP, Rock Mackie T. Tomotherapy and Other Innovative IMRT Delivery Systems. Seminars in Radiation Oncology. 2006 Oct ;16(4):199-208.
  • LINAC Design
    • Standing wave S band LINAC – 2 operating energies 1
      • 6 MV photon beam output for treatment
      • 3.5 MV photon for imaging
    • No flattening filter
      • Output increased to 8 Gy/min 1 at center of bore – 2 times that of periphery
      • The beam energy spectrum is more constant (< ± 5%)
      • There is less scatter contamination.
    • However a beam hardener and electron stopper is provided
  • Jaw Characteristics
    • 23 cm of 95% tungsten shielding is used in the linac support fixture and combination of primary collimator and jaws.
    • The average leakage from head is 0.01%.
    • Independent Y jaws have been provided – field width 1 – 5 cm at the isocenter.
      • Output in the fan beam drops dramatically below 1 cm due to loss of lateral electron equilibrium and partial source occlusion
    • A primary beam stop precludes need for a primary barrier
    • Main scatter is from the beam itself and minimal scatter from the head.
  • Machine Head configuration
  • Beam Characteristics: Projection
    • For treatment delivery the full rotation is divided into 51 projections .
    • Each projection is characterized by it's own leaf opening and closing pattern
    • Each projection covers an arc segment of 7 º .
    • Between each projections all leaves are closed for a short period of time – highly segmented step and shoot approach.
    • Constant dose rate for the LINAC assumed – but monitor chambers have inbuilt safety interlocks
  • Beam Characteristics: Pitch
    • The rotational fan beams overlap with each point seeing from 2 to 5 rotations or about 100 to 250 possible beamlets.
    • Three fan beam widths used – 1, 2.5 and 5 cm.
    • Pitch of the delivery : Defined as the fraction of the beam width that the couch translates (moves in or out) during each rotation of the gantry
    • The pitch can be defined and helps in offsetting the threading effect.
    • Typical values for the pitch are 0.25 - 0.5
  • Binary MLC Characteristics
    • Binary MLCs are provided – 2 positions – open or closed
      • Pneumatically driven 64 leaves
      • Open-close time of 20 ms
      • Width 6.25 mm at isocenter
      • 10 cm thick
      • Interleaf transmission – 0.5% in field and 0.25% out field
    • Maximum FOV = 40 cm
    LINAC Cone Beam Y jaw Y jaw Fan Beam Binary MLC
  • Couch Characteristics
    • Flat Couch provided allows automatic translations during treatment
    • Target Length long as 160 cm can be treated –
    • “Cobra action” of the couch limits the length treatable
    • Manual lateral couch translations possible
    • Automatic longitudinal and vertical motions possible
    • Possible to treat anywhere within a cylindrical volume 40 cm in diameter by 160 cm long
  • Workstation
    • Includes:
      • An operator station
      • Planning station
      • 32 CPU computer cluster attached to database server
      • Treatment machine
    • Parallel processor architecture for optimization of thousands of beamlets involved
    • System offers no contouring tools – rather contours have to imported from other TPS
  • Shielding Requirements
    • A tomotherapy primary beam shield is:
      • Reduced in width by a factor of almost 10
      • Increased in thickness by more than a tenth value layer in comparison to a conventional accelerator.
    • Furthermore, the secondary shielding requirements are enhanced by more than two tenth value layers with respect to conventional shielding demands.
    • However primary beam stopper included significantly reduces room shielding requirements.
    Balog J, Lucas D, DeSouza C, Crilly R. Helical tomotherapy radiation leakage and shielding considerations. Med. Phys. 2005 Mar 0;32(3):710-719.
  • Planning Process
  • Planning process
    • 3 D contouring done on treatment planning system – any DICOM-RT compatible TPS is capable for contouring
    • Contoured CT images pushed to TPS
    • Three factors are predefined before starting the calculations:
      • Pitch
      • Modulation Factor : Is the ratio of maximum to the mean leaf opening time for all non-zero leaf opening values per projection.
