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

Gräfe, James L.; Poirier, Yannick; Jacso, Ferenc; Khan, Rao; Liu, Hongwei; Villarreal-Barajas, J. Eduardo

doi:10.1120/jacmp.v15i4.4893

Cited by 17 publications

(20 citation statements)

References 10 publications

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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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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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“…This is done by measuring the depression or protrusion as the stand-off. A stand-off correction factor (SOC) is then calculated using the SSD and stand-off measured with the inverse square law or inverse cube law [15], depending on how large the stand-off is.…”

Section: Monitor Unit Calculationmentioning

confidence: 99%

An Internet of Things app for monitor unit calculation in superficial and orthovoltage skin therapy

Pearse

Chow

2020

IOPSciNotes

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Purpose: We developed an app for Internet of Things (IoT) device such as smartphone or tablet to calculate the monitor unit in superficial and orthovoltage skin therapy. The app can run both on the Windows and Android operation system. Methods: The IoT app was created based on the formula to calculate the monitor unit in skin therapy using kV photon beams. The calculation was based on databases of dose variables such as relative exposure factor and backscatter factor. The calculation also considered the stand-off and stand-in correction according to the inverse-square and inverse-cube law. Verification of the app was carried out by comparing the monitor unit results with those from hand calculations. Results: The frontend window of the app provided a user-friendly interface to the user for inputting prescription dose, beam and treatment setup variables. The user could save the calculation record electronically, generate a printout or send it to other radiation staff using the IoT. Verification of the app showing that deviation between the monitor units calculated by the app and by hand is insignificant. Conclusion: The verified IoT app can effectively calculate the monitor unit in superficial and orthovoltage skin therapy. The app takes advantages of all innate features of IoT such as real time communication, Internet access, data transfer and sharing.

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“…Previous studies have almost exclusively focused on dose variation with stand‐off at the central axis. While this shows that dose varies with stand‐off, it neglects the additional dose variation off‐axis which is compounded by the loss of dose due to increased source to surface distance (SSD).…”

Section: Introductionmentioning

confidence: 99%

“…While this shows that dose varies with stand‐off, it neglects the additional dose variation off‐axis which is compounded by the loss of dose due to increased source to surface distance (SSD). Also, many studies such as Gerig et al and Gräfe et al only investigated applicators which featured a plastic cap at the applicator end; thus, no stand‐in investigation was possible. Gräfe et al found no clinically significant variation of dose from the ISL up to 10 cm stand‐off with their investigated beam energies and applicators.…”

Section: Introductionmentioning

confidence: 99%

“…Also, many studies such as Gerig et al and Gräfe et al only investigated applicators which featured a plastic cap at the applicator end; thus, no stand‐in investigation was possible. Gräfe et al found no clinically significant variation of dose from the ISL up to 10 cm stand‐off with their investigated beam energies and applicators. Closed‐ended applicators of dimensions 4 × 4, 10 × 10, and 20 × 20 cm 2 were investigated at beam energies of 100, 150, and 200 kVp.…”

Section: Introductionmentioning

confidence: 99%

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

Cited by 17 publications

References 10 publications

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

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

An Internet of Things app for monitor unit calculation in superficial and orthovoltage skin therapy

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

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