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Organ doses from a proton gantry-mounted cone-beam computed tomography system characterized with MCNP6 and GATE
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.ORCID iD: 0000-0001-9039-4979
Stockholm University, Faculty of Science, Department of Physics.
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2018 (English)In: Physica medica (Testo stampato), ISSN 1120-1797, E-ISSN 1724-191X, Vol. 53, p. 56-61Article in journal (Refereed) Published
Abstract [en]

Purpose

To determine organ doses from a proton gantry-mounted cone-beam computed tomography (CBCT) system using two Monte Carlo codes and to study the influence on organ doses from different acquisition modes and repeated imaging.

Methods

The CBCT system was characterized with MCNP6 and GATE using measurements of depth doses in water and spatial profiles in air. The beam models were validated against absolute dose measurements and used to simulate organ doses from CBCT imaging with head, thorax and pelvis protocols. Anterior and posterior 190° scans were simulated and the resulting organ doses per mAs were compared to those from 360° scans. The influence on organ doses from repeated imaging with different imaging schedules was also investigated.

Results

The agreement between MCNP6, GATE and measurements with regard to depth doses and beam profiles was within 4% for all protocols and the corresponding average agreement in absolute dose validation was 4%. Absorbed doses for in-field organs from 360° scans ranged between 6 and 8 mGy, 15–17 mGy and 24–54 mGy for the head, thorax and pelvis protocols, respectively. Cumulative organ doses from repeated CBCT imaging ranged between 0.04 and 0.32 Gy for weekly imaging and 0.2–1.6 Gy for daily imaging. The anterior scans resulted in an average increase in dose per mAs of 24% to the organs of interest relative to the 360° scan, while the posterior scan showed a 37% decrease.

Conclusions

A proton gantry-mounted CBCT system was accurately characterized with MCNP6 and GATE. Organ doses varied greatly depending on acquisition mode, favoring posterior scans.

Place, publisher, year, edition, pages
2018. Vol. 53, p. 56-61
National Category
Radiology, Nuclear Medicine and Medical Imaging
Research subject
Medical Radiation Physics
Identifiers
URN: urn:nbn:se:su:diva-160350DOI: 10.1016/j.ejmp.2018.08.011ISI: 000445037300007OAI: oai:DiVA.org:su-160350DiVA, id: diva2:1249508
Available from: 2018-09-19 Created: 2018-09-19 Last updated: 2018-10-16Bibliographically approved
In thesis
1. Interlaced proton grid therapy: development of an innovative radiation treatment technique
Open this publication in new window or tab >>Interlaced proton grid therapy: development of an innovative radiation treatment technique
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Spatially fractionated radiotherapy, also known as grid therapy (GRID), has been used for more than a century to try to treat several kinds of lesions. Yet, the grid technique remains a relatively unknown and uncommon treatment modality nowadays. Spatially fractionated beams, instead of conventional homogeneous fields, have been used to exploit the experimental finding that normal tissue can tolerate higher doses when smaller tissue volumes are irradiated. This increase in tolerance with reducing beam size is known as the dose-volume effect. Despite the fact that targets were given inhomogeneous dose distribution, sometimes with some volumes receiving close to no dose, good results in the form of shrinking of bulky tumors have been observed in palliative treatments. The biological processes responsible for this effect are still under discussion, with several possible causes. However, numerous experiments on mice, rats and pigs have confirmed the existence of such effect, which in turn motivates the present development of grid therapy.While mainly photons have been used in grid therapy, proton and ion grid therapies are also emerging as potential alternatives. In this work, an innovative form of grid therapy was proposed. Grids of proton beamlets were interlaced over a target volume with the intention of achieving two main objectives: (1) to keep the grid pattern (made of adjacent high and low doses) from the skin up to the vicinity of the target while (2) delivering nearly homogeneous dose to the target volume. This interlaced proton grid therapy was explored with the use of different beam sizes, from conventional sizes deliverable at modern proton facilities, down to millimeter sized beams. Other considerations that would prevent its clinical use, such as the variable relative biological effectiveness of protons or the use of cone beam computed tomography, were also evaluated. The overall aim was to assess if, and how, such treatment modality could be applied clinically, from a physics and dosimetry point of view. While it presented several theoretical advantages, its potential issues of concern and limitations were also evaluated.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2018. p. 65
Keywords
proton therapy, grid therapy, spatially fractionated therapy, interlacing
National Category
Cancer and Oncology
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-160427 (URN)978-91-7797-442-0 (ISBN)978-91-7797-443-7 (ISBN)
Public defence
2018-11-09, CCK Lecture Hall, Building R8, Karolinska University Hospital, Solna, Stockholm, 09:00 (English)
Opponent
Supervisors
Available from: 2018-10-17 Created: 2018-09-24 Last updated: 2019-09-18Bibliographically approved
2. Out-of-field doses from proton therapy and doses from CBCT imaging: Risk of radiation-induced second cancer from modern radiotherapy
Open this publication in new window or tab >>Out-of-field doses from proton therapy and doses from CBCT imaging: Risk of radiation-induced second cancer from modern radiotherapy
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The use of ionizing radiation for treatment of cancer diseases is continuously increasing as patient survival is improving and new treatment techniques are emerging. While this development is beneficial for curing primary tumors, concerns have been raised regarding the unwanted dose contribution to healthy tissues of patients and the associated risk of radiation-induced second cancer (RISC). This is especially important for younger patients receiving radiotherapy more often than before and for whom the risk of developing RISC is elevated in comparison to the typical adult radiotherapy patient. In order to estimate the risk of RISC associated with modern radiotherapy and imaging, the associated radiation doses must be determined.

