Intrafractional Junction Shifts Utilizing Multileaf Collimation: A Novel CSI Planning Technique. Rodney Hood RT(R)(T)CMD

Transcription

1 Intrafractional Junction Shifts Utilizing Multileaf Collimation: A Novel CSI Planning Technique Rodney Hood RT(R)(T)CMD

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3 Happy Father s Day!

4 Quiet Room Beam me up Scotty!

5 What is CSI?

6 CSI-DURHAM!

7 CSI Craniospinal irradiation (CSI) is a technique used in radiation therapy to deliver a prescribed amount of radiation to the entire cranial-spinal axis to achieve curative measures in the treatment of intracranial tumors.

8 Why is CSI Necessary? Curative surgical excision of malignant intracranial neoplasms is rarely possible This tumor spreads malignant cells throughout the subarachnoid space via the cerebrospinal fluid (CSF) Entire central nervous system must be treated (CSF) Evans, A.E. et al. The Treatment of Medulloblastoma. J. Neurosurg. 72: ; 1990

9 CSI Craniospinal Irradiation Treats anywhere CSF flows Treatment fields typically include the brain to the thecal sac

10 CSF Cerebrospinal fluid Clear bodily fluid found in the brain and spine Produced by choroid plexus of the brain Provides mechanical and immunological protection for brain Cerebral auto regulation of cerebral blood flow Occupies the subarachnoid space between arachnoid and pia mater Constitutes the contents of the cisterns, sulci and brain ventricles as well as the central canal of the spinal cord.

11 Cerebrospinal Fluid (cont.) Brain produces around 500ml of CSF daily Fluid is constantly reabsorbed Usually about ml present at any one time CSF turns over around 4 times a day As always, all info ripped directly from Wikipedia

12 Craniospinal irradiation (CSI), combined with chemotherapy and surgery, is the current choice of treatment for malignant brain tumors - medulloblastoma - ependymoma - Germinoma (prophylactic)

13 CT Simulation Setup and Scanning

14 Wedge and sponge combination to extend mandible

15 Styrofoam pad used to build up patient to reduce cervical and thoracic spine curvature

16 Long Orfit mask to immobilize head and shoulders

17 CT zero marked along midline

18 No Styrofoam pad to build up spine

19 Styrofoam pad to build up spine

20 Why do we feather the junction? Uncertainty Possible setup error Possible patient movement

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22 Can you identify the pathology?

23 According to a study performed at Harvard Medical School that assessed the accuracy and precision of patient setup during treatment with a plan involving feathered fields, the magnitude of the spatial stochastic and systematic setup error was determined to be approximately 3 and 2 millimeters respectively. Holupka, E.J. et al. Effect of Set-Up Error on the Dose Across the Junction of Matching Cranial-Spinal Fields in the Treatment of Medulloblastoma. Int. J. Rad. Onc. Biol. Phys. 27: ; 1993

24 Moving the junction (feathering) is a common technique utilized in radiotherapy to smooth out any dose inhomogeneity across the junction of the abutting fields Doses along a border of two fields - too much dose (a hot spot) due to an overlap - under dosing (a cold spot) from too wide a gap The gap must be feathered in order to decrease the possibility of such hot or cold spots from occurring

25 Feathering 2 Types In order to evenly distribute dose along field junctions the junction/gap is moved 2 ways to feather Inter-fractional Intra-fractional

26 Difference between inter and intra The prefix inter means between The prefix intra means within

27 Inter-fractional Shifting the junction in-between treatments

28 Intra-fractional The fields decrease or increase during the treatment to move the field junction

29 Dosimetric Advantages Utilizing Intrafractional shifts is superior dosimetrically because it is much more forgiving for setup error and patient movement. When errors are introduced, hot and cold spots are reduced utilizing this technique

30 Treatment Advantages In conventional CSI treatments each junction shift requires a new setup Intrafractional treatments use the same plan throughout the treatment so the long initial setup only needs to be done on the first day Amount of imaging is reduced greatly

31 Field Setup Given the entire spine and brain must be treated, multiple fields must be used to encompass the entire volume comprising the brain and the spinal cord

32 Traditional method

33 FiF (Field-in-Field)

34 Brain field creation Set iso as low as possible while maintaining flash on vertex of skull (y2=20)

35 Brain field creation I try to set iso around c3

36 Brain field creation Set couch to match divergence

37 Brain field creation

38 Brain field creation Add MLC

39 Upper spine field creation Ensure no exit through mandible

40 Upper spine field creation Keep same x & y-coordinate ( left, right and ant/post ) as brain field

41 Upper spine field creation I like to set lower border at bottom of L2

42 Upper spine field creation Collimator of brain field is set using upper jaw size 18/100=.18(inv_tan)=10.2

43 Lower spine field creation I like to half beam this field

44 Lower spine field creation Set iso maintaining same x&y coordinates

45 Lower spine field creation Kick table to 270 Rotate gantry to match divergence of upper spine field Length of upper spine field will determine gantry angle Matching divergence is dosimetrically advantageous

