Characteristics of orthovoltage x-ray therapy beams at extended SSD for applicators with end plates

Li, X. Allen; Salhani, D.; M., C.

doi:10.1088/0031-9155/42/2/008

Cited by 13 publications

(22 citation statements)

References 12 publications

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“…The standoff correction shall be applied for closed‐ended applicators in order to reduce systematic dose errors for the orthovoltage X‐ray treatments. Our standoff findings reemphasize the results of Li et al (1) The standoff effect needs to be quantified for a given orthovoltage treatment unit; the radiation therapy planner can make an informed decision on the relevance of this parameter in their practice.…”

Section: Resultssupporting

confidence: 72%

“…2 is larger than that observed by Li et al (1) This may be due to the thicker acrylic endplate for our applicators (4 mm compared to 3.2 mm (1) ) and the use of the Markus chamber, which allows the effective point of measurement of the detector to be placed very close to the applicator end plate. In addition, Evans et al (11) observed no significant deviation from the ISL for closed‐ended applicators for a Gulmay D3300 unit.…”

Section: Resultsmentioning

confidence: 57%

“…The CC13 chamber has a 0.13 cc sensitive volume. This detector was chosen because of the similar volume compared to the 0.12 cc cylindrical ion chamber used by Li et al (1) The effective point of measurement for this chamber was taken as the center of the active volume and, therefore, the closest point of measurement for this chamber was 0.5 cm due to geometrical limitations of the detector size (limited by the detector radius and cable sheath). Since these are relative measurements performed in air, no effects due to chamber composition are expected since the beam quality in air does not change significantly within the short range of standoff distances used in the present study.…”

Section: Methodsmentioning

confidence: 99%

“…Li et al (1) have reported divergence from the expected ISL for closed‐ended applicators. It is also recommended by the AAPM Task Group protocol for kilovoltage X‐ray beams (TG61) that this effect be taken in account (2) .…”

Section: Introductionmentioning

confidence: 98%

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Assessing the deviation from the inverse square law for orthovoltage beams with closed‐ended applicators

Gräfe

Poirier

Jacso³

et al. 2014

J Applied Clin Med Phys

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In this report, we quantify the divergence from the inverse square law (ISL) of the beam output as a function of distance (standoff) from closed‐ended applicators for a modern clinical orthovoltage unit. The divergence is clinically significant exceeding 3% at a 1.2 cm distance for 4 × 4 and 10.15em×.15em10cm2 closed‐ended applicators. For all investigated cases, the measured dose falloff is more rapid than that predicted by the ISL and, therefore, causes a systematic underdose when using the ISL for dose calculations at extended SSD. The observed divergence from the ISL in closed‐ended applicators can be explained by the end‐plate scattering contribution not accounted for in the ISL calculation. The standoff measurements were also compared to the predictions from a home‐built kV dose computation algorithm, kVDoseCalc. The kVDoseCalc computation predicted a more rapid falloff with distance than observed experimentally. The computation and measurements agree to within 1.1% for standoff distances of 3 cm or less for 4.15em×.15em4cm2 and 10.15em×.15em10cm2 field sizes. The overall agreement is within 2.3% for all field sizes and standoff distances measured. No significant deviation from the ISL was observed for open‐ended applicators for standoff distances up to 10 cm.PACS numbers: 87.55.‐x, 87.55.kh

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Section: Resultssupporting

confidence: 72%

Section: Resultsmentioning

confidence: 57%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

See 2 more Smart Citations

Assessing the deviation from the inverse square law for orthovoltage beams with closed‐ended applicators

Gräfe

Poirier

Jacso³

et al. 2014

J Applied Clin Med Phys

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“…20 SSDs eff can be used instead of the nominal value to more accurately account for the effect of ISL in treatments with stand-offs, and this has been reported to be more important in the case of applicators closed with PMMA plates which were 3-4 mm thick. 21,22 In this work, the end plates of closed applicators had a thickness of 1 mm, and SSDs eff were determined for each applicator at 30, 50, 100 and 200 kV using cylindrical ionization chambers: depending on the field size, either a TW30013 (0.6 cm 3 ) or TM31010 (0.125 cm 3 ) chamber was used (both by PTWFreiburg, Freiburg, Germany). For each measurement, the effective point of measurement of the chamber in air was placed on axis and at the required distance from the applicator end.…”

