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Out-of-field doses from proton therapy and doses from CBCT imaging: Risk of radiation-induced second cancer from modern radiotherapy
Stockholm University, Faculty of Science, Department of Physics. (Medical Radiation Physics)
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: urn:nbn:se:su:diva-161044ISBN: 978-91-7797-470-3 (print)ISBN: 978-91-7797-471-0 (electronic)OAI: oai:DiVA.org:su-161044DiVA, id: diva2:1256301
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
List of papers
1. Modelling of a proton spot scanning system using MCNP6
Open this publication in new window or tab >>Modelling of a proton spot scanning system using MCNP6
2017 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 860, article id 012025Article in journal (Refereed) Published
Abstract [en]

The aim of this work was to model the characteristics of a clinical proton spot scanning beam using Monte Carlo simulations with the code MCNP6. The proton beam was defined using parameters obtained from beam commissioning at the Skandion Clinic, Uppsala, Sweden. Simulations were evaluated against measurements for proton energies between 60 and 226 MeV with regard to range in water, lateral spot sizes in air and absorbed dose depth profiles in water. The model was also used to evaluate the experimental impact of lateral signal losses in an ionization chamber through simulations using different detector radii. Simulated and measured distal ranges agreed within 0.1 mm for R90 and R80 , and within 0.2 mm for R50 . The average absolute difference of all spot sizes was 0.1 mm. The average agreement of absorbed dose integrals and Bragg-peak heights was 0.9%. Lateral signal losses increased with incident proton energy with a maximum signal loss of 7% for 226 MeV protons. The good agreement between simulations and measurements supports the assumptions and parameters employed in the presented Monte Carlo model. The characteristics of the proton spot scanning beam were accurately reproduced and the model will prove useful in future studies on secondary neutrons.

National Category
Other Physics Topics
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-161042 (URN)10.1088/1742-6596/860/1/012025 (DOI)000412324300025 ()
Conference
International Nuclear Science and Technology Conference (INST), Bangkok, Thailand, August 4-6, 2016
Available from: 2018-10-15 Created: 2018-10-15 Last updated: 2018-10-19Bibliographically approved
2.
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3. IMPACT OF IRRADIATION SETUP IN PROTON SPOT SCANNING BRAIN THERAPY ON ORGAN DOSES FROM SECONDARY RADIATION
Open this publication in new window or tab >>IMPACT OF IRRADIATION SETUP IN PROTON SPOT SCANNING BRAIN THERAPY ON ORGAN DOSES FROM SECONDARY RADIATION
2018 (English)In: Radiation Protection Dosimetry, ISSN 0144-8420, E-ISSN 1742-3406, Vol. 180, no 1-4, p. 261-266Article in journal (Refereed) Published
Abstract [en]

A Monte Carlo model of a proton spot scanning pencil beam was used to simulate organ doses from secondary radiation produced from brain tumour treatments delivered with either a lateral field or a vertex field to one adult and one paediatric patient. Absorbed doses from secondary neutrons, photons and protons and neutron equivalent doses were higher for the vertex field in both patients, but the differences were low in absolute terms. Absorbed doses ranged between 0.1 and 43 mu Gy. Gy(-1) in both patients with the paediatric patient receiving higher doses. The neutron equivalent doses to the organs ranged between 0.5 and 141 mu Sv. Gy(-1) for the paediatric patient and between 0.2 and 134 mu Sv. Gy(-1) for the adult. The highest neutron equivalent dose from the entire treatment was 7 mSv regardless of field setup and patient size. The results indicate that different field setups do not introduce large absolute variations in out-of-field doses produced in patients undergoing proton pencil beam scanning of centrally located brain tumours.

National Category
Radiology, Nuclear Medicine and Medical Imaging
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-160259 (URN)10.1093/rpd/ncy013 (DOI)000440983000055 ()30085315 (PubMedID)
Available from: 2018-09-19 Created: 2018-09-19 Last updated: 2018-10-16Bibliographically approved
4. Organ doses from a proton gantry-mounted cone-beam computed tomography system characterized with MCNP6 and GATE
Open this publication in new window or tab >>Organ doses from a proton gantry-mounted cone-beam computed tomography system characterized with MCNP6 and GATE
Show others...
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.

National Category
Radiology, Nuclear Medicine and Medical Imaging
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-160350 (URN)10.1016/j.ejmp.2018.08.011 (DOI)000445037300007 ()
Available from: 2018-09-19 Created: 2018-09-19 Last updated: 2018-10-16Bibliographically approved
5. Are IMRT treatments in the head and neck region increasing the risk of secondary cancers?
Open this publication in new window or tab >>Are IMRT treatments in the head and neck region increasing the risk of secondary cancers?
2014 (English)In: Acta Oncologica, ISSN 0284-186X, E-ISSN 1651-226X, Vol. 53, no 8, p. 1041-1047Article in journal (Refereed) Published
Abstract [en]

Background. Intensity-modulated radiation therapy (IMRT) has been increasingly employed for treating head and neck (H&N) tumours due to its ability to produce isodoses suitable for the complex anatomy of the region. The aim of this study was to assess possible differences between IMRT and conformal radiation therapy (CRT) with regard to risk of radiation-induced secondary malignancies for H&N tumours. Material and methods. IMRT and CRT plans were made for 10 H&N adult patients and the resulting treatment planning data were used to calculate the risk of radiation-induced malignancies in four different tissues. Three risk models with biologically relevant parameters were used for calculations. The influence of scatter radiation and repeated imaging sessions has also been investigated. Results. The results showed that the total lifetime risks of developing radiation-induced secondary malignancies from the two treatment techniques, CRT and IMRT, were comparable and in the interval 0.9-2.5%. The risk contributions from the primary beam and scatter radiation were comparable, whereas the contribution from repeated diagnostic imaging was considerably smaller. Conclusion. The results indicated that the redistribution of the dose characteristic to IMRT leads to a redistribution of the risks in individual tissues. However, the total levels of risk were similar between the two irradiation techniques considered.

National Category
Cancer and Oncology Physical Sciences
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-107626 (URN)10.3109/0284186X.2014.925581 (DOI)000340892900008 ()
Note

AuthorCount:3;

Available from: 2014-09-22 Created: 2014-09-22 Last updated: 2018-10-16Bibliographically approved

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