Outline Kilovoltage unit definition Calibration and commissioning - PDF document

Outline • Kilovoltage unit definition • Calibration and commissioning of an PRACTICAL ASPECTS OF COMMISSIONING AND orthovoltage unit CALIBRATION OF CLINICAL ORTHOVOLTAGE UNITS • Clinical applications Wamied Abdel-Rahman 1 , Li Heng • Quality assurance Liang 2 , Ismail Aldahlawi 1 , and Jan Seuntjens 3 1) Montreal General Hospital, Montreal, Quebec, Canada 2) SMBD Jewish General Hospital, Montreal, Quebec, Canada 3) McGill University, Montreal, Quebec, Canada McGill McGill Practical Medical Physics - AAPM 2009 1 Practical Medical Physics - AAPM 2009 2 Low energy photon radiotherapy Superficial vs. Orthovoltage (Based on F.M. Khan, The physics of radiation therapy, second Edition ) • Grenz-ray therapy – energy < 20 kVp Superficial Orthovoltage • Contact therapy HVL 0.01 mm -8.0 mm – 40-50 kVp BJR Supplement 25 HVL 0.5 mm – 4.0 mm Al – SSD < 2 cm (1996) Cu (approx. 6 -150 KV) – ~ 2 mA IAEA TRS-398 (2000) ≤ HVL 3.0 mm Al (100 kV) ≥ HVL 2.0 mm Al (80 kV) • Superficial therapy – 50-150 kVp – SSD = 15-20 cm AAPM TG-61 (2001) 40 kV - 100 kV 100 kV – 300 kV – ~ 5-8 mA • Orthovoltage therapy (“Deep” therapy) – 150-500 kVp – SSD ~ 50 cm – 10-20 mA McGill McGill Practical Medical Physics - AAPM 2009 3 Practical Medical Physics - AAPM 2009 4

Treatment applications Kilovoltage clinical application PDD (FS=10x10 cm 2 ) • Low energy photons • Skin cancer 100 6 MeV, SSD=100 cm – Advantages 90 9 MeV, SSD=100 cm 80 12 MeV, SSD=100 cm – Melanoma 50 kVp, SSD=22 cm, cone = 3 cm diam, 0.25 mm Al • Sharp penumbra 70 80 kVp, SSD= 40 cm, 2.45 mm Al 120 kVp, SSD= 40 cm, 3.75 mm Al 60 • Small lesions PD D (% ) 250 kVp, SSD= 40 cm, 2.02 mm Cu – Basal cell carcinoma (BCC) 50 Co-60, SSD = 80 cm • Less complexity of machine and treatment 40 30 – Squamous cell carcinoma (SCC) • Easy setup 20 10 – Disadvantages 0 0 1 2 3 4 5 6 7 8 9 10 • Penetrating beam Depth (cm) • Other skin lesions • Higher dose to the bone • Electron beam advantages: – Keloid treatment • Sharp falloff of the PDD • Better cosmetic outcome • Bone “sparing” • Brachytherapy advantages: • Better outcomes for some selected sites McGill McGill Practical Medical Physics - AAPM 2009 5 Practical Medical Physics - AAPM 2009 6 Gulmay D3225 Orthovoltage Unit • Gulmay Medical Limited, Chertsey, Surrey, UK – Floor mounted tube stand and treatment table – Comet MXR321 metal Ceramic x-ray tube assembly – 9 treatment filters and 1 warm up filter – 6 square applicators (SSD = 50 cm) Calibration and commissioning of an – 4 circular applicators (SSD = 20 cm) • X-ray beam specification orthovoltage unit – X-ray tube output limits: • 20-220 kV, 0-20 mA, 400-3000 W • kVp at the JGH : 80, 120, 180, and 220 kVp • Tube – Focal spot : ~7.5 mm – Target: Tungsten – Inherent filter: 0.8 ± 0.1 mm Be – Tube power max : 3000 W – Field coverage total: 40 o – Anode angle: 30 o – Weight: 11 Kg McGill McGill Practical Medical Physics - AAPM 2009 7 Practical Medical Physics - AAPM 2009 8

Gulmay D3225 Orthovoltage Unit Calibration Filters (9 + 1) • Protocol: – The American Association of Medical Physicists Task Group 61 (AAPM TG-61) • Requirements – Ionization chamber with an air kerma free in air calibration coefficient N k traceable to national standards Square (cm x 4x4 6x6 8x8 10x1 15x1 20x2 cm) SSD = 50 0 5 0 – N K can be calculated from the exposure cm Circular diameter 3 4 5 10 calibration coefficient N X : (cm), SSD = 20 cm N K = N X ( W / e ) air / (1- g ) McGill McGill Practical Medical Physics - AAPM 2009 9 Practical Medical Physics - AAPM 2009 10 Beam quality Beam quality specifier • Beam quality depends on: • Standards lab: HVL 1 and kVp for – Tube potential – Target material and determination of N k angle – Window material and thickness – Monitor chamber • Clinical unit: HVL 1 for determination other material and thickness parameters in the TG-61 formalism – Filtration material and thickness – Shape of collimator The physics of radiology, Johns & Cunningham – Source-chamber distance McGill McGill Practical Medical Physics - AAPM 2009 11 Practical Medical Physics - AAPM 2009 12

