Intro to MRL Study Guide I

basic components of a Unity MR-LINAC I

1.5T MR for clear target visualization

160 leaf MLC Linac

Onboard Magnetron

70 cm wide bore for patient comfort

basic components of Unity MR-Linac II

inner ring is MR scanner

outer ring is Linac

TPS is Monaco (based on monte carlo)

IMRT planning (no VMAT)

basic components of Unity MR Linac III

7 MV FFF

90 degree fixed collimator and does not rotate

MRL distance from target to isocenter

distance from target to isocenter is 143.5 cm due to the presence of two rings

MRL Y axis

Y axis limited to 22 cm due to poles of the magnet

what is required of personnel working around the MRL

must have level 1 MR certification

What are some of the technical challenges that engineers had to overcome to build the MR-linac?

an MR relies on very strong magnetic fields that can disrupt the RF pulses that the linac produces, resulting in linac failure

linac produces radiation that can damage MR components if not shielded

materials in the linac must be entirely non-ferromagnetic

linac produces particles that can be bent by the MR magnetic field

materials in the linac must be entirely non-ferromagnetic What are some of the technical challenges that engineers had to overcome to build the MR-linac?

to avoid being pulled apart by the MR or disrupting the images

What are some of the technical challenges that engineers had to overcome to build the MR-linac? the linac produces particles bent by the MRs magnetic field

these particles can be shielded inside the machine, but once they leave machine and enter patient, they cannot be shielded

physics modeling is required to determine how dose is affected by magnet’s pull

what is unique about MRL commissioning and QA 1

all components must be non-ferromagnetic

most tools used for QA do not the meet the non-ferrous standard so physics had to develop new devices and protocols

no lasers in the room

no lasers in the room for MRL-where are lasers normally found

normally mounted on the wall or in floor but there is a concern that lasers could interfere with magnet and affect image quality or laser could be projectile

the only laser used in MRL is

internal sagittal laser

what is the problem with only a single internal sagittal laser being used with MRL

single laser complicates patient setup and requires additional positioning

what is unique about MRL commissioning and QA II

MRL relies on adaptive workflow

volunteers without metal in their bodies underwent repeat MR imaging to calibrate the imaging system (MRI is non ionizing)

_________ and ______ validation complicated by the fact that…..

beam modeling and delivery system validation complicated by the fact that the electron beam in the linac is bent by the MR magnetic field

there is a lateral shift in the beam profile that must be accounted for

the machine relies on adaptive workflow: normal vs MRL

normally the RT set the patient up to positioning chosen during sim

MRL: plan is adjusted to match the patient position: adjust beam position to account for day to day patient and tumor changes

adapt to position (ATP) by MRL

online adaptive workflow adjusts tx plan based on rigid or deformable registrations by shifting the isocenter (virtual couch shift) to align with daily anatomy

what does adapt to position rely on

this is a quick adjustment that relies on rotating the original plan to the new daily position

what does adapt to position assess

anticipated inter and intrafractional setup and motion uncertainity

ATP is best suited when

anatomy has not changed much from sim

adapt to shape (ATS) for MRL

more time consuming process

team fully recontours anatomy and reoptimizes the dose

what compromises the MRL worflow

there is an offline portion and an online portion

offline portion of MRL workflow

offline portion is similar to conventional tx workflow

1. simulation (3D reference CT plus 3D reference MR Image)

2. planning

   1. use plan to evaluate offline adaptation: how should the plan be adapted to match reference images?

during the online portion of the mrl workflow

the patient is on the table

online portion of MRL workflow process

1. daily MR images acquired

2. adapt patient position to reference images

3. adapt to position original segments

4. adapt to position adapt segments

5. adapt to position optimize shapes

6. adapt to position optimize weight

7. THESE CHANGES ARE DONE WHILE THE PT IS ON TABLE

during the online portion-the patient is on the table: adapt the patient position to reference images-how is this done

adapt segments based on original segments by mathematically adjusting beam segments and weights

adapt to portion original segments-what is an original segment

distinct field opening that is a combination of shape created by leaves and assigned weight

when can adapt to position original segments be used

if there are very small differences in the daily MR compared to initial MR images (<3mm)

usually not achievable

adapt to position adapt segments

if the projected DVH differs significantly from original DVH, the leaves must be moved and dose recalculated with the new virtual isocenter

adapt to position-optimize shapes

leaves can be moved to better match bev projection of target based on adapted isocenter and dose is recalculated

adapt to position-optimize weights

segments weights can be optimized as well-this does not change the shapes (leaves)

inter-fraction (between fraction) uncertainties

patient position with regards to planning iso and tx beam

tumor position with regard to patient position

intra-fraction (within one fx)

tumor position with regards to patient position over time

how often i adaptation performed

every fraction, regardless of patient positional shifts

who is responsible for generating an adaptive plan

dosimetrist

who must be present for daily setup

physician (or resident), physicist, dosimetrist, therapist

who is responsible for daily MR assessment

therapist