Cone beam CT doses in radiotherapy patient positioning in Finland-prostate treatments.

Imaging parameters, frequencies and resulting patient organ doses in treatments of prostate cancer were assessed in Finnish radiotherapy centres. Based on a questionnaire to the clinics, Monte Carlo method was used to estimate organ doses in International Commission on Radiological Protection standard phantom for prostate, bladder, rectum and femoral head. The results show that doses from cone beam computed tomography imaging have reduced compared to earlier studies and are between 3.6 and 34.5 mGy per image for the above-mentioned organs and for normal sized patients. There still is room for further optimization of the patient exposure, as many centres use the default imaging parameters, and the length of the imaged region may not be optimal for the purpose.

RADIATION DOSE OF THE EYE LENS IN CT EXAMINATIONS OF THE BRAIN IN CLINICAL PRACTICE-THE EFFECT OF RADIOGRAPHER TRAINING TO OPTIMISE GANTRY TILT AND SCAN LENGTH.

Lenses are always exposed to radiation in brain computed tomography (CT) scans. However, the lens dose can be reduced by excluding lens from scanning area by optimising gantry tilt and scan length. The object of this study is to retrospectively analyse if the optimisation by gantry tilt and scan length have been adequate in the CT scan of the brain, and to prospectively analyse the effect of radiographer training to the quality of the CT examinations. This study was conducted in two parts. In all, 329 brain CTs performed in the Tampere University Hospital from 2017 to 2019 were revised retrospectively. The prospective part included 51 brain CT studies conducted in October 2021. Dose to the eye of the lens was modelled using CT-Expo using zero-degree beam angle and scan lengths to expose the lens either to the primary or scattered radiation. Non-zero gantry tilt had been used in a large proportion of the CT examinations in the retrospective setting, 84.8%. However, the lenses were successfully excluded from the scan area in only 1.8% of the examinations. In the prospective part, the gantry tilt was used in 98% of the studies and the proportion of successful examinations rose from 1.8 to 11.8%. The lens dose decreased significantly when the eyes were excluded from the imaging area. The modelled lens dose in the large retrospective part was 25.9 mGy (17.8-49.2 mGy) when the eyes were included and 1.5 mGy (0.4-1.9 mGy) when the eyes were excluded. The lens dose was similar in the small prospective part. Despite the gantry tilt is widely used, unnecessary lens irradiation occurs extensively because of suboptimal gantry tilt and scan length. The training of radiographers reduces the radiation exposure to the lens by more optimal gantry tilt and scan length.

The robustness of prostate radiotherapy for patients with hip prosthesis.

The aim of this study was to investigate prostate radiotherapy techniques for the patients with hip prosthesis in 4 different field setups. Volumetric Modulated Arc Therapy (VMAT) technique was used in 4 different cases: (1) using full VMAT arcs (VMAT_F); (2) same arcs as in case 1 but with avoidance sectors (VMAT_ASEC); (3) as case 2 but with the addition of a lateral static field through the prosthesis (VMAT_ASEC+STAT); (4) as in case 1 but with an automated structure avoidance option to avoid irradiation through the prosthesis (VMAT_ASTR). Fifteen previously treated prostate patients were retrospectively selected to this study. Treatment plans were created for all patients using all 4 techniques. The potential prosthesis misalignment in the treatment setup was modeled by moving the prosthesis 0.5, 1.0, and 1.5 cm ventrally and dorsally and recalculating the plans in each case. For VMAT_ASEC, the dose parameters for organs at risk were the highest and the dose coverage of the target volume was the poorest when compared to the other techniques. For VMAT_ASEC+STAT, the movement of the prosthesis changed the target dose distribution the most. VMAT_F and VMAT_ASTR fulfilled the planning criteria the best, even when the prosthesis was misaligned. VMAT_F radiated through the prosthesis more than VMAT_ASTR and increased the dose near the prosthesis surface when compared to VMAT_ASTR. VMAT_ASTR and VMAT_F were the most robust techniques for the patients with the hip prosthesis considering plan quality and the effect of positioning errors. The increased prosthesis surface dose with VMAT_F and possible dose calculation uncertainties favors the use of VMAT_ASTR.