Section: Focal Spotmentioning

confidence: 99%

Acceptance, commissioning and clinical use of the WOmed T-200 kilovoltage X-ray therapy unit

Aspradakis¹,

Zucchetti²

2015

BJR

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Objective: The objective of this work was to characterize the performance of the WOmed T-200-kilovoltage (kV) therapy machine. Methods: Mechanical functionality, radiation leakage, alignment and interlocks were investigated. Half-value layers (HVLs) (first and second HVLs) from X-ray beams generated from tube potentials between 30 and 200 kV were measured. Reference dose was determined in water. Beam start-up characteristics, dose linearity and reproducibility, beam flatness, and uniformity as well as deviations from inverse square law were assessed. Relative depth doses (RDDs) were determined in water and water-equivalent plastic. The quality assurance program included a dosimetry audit with thermoluminescent dosemeters. Results: All checks on machine performance were satisfactory. HVLs ranged between 0.45-4.52 mmAl and 0.69-1.78 mmCu. Dose rates varied between 0.2 and 3 Gy min 21 with negligible time-end errors. There were differences in measured RDDs from published data. Beam outputs were confirmed with the dosimetry audit. The use of published backscatter factors was implemented to account for changes in phantom scatter for treatments with irregularly shaped fields. Conclusion: Guidance on the determination of HVL and RDD in kV beams can be contradictory. RDDs were determined through measurement and curve fitting. These differed from published RDD data, and the differences observed were larger in the low-kV energy range. Advances in knowledge: This article reports on the comprehensive and novel approach to the acceptance, commissioning and clinical use of a modern kV therapy machine. The challenges in the dosimetry of kV beams faced by the medical physicist in the clinic are highlighted.

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Off‐axis dose distribution with stand‐in and stand‐off configurations for superficial radiotherapy treatments

Davey

Moore

Cleary

et al. 2019

J Applied Clin Med Phys

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Current practice when delivering dose for superficial skin radiotherapy is to adjust the monitor units so that the prescribed dose is delivered to the central axis of the superficial unit applicator. Variations of source‐to‐surface distance due to patient’s anatomy protruding into the applicator or extending away from the applicator require adjustments to the monitor units using the inverse square law. Off‐axis dose distribution varies significantly from the central axis dose and is not currently being quantified. The dose falloff at the periphery of the field is not symmetrical in the anode–cathode axis due to the heel effect. This study was conducted to quantify the variation of dose across the surface being treated and model a simple geometric shape to estimate a patient’s surface with stand‐in and stand‐off. Isodose plots and color‐coded dose distribution maps were produced from scans of GAFChromic EBT‐3 film irradiated by a Gulmay D3300 orthovoltage x‐ray therapy system. It was clear that larger applicators show a greater dose falloff toward the periphery than smaller applicators. Larger applicators were found to have a lower percentage of points above 90% of central axis dose (SA90). Current clinical practice does not take this field variation into account. Stand‐in can result in significant dose falloff off‐axis depending on the depth and width of the protrusion, while stand‐off can result in a flatter field due to the high‐dose region near the central axis being further from the source than the peripheral regions. The central axis also received a 7% increased or decreased dose for stand‐in or stand‐off, respectively.

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Characteristics of orthovoltage x-ray therapy beams at extended SSD for applicators with end plates

Cited by 13 publications

References 12 publications

Assessing the deviation from the inverse square law for orthovoltage beams with closed‐ended applicators

Assessing the deviation from the inverse square law for orthovoltage beams with closed‐ended applicators

Acceptance, commissioning and clinical use of the WOmed T-200 kilovoltage X-ray therapy unit

Off‐axis dose distribution with stand‐in and stand‐off configurations for superficial radiotherapy treatments

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