海外华人医学物理讲师团 – 放疗物理第二讲（2015）

Linac 构造与临床关系 -- Xiangsheng (Jason) Yan

1． 外形及厂家

2. Linac Head 构造

2.1) Beam Steering

2.2) Flattening Filter

2.3) Dual Ionization Chamber

2.4) Jaw & EDW

2.5) Light Field

2.6) MLC and all related issues

3．图像设备 (EPID, OBI, CBCT, IsoCal)

4．配件装备 (e- app, block, couch)

5. 可选设备 (SRS Cone, ExacTrac, Gating, etc.)

6. 验收 (if time allowed)

1. 外形及厂家

厂家：Varian(瓦里安)45%, Elekta(医科达: +GammaKnife)43%, Siemens 12.5%, 新华

(Shinva)12.5%、东软、大基医疗. Accuray(安科锐:Tomo+CyberKnife),

BrainLab(Vero). TPS: Eclipse(Varian), Pinnacle(Philips), iPlan(BrainLab), Monaco(Elekta), XKnife(Integra), RayStation(RayResearchLab), MultiPlan(CyberKnife), Tomo(Tomo), Oncentra(Elekta), BrachyVision\Variseed(Varian), etc.

2. Linac Head Structure.

2.1) Beam Steering:

Beam Steering example #1:

Solution: adjust the steering of the bending magnets, or the energy of the electron beam, or a combination of both. For any changes made, once profile is flat, must check energy and output!

Beam Steering example #2:

Example: center not aligned perfectly (0.15cm error), not observable on 30cm*30cm profile, but a big impact on SRS profile with 2cm cone size.

2.2) Flattening Filter:

Goal of Flattening Filter: making the profile of each photon energy flat at depth 10cm.

Why FFF (Flattening Filter Free)? Less demanding on field junction accuracy for RapidArc/VMAT technique.

2.3) Dual Ionization Chamber

--- safety: double check each other (e.g. sealed chamber leaking), backup if another fail. --- MU (Monitor Unit): 10cm*10cm field size, 100cm SSD, 1MU gives 1cGy at dmax (e.g. dmax =

1.5cm for 6X, 2.5cm for 10X, 3.5cm for 18X). --- backscatter effect: (from jaw if Varian Linac, from MLC if Elekta Linac)

--- chamber actually also has eight sectors, four for photons and four for electrons, to measure and give feedback of beam flatness and symmetry, hence servo can control the flatness and symmetry in real time at different gantry angles.

2.4) Jaw (EDW: Enhanced Dynamic Wedge) --- Physical Wedge vs. EDW (Varian): Physical wedge: too heavy, dangerous, limited # of angles, too much scatter

EDW: jaw closing, or opening to create various angled beams (jaw speed changes)

--- Virtual Wedge, Universal Wedge (Siemens, Elekta): jaw speed constant,

change dose rate; combining 60o wedged beam with non-wedged beam, with

different ratio of weighting, e.g. 50% 60o + 50% 0

o = 30

o wedged beam

2.5) Light Field --- Before first day of treatment, patient body marked with marker pen indicating treatment field. --- on first day of treatment, therapist align laser (at iso) to patient plan origin (tattoo or marker), then shift (x,y,z) to planned patient position, then the light field should align with marked field. --- patient alignment can be further confirmed/fine-tuned using EPID, or CBCT, or KV-pairs.

Linac acceptance (验收): light field need to be aligned with radiation field <1mm error, or <1%.

TrueBeam (newest Varian Linac): mirror on one of the carousel holes.

2.6) MLC (Multileaf Collimators 多叶准直器)

--- Why rounded leaf end?

--- IMRT (Intensity Modulated Radiation Therapy):

3．图像设备 (EPID, OBI, CBCT)

--- EPID: electronic portal imaging device. It’s amorphous silicon based (aS1000). Ithas an active

imaging area of 40 x 30 cm2 (at an SSD of 105 cm). The image matrix is created from an array of

1024 x 768 pixels. Resolution: 40cm/1024pixels = 0.4mm/pixel.

--- OBI (Onboard Imager), CBCT (Cone Beam CT):

--- talking between imaging device and Linac (IsoCal with Varian)

SSD Rod (visual) Start-shot using film (2D iso test) Winston-Luz test (3D iso test) IsoCal (automatic iso test)

Iso match: Gantry/Collimator/Couch iso Treatment iso = MV iso (EPID) KV iso (OBI, CBCT)

4．配件装备 (e- applicator, e- block, photon block, couch)

--- Couch Support: Tennis Board for body, and for head. Couch Rails.

5. 可选设备 (SRS Cone, ExacTrac, Gating, etc.) 5.1) why SRS cone: less penumbra, accurately centered by testing & adjusting.

Penumbra comes from: source size, collimation scatter, patient body scatter, SSD (Linac = 100cm, CybeKnife = 80cm, GammKnife = 40cm)

5.2) ExacTrac (from BrainLab, integrated into Varian system)

ExacTrac vs EPID: less dose, better image quality ExacTrac vs KV-Pairs: see above. ExacTrac vs CBCT:

--- only see bones, not tissue. Need helps with fiducial implant, or Calypso. --- Instantaneous images and 3D positioning.

ExacTrac: can be used combining Varian Gating technique.

5.3) Varian Gating