Intensity modulated radiotherapy (IMRT) for treatment of ... confirmed a 2.5cm x 2.5 cm x 2.5 cm mass

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    Carol Boyd

    March Case Study

    March 11, 2013

    Intensity modulated radiotherapy (IMRT) for treatment of post-operative high grade

    glioma in the right parietal region of brain

    History of Present Illness: Patient NH is a 65 year old male who presented in the emergency

    room with an onset of seizures in October 2009. A computed tomography (CT) scan showed a

    2.5 centimeter (cm) x 3.5 cm mass in the right posterior-parietal lobe with a small hemorrhagic

    component. The patient was admitted to the hospital for further work up of his brain mass.

    During his hospital stay, NH underwent a magnetic resonance imaging (MRI) scan that

    confirmed a 2.5cm x 2.5 cm x 2.5 cm mass in the right posterior-parietal lobe of the brain. A

    neurosurgeon consulted with the patient and his family about surgical intervention for the brain

    mass. The patient underwent a craniotomy on November 3, 2009 and the tumor was grossly

    resected. The surgical pathology report revealed a high grade glioblastoma multiforme (GBM).

    Upon further discussions at a multidisciplinary tumor board, it was recommended to the patient

    and his family for NH to undergo radiation therapy and chemotherapy for his diagnosis.

    In early December 2009, the patient was referred to the radiation oncology department for

    consultation of post operative radiation therapy to the brain. The radiation oncologist reviewed

    the patient’s medical records and prior imaging studies and recommended partial brain radiation

    therapy. Potential toxicities, complications and side effects of both early and late reactions were

    discussed with the patient and his family. The patient agreed to proceed with radiation therapy.

    Past Medical History: The patient has a past medical history of hypertension. Otherwise, the

    patient has been a healthy man up until the time of his brain tumor diagnosis. The patient denies

    any family history of cancer.

    Social History: The patient worked as a sandblaster and a painter for navy ships. NH has four

    children. The patient denies any alcohol use and denies any smoking history or illegal drug use.

    Medications: The patient’s list of medications includes Keppra, Hydrochlorothiazide and

    Temozolomide.

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    Diagnostic Imaging: In late October 2009, NH underwent a CT scan of the brain after an onset

    of seizures. The CT scan revealed a 2.5 cm round mass demonstrated in the right parietal lobe of

    the brain. An MRI imaging study performed on November 2, 2009 confirmed a 2.9 cm x 1.8 cm

    diameter lesion in the right-posterior, parietal region at the gray-white junction. There was no

    significant displacement of the septum pellucidum from the cranial midline. A craniotomy was

    performed on November 3, 2009 with a total resection of the gross brain tumor. The pathology

    of this resection revealed high grade gliobalstoma multiforme. A post-operative MRI performed

    on November 4, 2009 showed moderate transverse relaxation (T2) hypersensitivity in the margin

    and hypointensity from hemorrhage within the operative site. There was no contrast

    enhancement at the margin of the operative site.

    Radiation Oncologist Recommendations: After review of the patient’s medical records and

    previous imaging studies, the radiation oncologist recommended post-operative, partial brain

    radiation therapy. The radiation oncologist recommended a plan using a three dimensional

    conformal radiotherapy (3DCRT) technique to deliver 6000 centigray (cGy) in 30 fractions at

    200 cGy per fraction. Using a 3DCRT planning technique will achieve acceptable dose

    tolerances to normal critical structures and provide adequate target volume coverage. The

    physician also recommended a plan using an intensity modulated radiation therapy (IMRT)

    technique for comparison. In general, IMRT improves dose conformity and will allow higher

    radiation doses to be focused to regions within the tumor while sparing and minimizing dose to

    normal surrounding critical structures when compared to 3DCRT. 1

    The physician requested to

    use image guided radiation therapy (IGRT) to ensure that the patient is properly aligned on the

    treatment table as compared to the treatment plan coordinates.

