There has been an increasing interest in proton beam therapy (PBT) in recent years related to the advantageous depth-dose characteristics of proton beams compared to what is achievable with standard external photon beam radiotherapy (RT). With PBT, improved target dose conformity can be achieved together with a reduction in the dose to the organs at risk (OARs). This can for certain cases lead to an increased tumour control probability (TCP) at the same time as the probabilities for normal tissue complications (NTCP) and radiation-induced secondary cancers are reduced. However, there are challenges with PBT, in the form of uncertainties in the dose delivery to the patient, due to different influencing factors. These perturbing factors are contributing to the uncertainties during different steps in the RT flow process, from the treatment planning to the irradiation. In the present work, a comparative treatment planning study of PBT and photon RT for a few clinical liver- and stomach-cancer cases were performed with the aim of determining possible advantages of PBT. The treatment planning comparisons were performed by means of dosimetric evaluations and by use of tissue response models. The later included the calculation of TCP and NTCP as well as the assessment of risk of radiation-induced secondary cancer for the two compared RT techniques. A total of eleven patients previously treated with RT at Karolinska University Hospital were included in the study. Three of these patients had been treated for liver cancer and eight for stomach cancer. The photon plans which had been used in the real treatments at the hospital were taken as reference plans. The treatment planning for the liver cancer cases had been performed on conventional CT images, but 4D-CT images were used for target definition to account for the target motion. Three distinct CT images were used in the planning of the stomach cancer cases, the original CT image study on which the photon plans had been done and two CT image studies with artificially changed physical density for some of the internal organs to simulate different possible fillings of the stomach. The extra- or reduced gas filling was drawn on the CT slices by the radiation oncologist to estimate two worst-case scenarios for changes in density within the irradiated volume. The results indicate an improved target dose conformity, dose homogeneity and sparing of OARs for the PBT plans compared to the photon RT plans for the two clinical cases studied. The sparing of the OARs was also observed in the form of decreased NTCP for the PBT plans. The PBT plans showed to be worse than the photon plans when some structures were replaced by air and water. In the case of extra air there was a shift of the higher doses beyond the distal edge of the planned proton range which caused both an increase of the irradiated volumes of sensitive normal tissues and of the maximum doses to the OARs. In the case of extra water in the stomach, the maximum range of the protons was reduced causing target underdosage. The calculations of probabilities for radiation-induced secondary malignancies indicated a reduced risk for all the OARs with the proton plans for the liver cancer cases. For the stomach cancer cases, reduced risks were obtained for induction of cancer in the liver but an increased risk was calculated for the bowel(-)PTV, with the proton- compared to the photon-plans. The results of the calculations of risk for radiation-induced cancer in the kidneys were inconclusive. The assessment of risk of secondary cancer for other organs, not delineated in this work (to obtain the whole body risk), is needed in order to obtain more comprehensive and clinically useful results.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:su-132177 |
| Date | January 2014 |
| Creators | Mondlane, Gracinda |
| Publisher | Stockholms universitet, Fysikum |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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