| Abstract [eng] |
While radiation therapy is one of the most commonly implemented treatment methods for central nervous system cancer patients, treatment planning for this location requires great care and accuracy. Exceeding tolerance doses for organs at risk (OARs) due to patient positioning errors can result in short and long-term complications for the patient. Therefore, evaluation of dose planning risk is necessary to improve treatment methodologies and lower the risk of radiation-caused complications. This work consisted of several parts. First, set-up uncertainties of 64 CNS cancer patients for two radiation therapy treatment units (TRUEBEAM and TRILOGY) were evaluated. Then, planning organ at risk volume (PRV) margins were calculated using two different formulas. Calculated margins varied between 2.51 mm and 2.84 mm. However, due to limitations of the treatment planning system, 3 mm margins had to be used. In the next part, 27 approved treatment plans with a total prescribed dose of 60 Gy were modified by adding 3 mm PRV margins around the brain stem, chiasma and optic nerves. Modifying plans was necessary to evaluate dose planning risks for selected OARs arising from set-up errors. The evaluation showed that after modification, PRV constraints were exceeded in 55.56 % of plans for the brain stem, in 44.44 % of plans for chiasma, 18.52 % for the left optic nerve and in 22.22 % of plans for the right optic nerve. All 27 plans were re-planned according to a newly created protocol combining dose constraints for OARs implemented in the clinic and PRV recommendations from the literature. After re-planning, PRV constraints were met for all 27 plans and mean maximum doses were reduced by 3.08 Gy, 2.64 Gy, 2.27 Gy and 1.69 Gy to the brain stem, chiasma, right optic nerve and the left optic nerve, respectively. Implementation of PRV margins reduced dose planning risk without compromising planning target volume (PTV). After re-planning mean PTV coverage decreased by 0.17 %, from 97.83 % down to 97.66 %. No significant changes to the conformity index were observed. Lastly, after re-planning, the 15 highest-risk treatment plans were additionally investigated by generating uncertainty plans by shifting the isocenter by ±3 mm in x, y and z directions. Analysis revealed that patient set-up uncertainties in the z-direction result in the highest dose planning risk, especially, for chiasma. |
