Evaluation of inter-fraction prostate motion using kilovoltage cone beam computed tomography during radiotherapy

Aims

The success of delivering the prescribed radiation dose to the prostate while sparing adjacent sensitive tissues is largely dependent on the ability to accurately target the prostate during treatment. Kilovoltage cone beam computed tomography (CBCT) imaging can be used to monitor and compensate for inter-fraction prostate motion, but this procedure increases treatment session time and adds incidental radiation dose to the patient. We carried out a retrospective study of CBCT data to evaluate the systematic and random correction shifts of the prostate with respect to bones and external marks.

Materials and methods

A total of 449 daily CBCT studies from 17 patients undergoing intensity-modulated radiotherapy (IMRT) for localised prostate cancer were analysed. The difference between patient set-up correction shifts applied by radiation therapists (via matching prostate position in CBCT and planning computed tomography) and shifts obtained by matching bony anatomy in the same studies was used as a measure of the daily inter-fraction internal prostate motion.

Results

The average systematic and random shifts in prostate positions, calculated over all fractions versus only 10 fractions, were not found to be significantly different.

Discussion

The measured prostate shifts with respect to bony anatomy and external marks after the first 10 imaging sessions were shown to provide adequate predictive power for defining patient-specific margins in future fractions without a need for ongoing computed tomography imaging. Different options for CBCT imaging schedule are proposed that will reduce the treatment session time and imaging dose to radiotherapy patients while ensuring appropriate prostate cover and normal tissue sparing.     

Displacement of hepatic tumor at time to exposure in end-expiratory-triggered-pulse proton therapy

Purpose

This study is to evaluate reproducibility of hepatic tumors in end-expiration and end-inspiration on free-breathing, also measure shift of hepatic tumor location in pulsed proton beams exposure in end-expiration in order to estimate feasible planning target volume (PTV) margin.

Materials and methods

Pairs (1232) of anterior and lateral radiographs from 30 patients (628 end-expiration and 604 end-inspiration phases) were analyzed using fiducial markers adjacent to the tumors. By using the co-ordinates of the marker centroid of mass related to the isocenter, intrafractional variation was compared in end-expiration and end-inspiration, and a feasible PTV margin was generated using the measured motion.

Results

The average internal motion in end-expiration was 1.1 mm, which was significantly smaller than that in end-inspiration. The mean deviation from the plan was −0.1, 0.3, and 0.1 mm in the left-right (LR), cranio-cepharal (CC), and anterior-posterior (AP) directions, respectively. The estimated PTV margins were 3.2, 3.5, and 4.6 mm, in the LR, CC, and AP directions, respectively.

Conclusions

It was indicated that localization of the targets was more reproducibility in end-expiration than that in end-inspiration. Also, feasible and practical margin values were obtained. These should contribute accuracy of respiration synchronized proton radiotherapy for liver tumors.     

In vivo effects of LPS on B lymphocyte subpopulations. Migration of marginal zone-lymphocytes and IgD-blast formation in the mouse spleen

The aim of the present study was to analyze the effect of LPS on the localization and differentiation of splenic B lymphocytes. Therefore, we used a double immunoperoxidase technique which enabled us to detect both the IgM+ IgD- marginal zone lymphocytes and the IgM+ IgD+ follicular lymphocytes in the same tissue section. Next to a dramatic disappearance of the predominantly IgM+ IgD- lymphocytes in the marginal zone shortly after an intravenous injection of LPS, an increased number of these cells could be found in the splenic follicles. The present results strongly suggest that the IgM+ IgD- cells in the splenic follicles represent immigrating marginal zone lymphocytes, and not differentiating follicular B cells, because no IgM+ IgD- cells could be observed in the follicles of draining lymph nodes shortly after a subcutaneous injection of a similar amount of LPS. These observations support the suggestion that LPS induces a migration of marginal zone lymphocytes into the follicles. The present results also showed the formation of IgD plasmablasts in the inner PALS and around the terminal arterioles of the spleen after LPS administration. The induction of IgD plasmablasts appeared to be a specific effect of LPS which may be related to its toxic properties.     

Delineation of Margins for the Planning Target Volume (PTV) for Image-Guided Radiotherapy (IGRT) of Gastric Cancer Based on Intrafraction Motion

Background: Application of the image-guided radiotherapy (IGRT) system for gastric cancer involving daily verification of patient positioning on the treatment machine allows minimisation of geometrical errors as a consequence of intra- and inter-fraction motion. The purpose of this study was to define the intrafraction motion in gastric cancer patients during a treatment session based on the IGRT system and designation of margins around the clinical target volume CTV (internal target volume ITV) necessary to delineate the planning target volume (PTV). Methods: Twenty gastric cancer patients were analysed. The total radiation dose for each was 45Gy in 25 fractions within 5 weeks. The margins for the PTV were calculated according to van Herk (2004), Stroom and Heijmen (2002) and the International Commission on Radiation Units and Measurements (ICRU) Report 62 formulas based on craniocaudal (Y axis), laterolateral (X axis) and anteroposterior (Z axis) shifts. Results: Delineated margins for the PTV in gastric cancer with the three formulas applied were respectively 0.2, 0.2, and 0.2cm in the lateral plane, 0.3, 0.3, and 0.3cm in the craniocaudal plane and 0.3, 0.3, and 0.2cm in the anteroposterior plane. Conclusions: Recommended margins for the PTV in gastric cancer calculated in this study based on intrafraction motion are 0.3cm, 0.2cm and 0.3cm in the craniocaudal, lateral and anterioposterior directions, respectively. Use of the IGRT system corrects for the motions between factions and allows reduction in ITV-PTV margins. The main advantage of the smaller margins in comparison to the non-IGRT radiotherapy is a reduction in the probability of radiation complications.