      • Field width : Describes the fixed width of field – 1, 2.5 or 5 cm
  • Planning Process
    • The number of projections are fixed at 51 and all projections are utilized.
    • Using a small field size, smaller pitch and larger modulation factor lead to more conformal plans
    • Two sets of calculations are done:
      • Precalculation : More time consuming but an automatic overnight process.
        • Basic calculations describing the beamlets and their interaction with the patient are computed.
      • Optimization : Similar to the process of conventional IMRT where constraints and doses are prescribed using data from the precalculation stage. Less time consuming.
  • Planning Process
    • One important aspect of the TPS (v2.0) is that the system doesn't allow voxel overlap between two organs of the same type eg. two types of tumor volumes or two types of sensitive organs.
    • Where two organs of same type do overlap the dose volume information of the organ with higher priority will be displayed.
    PTV 1 PTV 2 If priority of PTV 1 is less than PTV 2 then its DVH wont be computed at all !!! If PTV 2 has a lesser priority but remains an overlapping organ for DVH calculation, volume inside PTV1 wont be taken into DVH calculation Saibishkumar EP, Jha N, Scrimger RA, MacKenzie MA, Daly H, Field C, et al. Sparing the parotid glands and surgically transferred submandibular gland with helical tomotherapy in post-operative radiation of head and neck cancer: A planning study. Radiotherapy and Oncology. 2007 Oct ;85(1):98-104.
  • Generated H&N Plan
  • Generated Plans
  • Cranio-caudal Penumbra
    • As radiation starts during patient motion when the target reaches the lower end of the fan beam – there is increased dose penumbra in the cranio-caudal region
    • This can be detrimental in situations where critical organs lie in close proximity in this direction – e.g. nasophayrnx
    • Efforts on way to reduce this effect by primary jaw shielding for the start of the treatment.
    Bauman G, Yartsev S, Rodrigues G, Lewis C, Venkatesan VM, Yu E, et al. A Prospective Evaluation of Helical Tomotherapy. International Journal of Radiation Oncology*Biology*Physics. 2007 Jun 1;68(2):632-641.
  • MV-CT Imaging and Adaptive RT
  • Imager Characteristics
    • Arc-shaped xenon detector has 738 channels, each with two ionization cavities filled with xenon gas and divided by 0.32 mm tungsten septa.
    • The detector array has a 110 cm radius of curvature
    • 540 out of 738 channels are used for the MVCT image reconstruction.
    • The source to detector distance is 145 cm.
    • Majority of photon beam interacts with tungsten septae which also prevent detector cross talk – greater effeciency
    • FOV = 40 cm
    • Maximum number of slices = 80
  • Image Characteristics
    • Allows higher image resolution than cone beam MV CT (3 cm diameter with 3% contrast difference)
    • Tissue heterogeneity calculations can be done reliably on the CT images as scatter is less (HU more reliable per pixel)
    • Not affected by High Z materials (implant)
    • Dose 0.3 – 3 Gy depending on slice thickness
    • 3 imaging modes: Coarse (12 mm), Normal (8 mm) and fine (4 mm) thickness.
  • Image Characteristics
    • FOV of 40 cm available in the tomotherapy MVCT system may lead to a degradation of image quality because the tissue outside the FOV is not properly accounted for in the reconstruction process
    • Typical result is ‘bowl’ artifacts - the reconstructed CT values are increased in the peripheral regions of the images
    • However sufficient information for checking setup even if the MV-CT image width is half that of the patient thickness.
