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Prostate cancer (PCa) is the second most common cancer diagnosis for men and the fifth leading cause of cancer-related death worldwide. A common treatment strategy for PCa is external beam radiation therapy (EBRT), where high doses of radiation are used to kill cancer cells. Recent developments in RT include maintaining acceptable side effects during intensified treatment over fewer treatment occasions (hypofractionation) and boosting the level of radiation to the gross tumor volume (GTV) visible on multiparametric magnetic resonance imaging (mpMRI) and positron emission tomography (PET). Without a histopathological (HP) reference, the dose distribution cannot be compared to the varying grades of aggressivity within the cancer, known as ISUP grade groups (IGGs). The aim of this master thesis project was to explore the dose distribution over IGGs using a gold standard HP reference and investigate the mitigating effects of rectal spacers, following a hypofractionated RT schedule with focal boosts.

The dataset consists of 15 patients, planned for radical prostatectomy. These patients harbored high-risk disease (IGG ≥ 4) in the GTV. HP evaluations following surgery resulted in physical slices of the prostate, showing the location and IGG of lesions. EBRT treatment plans that combined an ultra-hypofractionation strategy with a boost to the GTV were made. The dose distributions were evaluated by dose volume histograms (DVHs) over the target volumes, organs at risks (OARs) and the lesions. Robust evaluations of targets and OARs were performed by recalculating doses following translations of the patient by 2 mm in all directions. Similarly, lesions were shifted by 2 mm in all directions with respect to the nominal dose plan to estimate the sensitivity to motion. The effects of the translations were assessed by examining the impact on the DVHs and percentage of passed clinical goals.

Two viable dose plans for each patient were produced, one for a 10 mm spacer and one for 8 mm. Both plans fulfill all rectum goals. For the 10 mm plan, the average median dose (D50) was greater than the prescribed prostate dose (42.7 Gy) for all IGGs by atleast 1.1 Gy. The D50 of the higher grades (IGG 3, 4 and 5) were 47.5, 46.4 and 48.7 Gy, meaning that they were closer to the desired GTV dose 49.0 Gy than the prescribed prostate dose.

This thesis project showed that it is possible to reach high GTV doses while sparing the OARs and that the higher IGGs received a higher dose than the lower grades. Examining rectal dose depending on different spacer thicknesses allowed us to recommend a spacer thickness that is safe to use, which can provide increased patient comfort while saving time and resources.