Measurement of dose exposure of image guidance in external beam accelerated partial breast irradiation: Evaluation of different techniques and linear accelerators.

INTRODUCTION: Verifying the patient position is always an essential part of the treatment process, especially in hypofractionated treatments such as accelerated partial breast irradiation (APBI). The purpose of the study was to compare five image guidance techniques with respect to imaging dose and image quality. METHODS AND MATERIALS: We chose five types of imaging methods applicable for APBI and measured their dose exposure on four different accelerators (Synergy, TrueBeam, Artiste and CyberKnife). Absorbed dose was measured with ionization chamber in thorax phantom. Besides dose exposure image quality was also compared. RESULTS: The lowest dose exposure was measured with kV-kV planar imaging followed by kV-CBCT, MV-MV pair and MV-CBCT in ascending order. Average phantom dose with kV-kV image pair on CyberKnife was 0.01 cGy as the lowest and with MV-CBCT on Artiste was 7.11 cGy as the highest. Average dose exposures of MV-MV images with TrueBeam, Synergy and Artiste were 1.18 cGy, 2.13 cGy and 1.61 cGy, respectively, with similar image quality. For the same machines the doses of kV-CT imaging were comparable: 0.65 cGy, 0.65 cGy and 0.52 cGy, with some differences in image quality. MV-CBCT technique resulted in the highest dose and poorest image quality. CONCLUSIONS: In APBI the position of the patient and tumour bed can be verified with many tools. When fiducials are available, often 2D imaging is enough to achieve appropriate positioning and the kV-kV method is recommended. Imaging with 2.5MV can also be a good solution instead of 6MV. Without fiducials 3D images should be acquired and the recommended method is the kV-CBCT.

Dummy source digitization algorithm for reconstruction of flexible brachytherapy catheters with biplane images.

PURPOSE: The traditional brachytherapy catheter reconstruction with biplane images is based on digitizing radio-opaque markers with a pointing device on a film or on a screen. An algorithm to automate digitization of radio-opaque marker coordinates on biplane images is presented. MATERIAL AND METHODS: To obtain the marker coordinates in a proper sequence, instead of usual pair of reconstruction images, series of images were taken with insertion of radio-opaque markers consecutively into the catheters. The images were pre-processed to suppress the shield of anatomic structures. The determination of the marker coordinates is based on the detection of characteristic high gradient variation in pre-processed image profiles. The method was tested with six endometrial insertions performed with Simon-Norman catheters using our version of Heyman packing. RESULTS: 28 catheters of six treatment fractions were digitized, typically 10 markers per catheter. To obtain the marker coordinates, adjustment of two threshold levels on the pre-processed images were needed. The coordinates of the radio-opaque markers on the biplane projection images were obtained without positive or negative artefact. CONCLUSIONS: THE DUMMY SOURCE COORDINATES ON THE BIPLANE IMAGES WERE DIGITIZED IN A PROPER SEQUENCE: from the catheters' tip towards the end of the catheters. After the three-dimensional reconstruction of the catheters from the digitized coordinates, the geometry file was imported by the brachytherapy planning system for dose calculation. The method has the advantage to eliminate manual digitization of the dummy sources.

Multi-parametric fit method in reconstruction of brachytherapy needles.

PURPOSE: To adapt the multi-parametric fit method to reconstruct brachytherapy needles inserted with free hand technique for treatment of surface malignancies. METHODS: An alternative reconstruction method for brachytherapy (BT) needles is presented. The method is based on the digitized tip and end coordinates on pairs of posterior-anterior and posterior-oblique reconstruction images obtained with a non-isocentric C-arm. The needles tip and end coordinates are computed with the multi-parametric fit method that also incorporates the determination of the magnification factors of the reconstruction images. We tested the reconstruction accuracy with radio-opaque markers inserted into known positions. The range of C-arm angles that resulted in an accurate reconstruction was also investigated. We applied the method in treatments of the vulvar cancer using 3 to 5 pieces of BT needles located in a distance of approximately 1 cm. RESULTS: The phantom test showed largest difference between the reconstructed and expected distance between the simulated needles ±1.3 mm. In vulvar insertions, the reconstructed position of the BT needles obtained with different image pairs agreed within 3 mm. CONCLUSION: The reconstruction accuracy of the multi-parametric fit method in with proper imaging is suitable for the clinical use.

Modified Heyman packing performed with small number of Simon-Norman catheters.

PURPOSE: Modified Heyman packing method, performed usually with 6-11 flexible catheters is one out of many brachytherapy treatments. We described the insertion and dose planning method, with the use of 4-6 Simon-Norman catheters positioned to the fundus uteri, which is the most frequent source of the endometrial cancer. MATERIAL AND METHODS: We investigated the dose distribution of the main classes of regular catheter insertions with Simon-Norman applicators using dose profiles, obtained in lateral and axial direction at the fundus, and compared with the dose distribution of the Y-shaped catheter arrangements. RESULTS: The insertions with five and six Simon-Norman catheters produced higher dose at the fundus uteri, compared with the Y-shaped applicator. CONCLUSIONS: Insertions performed with small number of catheters permits to perform dosimetry optimisation for each treatment fraction.

Influence of different Fletcher-Suit applicator geometries on sagittal dose distribution.

