Using digitally reconstructed radiographs from MRI (MRI-DRR) to localize pelvic lymph nodes on 2D X-ray simulator-based brachytherapy treatment planning

Purpose: Many of the available brachytherapy treatment planning systems in developing countries are not equipped with CT [or MRI] simulator; therefore, 3D treatment planning cannot be performed. In this project a new procedure has been introduced for utilizing the 2D digitally reconstructed radiograph from MRI images in brachytherapy treatment planning. This procedure enables us to localize the tumor volume and delineate the extent of critical structures in vicinity of tumor volume. Methods: Pelvic lymph node chain position was delineated from transverse MRI images, and transferred into Digitally Reconstructed Radiograph [DRR] and then onto the X-ray images obtained from conventional simulator unit. These images were then imported to Brachytherapy treatment planning system to evaluate the dose to be applied to these organs in cervix Brachytherapy. The accuracy of the matching process was evaluated by phantom study, having known 3D geometric information and landmark assertion. Results: The statistical variations obtained from distance mismatch in phantom and patient studies were in the range of clinically applicable error of registration [< 2mm]. The results showed a large variation of the nodal dose when dose calculation is performed based on point B dose which is the geometrical reference point for calculating the dose to the pelvic lymphatic system. The result also shows that the dose to point B is usually underestimated to represent external iliac maximum dose, and overestimated for representation of external iliac minimum dose. Conclusion: The results indicated that the DRR images can produce comparable accuracies in tumor localization reported in 3D MRI or CT based treatment planning procedures. Therefore, this technique could be used as a feasible approach where a 3D treatment planning is not available

[ dosimetric evaluation of organs at risk in prostate radiation therapy using a MAGIC gel dosimeter]

Multiple fields and presence of hetrerogeneities create complex dose distributions that need three dimentisonal dosimetry. In this work, we investigated MR-based MAGIC gel dosimetry as a three-dimentional dosimetry technique to measure the delivered dose to bladder and rectum in prostate radiation therapy. A heterogeneous slab phantom including bones was made. Paired cubes in the phantom representing bladder and prostate and a cylindrical container representing rectum were filled with MAGIC gel and placed in the anthropomorphic pelvic phantom. The phantom was irradiated with four beams as planned using a treatment planning system [TPS]. Magnetic resonance transverse relaxation rate images were acquired and turned into dose distribution maps using a calibration curve. This calibration curve was obtained by linear fitting to R2 values of 4 test tubes against their given known doses. Image processing and data analysis were preformed in MATLAB 7 software. The gel dosimeter was validated using an ionization chamber. Dose maps and dose volume histograms [DVHs] were compared with dose distributions and DVHs of the TPS. Mean "distance-to-agreement" and mean "does difference" were 2.98 mm and 6.2%, respectively, in the comparison of profiles obtained from ionization chamber and gel dostimetry. Mean relative difference of DHVs between gel dosimetry and TPs data were 3.04%, 10.4% and 11.7%,for prostate, bladder and rectum, respectively. Gel dosimetry is a good method for three dimensional dosimetry although it has a low precision in high close gradient regions. This method can be used for evaluation of complicated dose distribution accuracy in 3D conformal radiotherapy, especially in presence of heterogeneities

Plaque radiotherapy in recurrent or incomplete-excised conjunctival squamous cell carcinoma and melanoma

To assess the results of brachytherapy in patients with recurrent or incomplete excised conjunctival squamous cell carcinoma [SCC] and malignant melanoma. Three patients underwent brachytherapy of one eye and one patient underwent brachytherapy of both eyes with ruthenium-106 [RU-106] plaques, all of them had a history of incomplete resection or recurrence of the tumor after surgery. All patients were male with an average age at diagnosis of 54 years [range, 34-76 years].The shape and the size of plaques were determined based on location and size of the suspected area. The plaque was inserted to deliver a target dose of 80-100 Gy in the region of conjunctival malignancy. The diagnosis was squamous cell carcinoma in three eyes and conjunctival melanoma in two eyes. All patients had surgical history of one to three previous excisions with or without cryotherapy before brachytherapy. There were microscopic residual tumors after excision in 2 eyes and recurrent lesion was evident in 3 other eyes. A mean dose of 95 Gy was delivered to the tumor bed. Complete tumor regression without any evidence of recurrent lesion was obtained in all five eyes. The patients were followed for 32 months on average [range, 18-42 months]. No radiation related complication was detected, with an exception of a dry eye in the last follow up. Brachytherapy with RU-106 plaque is an alternative method for treatment of selected patients with recurrent or residual conjunctival SCC and melanoma