  • Matching of images
    • Two types of matching algorithms:
      • Automatic – mutual information algorithm
      • Manual – translations and rotations
    • Calculated couch shifts adjusted automatically except for lateral translations
    • Limit of manual lateral adjustment = 2.5 cm
    • Roll correction possible – adjust gantry start position
  • MV-CT Images
  •  
  • Cone beam CT vs Tomo MV CT
    • Cone beam CT on C-arm gantry designs restricted to 1 rotation per second due to collisional considerations
    • In Tomotherapy CT length of 1.2 cm can be imaged in 10 sec or 1 rotation
    • Acquisition occurs automatically
    • In KV cone beam CT takes 1.7 min to acquire 285 projections and few minutes to reconstruct
    • However, this effort would yield 256 slices, almost twice as many slices per minute as the tomotherapy unit.
  • Adaptive Planning process
    • 3 categories of dose data are required for adaptive treatment
      • Dose Prediction : Display of changed dose due to changes in setup and anatomy from fraction to fraction – it is a pretreatment verification process and an online process.
      • Dose Verification : Uses the MV CT to verify the dose distribution – it is an offline process so can incorporate data acquired during the treatment.
      • Dose Reconstruction : This process includes determination of the actual dose received at the time of treatment – done from sinogram data acquired by the Xe detectors that work at the time of treatment – it is again an offline process.
  • ART: Concept
    • Conventional R x
    • Sample Population based margins
    • Accommodates variations of setup for the populations
    • No or infrequent imaging
    • Largest margin
    • Offline ART
    • Individual patient based margins
    • Frequent imaging of patients
    • Estimated systemic error corrected based on repeated measurements
    • A small margin kept for random error
    • Plans adapted to average changes
    • Online ART
    • Individual patient based margins
    • Daily imaging of patients
    • Daily error corrected prior to the treatment
    • Smallest margin required
    • Plans adapted to the changing anatomy daily!
    1. 2. 3.
  • ART: Why ? Due to a change in the contours (e.g. Weight Loss) the actual dose received by the organ can vary significantly from the planned dose despite accurate setup and lack of motion.
  • Importance: Adaptive RT
    • Without replanning in Head and neck cancer:
      • Dose to 95% target volume reduces in 92% patients
      • Magnitude of dose reduction: 0.2 – 7.4 Gy
      • Spinal cord dmax increases in all patients
      • Magnitude of increase: 0.2-15.4 Gy
      • Brain stem dmax increased in 82% patients.
      • Magnitude of increased by 0.6-8.1 Gy
    Hansen EK, Bucci MK, Quivey JM, Weinberg V, Xia P. Repeat CT imaging and replanning during the course of IMRT for head-and-neck cancer. Int J Radiat Oncol Biol Phys. 2006 Feb 1;64(2):355-62.
  • ART: Problem Real time adaptive RT is not possible “today”
  • ART: Steps..
  • ART: Steps
  • Clinical Planning and Treatment Experience
  • Atypical Treatment Planning
    • Some of the atypical radiation techniques that have been planned using HT are:
      • Craniospinal irradiation
      • Total marrow irradiation
      • Total Lymphoid irradiation
  • Total Marrow Irradiation
    • TMI to 10 Gy was delivered as part of a tandem transplant regimen using HT to the 53-year-old patient with multiple myeloma
      • After TMI, the patient experienced the expected blood count nadir, followed by successful engraftment
      • Grade 2 nausea and grade 1 emesis occurred only briefly on day 2 of TMI
      • Skin erythema, oral mucositis, esophagitis, and enteritis were not observed
      • Organ doses were substantially lower than those associated with standard TBI
      • TMI and TMLI (Total Marrow & Lymphoid irradiation) potential alternative to TBI and targeted radio-immunotherapy
    Wong JYC, Liu A, Schultheiss T, Popplewell L, Stein A, Rosenthal J, et al. Targeted total marrow irradiation using three-dimensional image-guided tomographic intensity-modulated radiation therapy: an alternative to standard total body irradiation. Biol Blood Marrow Transplant. 2006 Mar ;12(3):306-15.
  • Craniospinal Radiation Penagaricano JA, Papanikolaou N, Yan Y, Youssef E, Ratanatharathorn V. Feasibility of cranio-spinal axis radiation with the Hi-Art tomotherapy system. Radiotherapy and Oncology. 2005 Jul ;76(1):72-78. Results : When considering D50% and D10%, CSA-TOMO has a dosimetric advantage over CSA-RT for most organs at risk. The body integral dose was higher for the CSA-TOMO plan by approximately 6.5% .
