Dosimetric Advantages of VMAT TBI Plans: A Direct Comparison to Conventional TBI Plans

PURPOSE/OBJECTIVE(S)

Total body irradiation (TBI) continues to play an integral role in the conditioning of patients undergoing bone marrow transplantation. Historically, conventional TBI (C-TBI) has been delivered based on a clinical plan without CT simulation. However, volumetric modulated arc therapy TBI (V-TBI) is emerging as an alternative method to deliver TBI in a contemporary fashion. We aimed to compare these two methods of TBI delivery.

MATERIALS/METHODS

Patients undergoing treatment with V-TBI were identified. C-TBI plans were created using their existing simulation CT images. Patient thickness was measured on the scan and compensators such as lead sheets to attenuate dose in areas with less separation and lung blocks drawn on digitally reconstructed radiographs (DRR) were added when necessary. A 3D dose distribution was then calculated allowing for the direct comparison between C-TBI and V-TBI on the same patient using the same CT image set. Dosimetric data from each plan including target volume coverage, dose homogeneity, absolute max dose, and dose to lungs were recorded.

RESULTS

V-TBI and C-TBI plans for a total of four patients were preliminarily analyzed. Two patients were prescribed 200 cGy in a single fraction, while the other two were prescribed 1200 cGy in eight fractions. V-TBI resulted in a more favorable dosimetry for all four patients in most evaluated metrics including dose coverage, dose homogeneity, and lung dose (Table 1). V-TBI did result in an increased absolute maximum dose to all four patients compared to c-TBI, but still met the desired constraint of D0.03cc<125%.

CONCLUSION

V-TBI resulted in more favorable dosimetry for all four patients compared to C-TBI. To our knowledge, this is the first direct dosimetric comparison between the two methods. Analysis of 8 more V-TBI cases is currently underway. In the future, we plan to design a prospective study to evaluate the clinical outcomes of patients undergoing V-TBI vs C-TBI.

Dose Coverage Variation Caused by Setup Uncertainties in VMAT-TBI Treatment

PURPOSE/OBJECTIVE(S)

Volumetric Modulated Arc Therapy Total Body Irradiation (VMAT TBI) offers several advantages over conventional TBI techniques, including reduced cost of the treatment room (a normal-sized vault versus a large shielded vault), increased patient comfort during treatment, lung sparing without the need for physical blocks, and a more homogeneous dose distribution. In VMAT-TBI treatment, plans at multiple isocenters are utilized to cover the entire body, and patients are positioned with the aid of image guidance. However, aligning the patient's entire body during setup can be challenging due to its pliability, and the setup time is heavily dependent on the tolerance allowed by the image guidance. Therefore, studying the variation in dose coverage caused by setup uncertainties in VMAT-TBI treatments can help optimize the clinical workflow and determine the optimal tolerance for patient positioning.

MATERIALS/METHODS

New plans were generated to simulate the uncertainties that occur during treatment setup for each patient. These plans were created by shifting the original VMAT TBI plans at the head, chest, abdomen (or pelvis) isocenters by 5mm and 1cm in the left-right (LR), inferior-superior (IS), and anterior-posterior (AP) directions, respectively. Dose DicomRT files were exported, and the dose change due to the shifts was analyzed. The statistical quantification of the percentage of the body that experienced a dose change of over 2%, 5%, and 10% of the prescription due to the shifts from the original plans was calculated for all patients. Histograms were generated, showing the percentage of body getting dose change of 1-2%, 2-3%, 3-5%, 5-7%, 7-10%, 10-15%.

RESULTS

The table below displays the percent volume receiving a dose change of 2%, 5%, and 10% of the prescription for a 5mm shift. Among the shift directions, the dose change is most sensitive in the IS direction, with similar impact observed the in LR and AP directions. Among different sites, the chest experiences the largest dose change, followed by the pelvis. For a 5mm shift in the IS direction, the average percent volume receiving a dose change of 2%, 5%, and 10% in the chest is 9.25%, 2.64%, and 0.27%, respectively. For a 1cm shift, the numbers are 12.23%, 6.75%, and 1.29%. In the pelvis (abdomen), these values are 9.03%, 1.67%, and 0.17% for a 5mm shift and 13.28%, 6.1%, and 0.85% for a 1cm shift. For head plans, the values are 2.72%, 0.9%, and 0.14% for a 5mm shift and 3.77%, 1.66%, and 0.53% for a 1cm shift.

CONCLUSION

Accurate alignment in the chest region is crucial in VMAT TBI treatment. Efforts should be made to minimize shifts over 1cm in the IS direction.

Bilateral ossifying fibroma of the maxillary sinus

A case involving a 35-year-old man with massive, bilateral, slow-growing ossifying fibromas in the maxillary sinuses resulting in facial deformity and orbital compression is discussed. The historical difficulty in categorizing fibro-osseous lesions and the importance of clinical, radiographic, and surgical findings relating to the proper diagnosis and treatment are reviewed. The use of the CT scan to delineate the extent of the pathologic lesion is demonstrated.

Spontaneous tooth exfoliation and osteonecrosis following a herpes zoster infection of the fifth cranial nerve

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