INTRODUCTION: One of the brachytherapy treatment modality of cervix carcinomas is insertion with Fletcher-Suit (FS) applicator. Depending on the patient anatomy and pathology and on the construction of the FS applicator different geometrical arrangements in ovoid separation, in ovoids' sagittal level with respect to the tandem were experienced. The multiple insertions show minor differences in applicator geometries. The aim of the study is to evaluate the influence of main geometrical parameters: the ovoid separation, symmetry and the ovoids' sagittal shift on dose distribution in different FS applicator arrangements. We tested the effect of dwell time settings in improvement of dose distribution of less adequate insertions. We also investigated the effect of inter-fractional variation of applicator geometry. MATERIALS AND METHODS: We considered 73 treatment fractions of 22 patients. All insertions were performed by the same gynaecologist with the same type of FS applicator, while the treatment plans were generated by the same physicist using the same treatment planning method. We compared the sagittal dose distribution of different FS applicator geometries with dose levels at two applicator points, defined 2 cm apart from the tandem towards the bladder and rectum. We computed the Pearson correlation coefficients between the dose levels at the applicator points and the ovoid separation, symmetry and the ovoids' sagittal shift. We also investigated the effect of dwell time settings in ovoids in order to decrease the dose to organs at risk. The inter-fractional variation of the FS applicator geometries and the influence on the dose levels at the two applicator points were also tested. RESULTS AND CONCLUSIONS: Strong correlation was found between the ovoid separation and dose values to applicator points defined in sagittal direction of FS applicator arrangements. Also strong correlation was between the ovoids' sagittal position with respect to the tandem and the applicator point defined towards the rectum, while the ovoid symmetry had no influence on the sagittal dose distribution. The standard deviations of inter-fractional variation of the ovoid separation and the ovoids' sagittal position were within +/-5.2 mm and +/-10.2 mm respectively. The inter-fractional variations in FS applicator geometry resulted in variation in dose levels at the applicator points +/-0.8 Gy typically, while the largest value was +/-1.6 Gy.

The consistency of Fletcher-Suit applicator geometry and of the rectal probe's position in high dose rate brachytherapy treatment fraction of cervix carcinoma.

PURPOSE: The dose values computed with the treatment planning system and the in vivo dose measurements with semiconductor detectors in rectum during the high dose rate brachytherapy treatment fraction of the cervix carcinoma are occasionally significantly different. We've investigated the consistency of the Fletcher-Suit applicator geometry and the in vivo rectal probe's position stability during the high dose rate brachytherapy treatment fraction. MATERIAL AND METHODS: The patient lied in a lithotomic position during a biplane reconstruction images, throughout the treatment planning and dose administration. We obtained post-treatment reconstruction images and prepared a post-treatment plan. The amount of 14 treatment fractions of 10 patients were considered in the study. Two methods were applied: evaluation of the difference of reconstructed pre-treatment and post-treatment applicator points and rectal probe's detectors being relevant to the co-ordinate system fixed to the applicator, and estimation of applicators and rectal probe's reallocation with respect to the pelvic bones with registration of pre- and post-treatment reconstruction images. RESULTS: We've experienced good consistency in the Fletcher-Suit applicator geometry in all treatment fractions. 70% of them presented small variation in the rectal probe's position, while the rest showed significant shift in the applicator or rectal probe's position with regard to the pelvic bones.

Multiparametric fit method in reconstruction of Fletcher-Suit-Delclos applicator.

In our institution we have introduced the variable angle reconstruction method with a nonisocentric C-arm as brachytherapy localizer in gynecological treatments. The main advantage of this method is that on the posterior oblique images the ovoids of the Fletcher-Suit-Delclos (FSD) applicator are clearly separated. The main problem with the nonisocentric machine is that the magnification factor is not known exactly and varies with the C-arm angle. For generation of treatment plans with a commercial brachytherapy planning system the magnification factors of the reconstruction images have to be determined previously. We present a multiparametric fit method (MPF) that is suitable for reconstruction of FSD applicators and that utilizes pairs of posterior-anterior and lateral or preferably posterior oblique reconstruction images obtained by a C-arm. The MPF reconstruction relies on the known geometry of the tandem and the ovoid. The actual FSD insertion is reconstructed by adaptation of the geometry of tandem and ovoid by translations and rotations. The fixation mechanism limits the possible excursions of the FSD applicator's parts. The limited freedom of excursions and the determination of the magnification factors are also incorporated.

Dose distribution of modified heyman packing.

Introduction. One of the treatment modalities of brachytherapy of endometrial carcinomas is the modified Heyman packing technique.The aim of the study is to assess the dose distributions of regular arrangements of modified Heyman packings. Materials and Methods. We reconstructed the catheters with biplane X-ray images and optimised the dose distribution with defining'dose points' the uterine wall thickness apart from the outermost left and right catheters. We fitted the reconstructed catheters with the diagnostic sagittal MR scan obtained prior to the first insertion. We checked the dose distribution defining 'patients points' on sagittal contour of the uterus. Results and Conclusions. The treatment plans optimised 'on dose points and geometry' resulted in a satisfactory dose distribution, however systematic overdosed and under-dosed regions have been experienced.

To what extent can digital images obtained with a non-isocentric C-arm be used for brachytherapy treatment planning in gynaecology.

BACKGROUND AND PURPOSE: The non-isocentric C-arm X-ray fluoroscopy unit is not typically used as a brachytherapy localizer. The main aim of the study is to examine the extent to which the potential of a mobile C-arm unit as a brachytherapy localizer for gynaecological treatments with Fletcher-Suit applicator can be used. PATIENTS AND METHODS: With a C-arm, located in the brachytherapy treatment room in situ reconstruction images are made, eliminating the transportation of the patient to the X-ray room. The brachytherapy treatment planning is based on the frame grabbed images using variable angle reconstruction. RESULTS: The method of making a pair of reconstruction images with the C-arm is described. The accuracy of the reconstruction has been examined with phantom studies and therapy plans of Fletcher-Suit applications of different variable angles.