46 Angled Lower Spine Field

47 Angled lower spine vs. Pa low spine PA fields converging anterior to cord Post field matched to upper spine field divergence

48 Don t forget the Golden Rule ALARA As Low As Rodney Achieves!

49 Setup Field Junctions First decide # of junctions needed This is dependent on total dose We feather every 9Gy or once a week So a 27Gy course would require 2 junction shifts Jaws must be opened to largest aperture size because we will be merging fields to create a step and shoot delivery Utilizing 1cm junctions feathering would require 2cm of field overlap We use a 5mm safety gap so the overlap region is 1.5cm at both junctions 1.5cm overlap

50 Setup Brain Subfields First subfield is open to largest aperture size For next subfield pull two MLC s to decrease aperture size by 1cm For the last subfield pull an additional 2 MLC s to decrease aperture by another 1cm Fields are closed by 1cm to match the larger MLC in upper spine field Dose is calculated and subfields are merged

51 Subfields merged

52 Setup Upper Spine Fields Setup upper spine field so the aperture shrinks on both superior and inferior borders

53 Setup Lower Spine Fields Setup lower spine field similar to upper but only modify aperture at superior aspect

54 Dose Calculation Dose is calculated to midplane of brain fields Dose to each spine field is normalized to a specific depth by the Rad Onc by averaging depths along the spinal axis. Two separate spine fields are desirable when spinal canal depths vary greatly from upper spine and lower spine segments

55 Gap Calculations

56 Upper/Lower spine Junction verification During planning the PA upper spine field as well as the PSO lower spine fields are set to gantry 0 The distance between the lower border of the upper field and the upper border of the lower field is measured in the TPS I also like to contour the location of the BB s so that DRR s can be created from the treatment fields This eliminates hand calculations

57 Calculated Gaps Gap and Couch Position Report

58 Single Spine Field

59 Treatment

60 First Day (Dry Run) Pt is setup identical to CT simulation. Make indicated iso shifts Start with the brain and work inferior From this iso all shifts are longitudinal only Image brain (I prefer KV) Adjust pt as needed-i.e. shifts or pt adjustment Make calculated inferior shift from brain iso to upper spine field Image and verify match Make calculated shift from upper spine iso to lower spine field Image lower spine No adjustment to spine iso s are made only pt postition

61 Dry Run (cont.) After all initial kv imaging is complete an MV of each tx field is taken After MV imaging of the brain a BB is placed on the center of the inferior border We now shift inferiorly to the upper spine field The field is imaged to verify blocks and the position of the BB s placed on the inferior aspect of the brain field This field is now rotated to 0 and the field is drawn onto the patient A BB is placed at the inferior aspect of this field We now shift to the lower spine field This field is imaged to verify blocks and the location of the BB. This field is rotated to 0 and drawn on The anterior skin gap is measured between the upper and lower spine field and verified against the calculated distance

62 Junction Verification-Brain/Upper Spine BB s are placed on inferior border of brain field, portal image of upper spine field is taken and BB distance is verified in off-line review.

63 Anterior Skin Gap Verification Rotate gantry to 0 and mark field edges on skin verify the gap on the skin against the numbers instructed

64 Upper/lower spine verification

65 5mm gap Dosimetry No gap

66 Dosimetry-evaluating junction doses

67 Aligned/Misaligned Static 1cm overlap

68 Aligned/Misaligned FiF 1cm overlap

69 Introduced Errors Perfect Alignment Dose (cgy) FiF Static Distance (cm)

70 Introduced Errors 5mm Gap Dose (cgy) FiF Static Distance (cm)

71 Introduced Errors 5mm Overlap Dose (cgy) Static FiF Distance (cm)

72 Introduced Errors 1cm Overlap Dose (cgy) FiF Static Distance (cm)

73 Introduced Errors 1cm Gap Dose (cgy) FiF Static Distance (cm)

74 TREATMENT VERIFICATION Gotta Be Right! 74

75 Delivery Verification-Portal Dosimetry Brain Upper Spine Lower Spine

76 Delivery verification Be mindful of where MLC are parked

77 Park MLC outside of patient or under jaw

78 Brain/Spine Junction Film Dosimetry

79 Spine/Spine Junction Film Dosimetry

80 Conclusion Shifting the junction intrafractionally reduces treatment setup time by limiting the number of setups needed. This technique is also superior dosimetrically because it reduces the variation in dose across the junction. Lastly, shifting in this manner is more forgiving to setup error and patient movement.

81 Questions?

82 Acknowledgements Justus Adamson, Ph.D. Taoran Li, Ph.D. Irina Vergalasova, Ph.D. Kevin Kirby, CMD Scott Green, CMD dui8novkqbq