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2 Polish Journal of Medical Physics and Engineering

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Evaluating the impact of anatomical changes on dose distributions in head and neck cancer

Sobajtis Aleksandra, Kukołowicz Paweł, Iwanowska-Chomiak Beata

Polish Journal of Medical Physics and Engineering

2023

Vol. 29, Iss. 3

156--164

Introduction: Thanks to modern IGRT procedures, it is possible to track changes in the patient's anatomy and thus calculate the dose distribution for the current anatomical conditions of the patient. This allows the scheduled dose to be compared with the delivered dose. In the case of large discrepancies, it is possible to improve the treatment plan. Radiotherapy, during which the treatment plan is modified, resulting from changes in anatomy, is referred to as adaptive radiotherapy. Material and methods: This study was performed for 30 patients with H&N cancer at the University Hospital in Zielona Góra. All patients were treated with VMAT. The Simultaneous Integrated Technique was used. In each treatment session, set-up verification was performed. Alternating every other day, the CBCT and two orthogonal portal images were made, and position correction prior to each session was performed. For all patients, new planning CT was made after the 11th and 22nd treatment sessions. Dose distributions with the initial plan on CT11 and CT22 were calculated. The initial dose-volume histograms DVH0 were compared with dose-volume histograms DVH11 and DVH22 calculated on CT11 and CT22. Results: We compared the dose distribution in the CTVs and in the most important organs at risk obtained for initial anatomy and dose distributions calculated with the initial plan on the CTs performed after the second and the fourth week of irradiation. The differences between mean doses and V95% to GTV obtained for the initial CT and two other CTs were small. For a few CTs, the values of V95% were smaller by more than 5% points. In most patients, the mean dose in salivary glands increased during treatment. Conclusions: Anatomical changes occurring during radiotherapy in patients with head and neck cancers have little influence on the dose deposited in the Clinical Target Volume. Adaptive therapy may be of particular importance if relapse occurs and re-irradiation.

Determination of the CTV-PTV margin for prostate cancer radiotherapy depending on the prostate gland positioning control method

Sobajtis Aleksandra, Kukołowicz Paweł

Polish Journal of Medical Physics and Engineering

2020

Vol. 26, Iss. 4

169--179

Objective: The objective of the study was to determine the correct CTV-PTV margin, depending on the method used to verify the PG position. In the study, 3 methods of CBCT image superimposition were assessed as based on the location of the prostate gland (CBCT images), a single gold marker, and pubic symphysis respectively. Materials and methods: The study group consisted of 30 patients undergoing irradiation therapy at the University Hospital in Zielona Góra. The therapy was delivered using the VMAT (Volumetric Modulated Arc Therapy) protocol. CBCT image-based superimposition (prostate-based alignment) was chosen as the reference method. The uncertainty of the PG positioning method was determined and the margin to be used was determined for the CBCT-based reference method. Then, changes in the position of the prostate gland relative to these determined using the single marker and pubic symphysis-based methods were determined. The CTV-PTV margin was calculated at the root of the sum of the squares for the doubled value of method uncertainty for the CBCT image-based alignment method and the value of the difference between the locations of planned and actual isocenters as determined using the method of interest and the CBCT-based alignment method for which the total number of differences accounted for 95% of all differences. Results: The CTV-PTV margins to be used when the prostate gland is positioned using the CBCT imaging, single marker, and pubic symphysis-based methods were determined. For the CBCT-based method, the following values were obtained for the Vrt, Lng, and Lat directions respectively: 0.43 cm, 0.48 cm, 0.29 cm. For the single marker-based method, the respective values were 0.7 cm, 0.88 cm, and 0.44 cm whereas for the pubic symphysis-based method these were 0.65 cm, 0.76 cm, and 0.46 cm. Conclusions: Regardless of the method, the smallest margin values were obtained for the lateral direction, with the CBCT-based method facilitating the smallest margins to be used. The largest margins were obtained using the single marker-based alignment method.