    The Plan (prescription): The radiation oncologist’s treatment recommendation to the medical

    dosimetrist was to begin a plan using a 3DCRT technique to a dose of 6000 cGy to the planning

    target volume (PTV). Since the PTV was in close proximity to the brainstem, the radiation

    oncologist suggested a radiation treatment plan utilizing an IMRT technique to see if the IMRT

    plan can better spare the dose to the brainstem. Both planning technique will be compared

    through dose distribution and the dose volume histogram (DVH).

    Patient Setup / Immobilization: In mid December 2009, NH underwent a CT simulation scan

    for radiation therapy treatment. Immobilization is very important especially when there are

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    multiple critical structures to avoid during treatment. Effective immobilization can be achieved

    with the use of a head holding device. 2 The patient was supine on the CT simulation table. The

    patient’s head rested on a Silverman headrest fixed to a Civco S frame head holder. To

    immobilize the patient for radiation therapy treatment, a thermoplastic face mask with

    reinforcement strips was customized and molded to the patient’s head. Fiducials were then

    placed on the patient’s mask to aid in positioning during treatment (Figure 1). The patient’s

    arms were placed by his side and a knee sponge was placed under the patient’s knees for comfort

    (Figure 2). Once the patient was immobilized, a CT scan was initiated. After the radiation

    oncologist reviewed and approved the CT simulation scan, the radiation therapist took photos of

    the patient’s position and recorded positioning parameters to document the patient’s treatment

    position.

    Anatomical Contouring: The CT images were transferred to the Phillip’s Pinnacle 8.0m

    radiation therapy treatment planning system (TPS) in preparation for volume contouring and

    treatment planning. The medical dosimetrist fused the pre and post-operative MRI imaging

    studies into the TPS. The radiation oncologist contoured the operative site with the aid of the

    fused post-operative MRI scan and labeled this volume the clinical target volume (CTV). A 2.5

    cm margin was created around the CTV to form the planning target volume (PTV). The medical

    dosimetrist was instructed to contour the organs at risk (OR) which included the lens of the eyes,

    the retinae, the brainstem, the optic nerves, the optic chiasm and the brain. After the radiation

    oncologist reviewed and made adjustments to the OR, a planning goal sheet was given to the

    medical dosimetrist and treatment planning was initiated.

    Beam Isocenter / Arrangement: The medical dosimetrist began with a 3DCRT planning

    technique and placed the isocenter in the middle of the PTV (Figure 3). A 3DCRT plan may

    increase the target dose coverage, reduce the volume of normal tissue receiving high dose and

    protect crucial organs from unnecessary exposure. 1 Gantry angles were carefully chosen to avoid

    going through healthy brain tissue. The medical dosimetrist used the critical organs in a 3D

    render mode of the TPS to aid in creating beams to avoid entering or exiting through critical

    structures. The beam arrangement for the 3DCRT plan included gantry angles of 310°, 243° and

    154° respectively. There was a 0.7 -1.0 cm block margin created around the PTV for each beam.

    A combination of 6 megavoltage (MV) and 18 MV energies were use to optimize the treatment

    plan. Thirty and 45° wedges were also used to optimize the 3D plan. A comparison trial

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    treatment plan was done using an IMRT planning technique. The isocenter used in the 3DCRT

    plan was also used in the IMRT plan (Figure 4). Using an IMRT planning technique will allow

    higher radiation doses to be focused to regions within the tumor while sparing and minimizing

    dose to surrounding critical structures. Five gantry angles were configured which included beam

    angles of 160°, 88°, 16°, 304°and 232° respectively. Once the medical dosimetrist configured

    the gantry angles, the planning objectives, dose constraints and dose prescription were entered

    into the IMRT parameters portion of the TPS. The field apertures for the IMRT plan were

    determined automatically by the treatment parameters entered into the TPS.

    Treatment Planning: The radiation oncologist defined the dose prescription and planning

    objectives for the 3DCRT and IMRT plan. The tolerance of the normal tissues of the central

    nervous system (CNS) was carefully observed as it limits the dose of radiation that can be safely

    delivered. 3 The objective was to configure a 3D plan through the use of multiple non-coplanar

    fields to the defined target volume and adjust beam weighting, wedge orientation and placement,

    to minimize cortical dose and maximize dose homogeneity within the high-dose region while

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