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Title: SU-F-T-75: Dosimetry Considerations in the Use of Hanging-Eye Block for Lesions of the Conjunctiva

Abstract

Purpose: Superficial lesions in the conjunctiva are frequently treated with en face electrons, using a hanging block to spare dose to the lens of the eye. Dose to the tumor and lens depend on the design and setup of the block and supporting apparatus. We performed in phantom measurements in order to characterize the dose sparing effects of the block as well as the under-dosing effect under the supporting apparatus for 6 MeV treatment. Methods: The commercial hanging block studied uses a 1.2 cm diameter tungsten cylinder supported by a 3 mm diameter acrylic rod. Point dose measurements under the hanging block, under an unblocked part of the field, and under the acrylic rod were performed using MOSFET detectors. In addition, EBT3 film was used for both PDD and profile measurements at a depth in phantom of 3 mm for both 105 and 103 cm SSD. Results: MOSFET measurements reported a dose reduction of 95% under the tungsten block when using an SSD of 103 cm, and 86% when using an SSD of 105 cm at a depth in phantom of 3 mm. Film measurements showed that the area under the acrylic rod may be under-dosed by as much asmore » 30% when using 103 SSD. MOSFET measurements confirmed that when using an SSD of 103 cm, the area under the acrylic rod is under-dosed by up to 30% at 3mm depth, compared to the unblocked part of the field. Conclusion: The effectiveness of the commercial hanging block apparatus depends on setup, with 95% lens sparing possible with an SSD of 103 cm. This short SSD is necessary for sharp penumbra. At this SSD, substantial under-dosing under the acrylic support rod is possible. This must be mitigated with either feathering, or using an alternative method of support for the tungsten block.« less

Authors:
; ; ; ; ; ;  [1]
  1. Northwestern Memorial Hospital, Chicago, IL (United States)
Publication Date:
OSTI Identifier:
22642323
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; CONJUNCTIVA; CRYSTALLINE LENS; DOSIMETRY; MOSFET; NEOPLASMS; PHANTOMS; RADIATION DOSES; TUNGSTEN