Patients undergoing radiotherapy receive in-field doses from the primary beam but also out-of-field doses originating from secondary radiation produced in the beamline and within the patient. Over the last years, the use of proton pencil beam scanning (PBS) therapy has rapidly increased due to its potential to reduce the in-field doses to healthy tissues in comparison to photon therapy. One of the drawbacks with proton therapy is the production of neutrons capable of travelling large distances and depositing out-of-field doses to organs located far from the primary treatment field. The dose reduction associated with proton PBS therapy could consequently be affected by the out-of-field doses originating from secondary radiation.

The sharp dose gradients associated with modern treatment techniques, such as photon intensity-modulated radiotherapy (IMRT) and proton PBS therapy require more frequent and accurate patient imaging in comparison to conventional treatment techniques such as three-dimensional conformal radiotherapy (CRT). Setup verification images could be acquired with cone-beam computed tomography (CBCT) producing three-dimensional patient images at the cost of an increased patient dose in comparison to planar x-ray imaging. Concerns have been raised regarding the cumulative patient doses from repeated CBCT imaging versus the dose-saving benefits associated with modern radiotherapy techniques like IMRT and proton PBS.

In this thesis, a study on the in-field and out-of-field doses to healthy tissues from photon IMRT and CRT treatments of head and neck tumors showed that the risk of RISC was unaffected by the employed treatment technique and indicated that the lifetime risk of cancer induction was of the order of 1-2%.

Results from measurements and Monte Carlo simulations showed that the out-of-field absorbed doses and equivalent doses associated with proton PBS treatments of brain tumors were up to 60 µGy/Gy and 150 µSv/Gy, respectively. The risk of RISC associated with these out-of-field doses was in the range of approximately one induced cancer in ten thousand treated patients. A simulation study on the doses from a proton gantry-mounted CBCT system showed that repeated CBCT imaging could result in cumulative organ doses of almost 2 Gy. The conclusion from these studies is that the dose-sparing effects of proton PBS therapy are not overshadowed by the out-of-field doses originating from secondary radiation for brain tumor treatments, but that the cumulative doses from repeated CBCT imaging could have a relevant impact on the overall dose reduction.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2018. p. 55
National Category
Other Physics Topics
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-161044 (URN)978-91-7797-470-3 (ISBN)978-91-7797-471-0 (ISBN)
Public defence
2018-11-30, Rehabsalen, Norrbacka, S2 plan 01, Karolinska Universitetssjukhuset, Solna, Stockholm, 09:00 (English)
Opponent
Supervisors
Available from: 2018-11-07 Created: 2018-10-16 Last updated: 2018-11-02Bibliographically approved

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