Medical Linear Accelerators in Radiation Therapy. Haijun Song, Ph.D. Dept. of
Radiation Oncology. Duke University Medical Center ...
Medical Linear Accelerators in Radiation Therapy
Haijun Song, Ph.D. Dept. of Radiation Oncology Duke University Medical Center
MV Linear Accelerator
Anatomy of a Linac
Anatomy of a Linac
Beam Mofiers Cone MLC (Multi-Leaf Collimator)
Radiosurgery with Linac
MLC
Conformal RT (3DCRT) & Intensity Modulation RT (IMRT)
Conformal RT (3DCRT) & Intensity Modulation RT (IMRT)
Dose Map delivered by IMRT
Dose Map delivered by IMRT
Dose Map delivered by IMRT
Image Guided RT (IGRT)
Respiratory Gating RT
Tomotherapy and Robotic
Difference among Radiation Sources Factors affecting radio-bio response • Gamma vs. Electron Among Gamma sources • Photon Energy Spectrum • Dose Distribution • Dose Rate
Radioactive Isotopes Example: Cs-137 • single dominant energy, 0.66 MeV • Activity: 6000 Ci and 100 Ci, dual source • Dose rate range: 0.001 - 10 Gy/min
Radioactive Isotopes Example: Ir-192 • Complex energy spectrum: 30 gammas and 40 x rays. • Dose rate, 0.01 - 5 Gy/min being used RT. Ir-192 photon spectrum 1000
Percent per decay
100 10 1 0.1 0.01 0.001 0.001 0.0001
0.01
0.1 E (MeV)
1
10
From kV to MV Commonly: 50 – 300 kVP Beam Quality: HVL a few mm Al to Cu. Dose Rate: around Gy/min
From kV to MV Commonly: 6 MV -25 MV Beam quality: PDD10 in water, 67% and up. Dose Rate: around Gy/min
X-Ray tube vs. Linac
Acce. Tube: Travelling Wave
X-Ray Production Side emission vs. straight through emmission
Photon energy spectrum from a 6 MV beam Fl uence/ MeV/ I ndiFlcent el ect uence/ MeV/r on I ndi cent el ect r on 1. 40E- 04 1. 20E- 04 1. 00E- 04 8. 00E- 05 6. 00E- 05 4. 00E- 05 2. 00E- 05 0. 00E+00
0
1
2
3
4
5
6
Depth Dose Curves
Dose Distribution: One beam/parallel beams
Dose Distribution: One beam/parallel beams
MV photon and electron beams
Depth Dose of e Beams 100 90
Percent Depth Ionization
80 70 60
6
8
10
12
15 18 M eV
50 40 30 20 10 0 0
1
2
3
4
5
6
7
D e p th ( c m ) in W a te r
8
9
Dose Measurement DwQ = M * kQ N D60,wCo Ion chamber and electrometer In water for MV photon or electron beams In air for KV beams Tracable to NIST Co-60 standard beam
Buildup region and skin sparing Compton e1
1
Compton e2
1
1
1
1
1
1
1
Compton e3 Compton e4
1 Depth in water Æ
Total dose
1
2
3
3
Treating Superficial targets Bolus: tissue equivalent, soft to conform to skin 100
B
dmax
Percent Depth Dose
80
60
S 40
20
0 0
1
2
Depth in Water (cm)
3
4
Dose Distribution: One beam/parallel beams
Buildup region and skin sparing continued Skin sparing also means a target at shallow depth will not get adequate dose. For small animals, “sparing” of a couple of centimeter from skin may “spare” the whole animal. What do we do when we need to treat superficial target?
Bolus
Treating Superficial targets Bolus: tissue equivalent, soft to conform to skin 100
B
dmax
Percent Depth Dose
80
60
S 40
20
0 0
1
2
Depth in Water (cm)
bolus
3
4
Treating Superficial targets Bolus: tissue equivalent, soft to conform to skin 100
B
dmax
Percent Depth Dose
80
60
S 40
20
0 0
1
2
Depth in Water (cm)
bolus
3
4
Treating Superficial targets MeV range electron beams /w bolus
kV (suferfical units, orthovoltage units) MV photon beam with bolus
How useful are MeV electron beams for small animal irradiations?
What can Rad Onc Offer Rad Biology All offered to human can be offered to other species. Imaging: CT, MRI Precise and conformal dose coverage: 3DCRT, IMRT, IGRT, Respiratory Gating, radiosurgery, electron irradiation. Precision and conformality means sparing healthy tissue, too. And More: Brachytherapy, hyperthermia, proton, neutron.
Acce. Tube: Standing Wave
Acce. Tube: Short Standing Wave