Citation Formats

Grelewicz, Z, Lee, B, Cutright, D, Kang, Z, Gopalakrishnan, M, Sathiaseelan, V, and Zhang, H. SU-F-T-75: Dosimetry Considerations in the Use of Hanging-Eye Block for Lesions of the Conjunctiva. United States: N. p., 2016. Web. doi:10.1118/1.4956211.
Grelewicz, Z, Lee, B, Cutright, D, Kang, Z, Gopalakrishnan, M, Sathiaseelan, V, & Zhang, H. SU-F-T-75: Dosimetry Considerations in the Use of Hanging-Eye Block for Lesions of the Conjunctiva. United States. doi:10.1118/1.4956211.
Grelewicz, Z, Lee, B, Cutright, D, Kang, Z, Gopalakrishnan, M, Sathiaseelan, V, and Zhang, H. 2016. "SU-F-T-75: Dosimetry Considerations in the Use of Hanging-Eye Block for Lesions of the Conjunctiva". United States. doi:10.1118/1.4956211.
@article{osti_22642323,
title = {SU-F-T-75: Dosimetry Considerations in the Use of Hanging-Eye Block for Lesions of the Conjunctiva},
author = {Grelewicz, Z and Lee, B and Cutright, D and Kang, Z and Gopalakrishnan, M and Sathiaseelan, V and Zhang, H},
abstractNote = {Purpose: Superficial lesions in the conjunctiva are frequently treated with en face electrons, using a hanging block to spare dose to the lens of the eye. Dose to the tumor and lens depend on the design and setup of the block and supporting apparatus. We performed in phantom measurements in order to characterize the dose sparing effects of the block as well as the under-dosing effect under the supporting apparatus for 6 MeV treatment. Methods: The commercial hanging block studied uses a 1.2 cm diameter tungsten cylinder supported by a 3 mm diameter acrylic rod. Point dose measurements under the hanging block, under an unblocked part of the field, and under the acrylic rod were performed using MOSFET detectors. In addition, EBT3 film was used for both PDD and profile measurements at a depth in phantom of 3 mm for both 105 and 103 cm SSD. Results: MOSFET measurements reported a dose reduction of 95% under the tungsten block when using an SSD of 103 cm, and 86% when using an SSD of 105 cm at a depth in phantom of 3 mm. Film measurements showed that the area under the acrylic rod may be under-dosed by as much as 30% when using 103 SSD. MOSFET measurements confirmed that when using an SSD of 103 cm, the area under the acrylic rod is under-dosed by up to 30% at 3mm depth, compared to the unblocked part of the field. Conclusion: The effectiveness of the commercial hanging block apparatus depends on setup, with 95% lens sparing possible with an SSD of 103 cm. This short SSD is necessary for sharp penumbra. At this SSD, substantial under-dosing under the acrylic support rod is possible. This must be mitigated with either feathering, or using an alternative method of support for the tungsten block.},
doi = {10.1118/1.4956211},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • The planning and subsequent treatment of inclined lesions is difficult when non-vertical or non-horizontal beams are to be employed. Rotations of the patient support assembly and the collimator further complicate the planning. The present work describes an empirical approach that is relatively simple (requires protractor and calculator). Frequently, plans must be generated in planes other then the central axis and a caution about the location of the SAD point will be discussed.
  • A technique for orbital radiotherapy is presented consisting of an anterior, appositional electron beam with a hanging lens block. The beam was modified by introducing two 1.6 mm thick plastic spoilers, at about 3 cm and 15 cm from the lens, to boost in-scattering of electrons under the block. The 9 mm diameter, 2 cm long stainless steel cylindrical block was suspended 0.5-1.0 cm above the eye. The authors performed film, TLD (Thermo Luminescent Dosimetry), and diode dosimetry to determine the dose fill-in behind the lens. The introduction of the spoilers dramatically changed the dose distribution. The maximum dose undermore » the block increased from 66% to 85% of the open field dose. Moreover, the dose to the posterior surface of the globe directly underneath the block, at a depth of 3 cm, increased from 48% to 76% of maximum dose, while the dose to the lens was still below 20%. This is a simple and easily reproducible treatment and is an improvement on a previously described technique. The dose distribution is adequate for cases where the target volume surrounds and is posterior to the globe.« less
  • Purpose: Kilo-voltage (kV) photons and low megavoltage (MeV) electrons are the most common options for treating small superficial lesions, but they present complex dosimetry. Using a tertiary lead shield may protect the surrounding critical structures. Our goal was to quantitatively evaluate the dosimetric impact resulting from applying tertiary shields on superficial lesions. Method: We directly compared the beam characteristics of 80 kV (0.8 mm Al) photon setup abutting the water phantom surface and 6 MeV electron setup at 100 cm SSD. Profiles and depth doses were acquired using a 3D scanning water tank and an ion chamber (active volume 0.01more » cm{sup 3}). Beam profiles were scanned at Dmax. Three lead sheets (2 mm thickness) with 2.7, 2.2, and 1.6, cm diameter circular cutouts were fabricated and placed at the water surface for both photon and electron fields. Results: The penumbra (80% – 20%) of the open 4×4 cm{sup 2} electron insert was 10.7 mm, compared to an average of 7.2 mm with the tertiary cutouts. The penumbra of the open kV photon beam was 2.8 mm compared to an average of 1.8 mm with the tertiary cutouts. For field widths 2.7, 2.2, and 1.6 cm, the flatness of the electron beams was 16%, 17.3%, and 21%, respectively, and for the kV photon beams was 1.4%, 2.3%, 3.3%, respectively. The electron depth dose (PDD) shifted shallower and the photon PDD shifted deeper as the field size became smaller. Conclusion: The penumbra of small electron fields can be improved by adding tertiary lead shields. Both modalities are clinically feasible; however, kV photons still offer sharper penumbra and better flatness than that of 6 MeV electrons with tertiary shielding. Thus, kV photons may still be a superior option for small superficial lesions.« less