  • Dosimetric Comparison Fiorino C, Dell'Oca I, Pierelli A, Broggi S, Martin ED, Muzio ND, et al. Significant improvement in normal tissue sparing and target coverage for head and neck cancer by means of helical tomotherapy. Radiotherapy and Oncology. 2006 Mar ;78(3):276-282.
  • Dosimetric Comparison Fiorino C, Dell'Oca I, Pierelli A, Broggi S, Martin ED, Muzio ND, et al. Significant improvement in normal tissue sparing and target coverage for head and neck cancer by means of helical tomotherapy. Radiotherapy and Oncology. 2006 Mar ;78(3):276-282.
  • Clinical Results: Bauman et al Bauman G, Yartsev S, Rodrigues G, Lewis C, Venkatesan VM, Yu E, et al. A Prospective Evaluation of Helical Tomotherapy. International Journal of Radiation Oncology*Biology*Physics. 2007 Jun 1;68(2):632-641.
  • Clinical results: Bauman et al
    • In 61 patients treated with HT – the plan was deemed acceptable in 80%
      • 11 patients could not complete 80% of their treatment with HT – due to machine downtime
      • Approximately 90% of the planned fractions could be delivered with HT
    • Median times:
      • Positioning: 20 minutes (11 – 82 min)
      • Treatment: 6 minutes (2 – 68 min)
      • Median overall treatment time similar
    • 78% patients were satisfied or very satisfied - Patients were least satisfied with treatment duration.
  • Clinical Results: Bauman et al
    • OAR DVH metrics:
      • A total of 566 clinically relevant OAR DVH points compared
        • In 59% of these comparisons, HT OAR metrics were as good as or better than the 3D-CRT metric
        • In 41% 3DCRT was better – however improvement minimal to the tune of 1 – 2 Gy in most cases
      • HT better than 3DCRT for:
        • 53% of head/neck OAR comparisons
        • 55% of thoracic OAR comparisons
        • 72% of abdomen/pelvis OAR comparisons.
  • Non coplanar treatment
    • 10 patients with skull base tumors – stereotactic radiotherapy planned 50-50.4 Gy
    • HT compared with noncoplanar IMRT
      • PITV ratio reduced with LINAC based plans – 2.22 for HT vs 1.44 for LINAC
      • Inhomogeneity Index was higher as compared to LINAC based plans (0.1 vs 0.08)
      • 13% - 540% increase in low dose isodose volumes with HT
      • Where the disease spread inferiorly HT was considered superior.
    Soisson ET, Tome WA, Richards GM, Mehta MP. Comparison of linac based fractionated stereotactic radiotherapy and tomotherapy treatment plans for skull-base tumors. Radiotherapy and Oncology. 2006 Mar ;78(3):313-321.
  • Concerns with HT
    • 4 major concerns identified by Bauman and Sterzing et al in their series were:
      • Increase in the integral dose to normal tissues due to the rotational arc nature of the treatment
      • Increased penumbra in the cranio-caudal direction due to inherent problem with machine design
      • In certain situations noncoplanar beam arrangements may give potentially superior results.
      • Extra dose of the order of 0.6–2 cGy per MV-CT depending on the chosen pitch can lead to increased dose to normal tissues with unknown consequences.
  • Conclusions
    • Helical tomotherapy is a new paradigm at delivering megavoltage radiation therapy
    • Machine designed with express purpose of delivering Image guided IMRT
    • Cost benefit remains to be determined
    • Several issues remain of concern
    • Novel treatment techniques now possible
    • New advances may render present technology obsolete:
      • Volumetric Modulated Arc Therapy - Varian
      • IMPT (Helical / C-arm based)
      • Cobalt Tomotherapy with integrated MRI (developmental)