Over-response of synthetic microDiamond detectors in small radiation fields

Ralston, Anna; Tyler, Madelaine; Liu, Paul; McKenzie, David R.; Suchowerska, Natalka

doi:10.1088/0031-9155/59/19/5873

Cited by 81 publications

(90 citation statements)

References 14 publications

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“…The correction factors resulted ≈2% lower than those proposed by Ralston et al 31 16 on a CyberKnife system. Our dose response ratios thus result in an acceptable agreement with the already published ones, bearing in mind the different conditions in which those results were obtained.…”

Section: Discussioncontrasting

confidence: 64%

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Evaluation of a synthetic single‐crystal diamond detector for relative dosimetry on the Leksell Gamma Knife Perfexion radiosurgery system

Mancosu¹,

Reggiori²,

Stravato³

et al. 2015

Medical Physics

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Purpose: To evaluate the new commercial PTW-60019 synthetic single-crystal microDiamond detector (PTW, Freiburg, Germany) for relative dosimetry measurements on a clinical Leksell Gamma Knife Perfexion radiosurgery system. Methods: Detector output ratios (DORs) for 4 and 8 mm beams were measured using a micro- Diamond (PTW-60019), a stereotactic unshielded diode [IBA stereotactic field detector (SFD)], a shielded diode (IBA photon field detector), and GafChromic EBT3 films. Both parallel and transversal acquisition directions were considered for PTW-60019 measurements. Measured DORs were compared to the new output factor reference values for Gamma Knife Perfexion (0.814 and 0.900 for 4 and 8 mm, respectively). Profiles in the three directions were also measured for the 4 mm beam to evaluate full width at half maximum (FWHM) and penumbra and to compare them with the corresponding Leksell GammaPlan profiles. Results: FWHM and penumbra for PTW-60019 differed from the calculated values by less than 0.2 and 0.3 mm, for the parallel and transversal acquisitions, respectively. GafChromic films showed FWHM and penumbra within 0.1 mm. The output ratio obtained with the PTW-60019 for the 4 mm field was 1.6% greater in transverse direction compared to the nominal value. Comparable differences up to 0.8% and 1.0% for, respectively, GafChromic films and SFD were found. Conclusions: The microDiamond PTW-60019 is a suitable detector for commissioning and routine use of Gamma Knife with good agreement of both DORs and profiles in the three directions

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Section: Discussioncontrasting

confidence: 64%

“…As reported in two recent papers, indeed, the physical density of the detector is the dominant parameter in small fields. 30,31 In large fields, instead, the mass energy absorption coefficient dominates. If no volume averaging were present, there would be an over-response of finite size detectors in such small fields.…”

Section: Discussionmentioning

confidence: 99%

Evaluation of a synthetic single‐crystal diamond detector for relative dosimetry on the Leksell Gamma Knife Perfexion radiosurgery system

Mancosu¹,

Reggiori²,

Stravato³

et al. 2015

Medical Physics

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“…Diamond detectors, which have a sensitive volume of density 3.5 g/cm 3 , have been reported to overrespond in small fields 16 , 39 , 41 , 42 . Monte Carlo simulations have shown that the magnitude of overresponse is proportional to the mass density of the material in the volume of interest, with a larger overresponse determined for a voxel of the same size in a radiation beam for diamond than for silicon in a 5 mm field for 15 MV (38) .…”

Section: Discussionmentioning

confidence: 99%

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators

Tyler

Liu

Lee

et al. 2016

J Applied Clin Med Phys

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Flattening filter‐free (FFF) beams are becoming the preferred beam type for stereotactic radiosurgery (SRS) and stereotactic ablative radiation therapy (SABR), as they enable an increase in dose rate and a decrease in treatment time. This work assesses the effects of the flattening filter on small field output factors for 6 MV beams generated by both Elekta and Varian linear accelerators, and determines differences between detector response in flattened (FF) and FFF beams. Relative output factors were measured with a range of detectors (diodes, ionization chambers, radiochromic film, and microDiamond) and referenced to the relative output factors measured with an air core fiber optic dosimeter (FOD), a scintillation dosimeter developed at Chris O'Brien Lifehouse, Sydney. Small field correction factors were generated for both FF and FFF beams. Diode measured detector response was compared with a recently published mathematical relation to predict diode response corrections in small fields. The effect of flattening filter removal on detector response was quantified using a ratio of relative detector responses in FFF and FF fields for the same field size. The removal of the flattening filter was found to have a small but measurable effect on ionization chamber response with maximum deviations of less than ±0.9% across all field sizes measured. Solid‐state detectors showed an increased dependence on the flattening filter of up to ±1.6%. Measured diode response was within ±1.1% of the published mathematical relation for all fields up to 30 mm, independent of linac type and presence or absence of a flattening filter. For 6 MV beams, detector correction factors between FFF and FF beams are interchangeable for a linac between FF and FFF modes, providing that an additional uncertainty of up to ±1.6% is accepted.PACS number(s): 87.55.km, 87.56.bd, 87.56.Da

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“…If the dose distribution across the circular integration area has a circular symmetry, then an alternative integration method using high‐resolution scanned dose profiles can be employed 15, 25, 26. Table 1 lists the type of detectors used for each instance where we used this method, a general description of which is given here.…”

Section: Methodsmentioning

confidence: 99%

Commissioning of a PTW 34070 large‐area plane‐parallel ionization chamber for small field megavoltage photon dosimetry

Kupfer

Lehmann

Butler³

et al. 2017

J Applied Clin Med Phys

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PurposeThis study investigates a large‐area plane‐parallel ionization chamber (LAC) for measurements of dose‐area product in water (DAP w) in megavoltage (MV) photon fields.MethodsUniformity of electrode separation of the LAC (PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high‐resolution microCT. Signal dependence on angle α of beam incidence for square 6 MV fields of side length s = 20 cm and 1 cm was measured in air. Polarity and recombination effects were characterized in 6, 10, and 18 MV photons fields. To assess the lateral setup tolerance, scanned LAC profiles of a 1 × 1 cm2 field were acquired. A 6 MV calibration coefficient, ND ,w, LAC, was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAP w. Additional calibrations in 10 × 10 cm2 fields at 6, 10, and 18 MV were performed.ResultsElectrode separation is uniform and agrees with specifications. Volume‐averaging leads to a signal increase proportional to ~1/cos(α) in small fields. Correction factors for polarity and recombination range between 0.9986 to 0.9996 and 1.0007 to 1.0024, respectively. Off‐axis displacement by up to 0.5 cm did not change the measured signal in a 1 × 1 cm2 field. ND ,w, LAC was 163.7 mGy cm−2 nC −1 and differs by +3.0% from the coefficient derived in the 10 × 10 cm2 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV.ConclusionsThe LAC requires only small correction factors for DAP w measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm2 fields, but beam incidence must be kept as close to normal as possible. Calibration in 10 × 10 fields is not recommended because of the LAC's over‐response. The accuracy of relative point‐dose measurements in the field's periphery is an important limiting factor for the accuracy of DAP w measurements.

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Over-response of synthetic microDiamond detectors in small radiation fields

Cited by 81 publications

References 14 publications

Evaluation of a synthetic single‐crystal diamond detector for relative dosimetry on the Leksell Gamma Knife Perfexion radiosurgery system

Evaluation of a synthetic single‐crystal diamond detector for relative dosimetry on the Leksell Gamma Knife Perfexion radiosurgery system

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators

Commissioning of a PTW 34070 large‐area plane‐parallel ionization chamber for small field megavoltage photon dosimetry

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