Formalism: TG-61 Dosimeters • Parallel-plate chambers: • In air method: – below 70 kV – Measurement performed in air – 40 kV ≤ Tube potential ≤ 300 kV • Cylindrical chambers – For 70 kV – 300 kV • In phantom method: Ionization – Measurement performed in chamber • Appropriate buildup should be used to eliminate Water the effect of contaminating electrons – Size: At least 30 × 30 × 30 cm 3 – 100 kV < Tube potential ≤ 300 kV McGill McGill Practical Medical Physics - AAPM 2009 13 Practical Medical Physics - AAPM 2009 14 Determination of HVL: TG-61 Determination of HVL recommendations • 1 st HVL: thickness of a specified attenuator • Diaphragm that reduces the air-kerma rate in a narrow – Beam diameter ≤ 4 Monitor beam to ½ its original value. cm chamber 50 cm – Thickness must Attenuator material attenuate primary – Measurement of the variation with the beam to 0.1 %. attenuator thickness of the air-kerma rate at a diaphragm point in a “scatter free” and narrow beam. 50 cm – Detectors with sufficient build-up should be Ionization chamber used to eliminate the effect of contaminating (detector) electrons. McGill McGill Practical Medical Physics - AAPM 2009 15 Practical Medical Physics - AAPM 2009 16

Determination of HVL: TG-61 Determination of HVL: TG-61 recommendations recommendations • Monitor chamber • Attenuator – Used to correct for – High-purity material Monitor Monitor kerma rate (99 %). chamber chamber 50 cm 50 cm variations Attenuator Attenuator – Thickness material material – Should not perturb measured with an the narrow beam. accuracy of 0.05 diaphragm diaphragm mm. – Should not be affected by the 50 cm 50 cm attenuator material Ionization Ionization chamber chamber (detector) (detector) McGill McGill Practical Medical Physics - AAPM 2009 17 Practical Medical Physics - AAPM 2009 18 HVL measurement HVL measurement McGill McGill Practical Medical Physics - AAPM 2009 19 Practical Medical Physics - AAPM 2009 20

Determination of N k for a clinical beam (TG-61 Determination of N k in the clinic (TG-61 recommendation) recommendation) • Use of kVp and HVL • ADCL’s provide calibration coefficients – Ideal : Obtain N k for for specific beam the same kVp and qualities that are HVL beam that grouped into: matches the user clinical beam – Lightly filtered ( L series ) – Medium filtered ( M series ) – Practical : Obtain N k for two beams – Heavily filtered ( H with the same kVp series ) but two HVLs and interpolate using the HVL for the clinical beam • Interpolation based on HVL may only be performed within the same series McGill McGill Practical Medical Physics - AAPM 2009 21 Practical Medical Physics - AAPM 2009 22 Percentage depth dose Calibration: Setup • Chamber: NE 2571 farmer type • Measurement medium: cylindrical chamber water • Method: in-air • Output specification point: • Instrument: NACP01 – 0 cm depth in water at the cone end parallel plate chamber – Inverse square factor is - Window thickness = 90 required for closed end cones mg/cm 2 to correct for chamber position - Electrode spacing = 2 mm - Effective point of measurement = 1.9 mm McGill McGill Practical Medical Physics - AAPM 2009 23 Practical Medical Physics - AAPM 2009 24

PDD vs. SSD Beam profiles: Inline and crossline Crosslin e PDD VS SSD Inline McGill McGill Practical Medical Physics - AAPM 2009 25 Practical Medical Physics - AAPM 2009 26 Back scatter factors Dose to tissue McGill McGill Practical Medical Physics - AAPM 2009 27 Practical Medical Physics - AAPM 2009 28

SSD vs potential errors 1 mm SSD error SSD (cm) SSD (cm) 20 20 50 50 ISL ISL (20.1/20) 2 (20.1/20) 2 =1.010 =1.010 (50.1/50) (50.1/50) 2 2 =1.004 =1.004 Error(%) Error(%) 1.0% 1.0% 0.4% 0.4% Clinical application SS D McGill McGill Practical Medical Physics - AAPM 2009 29 Practical Medical Physics - AAPM 2009 30 Custom Shielding Cavity filling •Entrance shielding: lead sheet, Cerrobend cutout •Exit shielding: lead, tungsten (eye shielding) (coated) McGill McGill Practical Medical Physics - AAPM 2009 31 Practical Medical Physics - AAPM 2009 32

Quality Assurance Physics QA Quality assurance McGill McGill Practical Medical Physics - AAPM 2009 33 Practical Medical Physics - AAPM 2009 34 Conclusions • The AAPM TG-61 is a protocol for reference dosimetry of low energy photon beams (40 kV – 300 kV). • The effective point of measurement for parallel-plate and cylindrical ionization chambers is the center of the sensitive air cavity. • If the dose close to or at the surface is of interest, the in-air method should be used. • If the dose at depth is of interest, the in-phantom method should be used. McGill Practical Medical Physics - AAPM 2009 35