Can we skip the simulation step for radiation treatment planning in patients with symptomatic bone metastases?

255 Background: Patients with bone metastases may require urgent radiation treatment (RT) due to severe pain, neurologic or other symptoms. Conventional approaches typically involve a 3-step process: initial consultation, followed several days later by a planning CT (pCT) simulation and dosimetric planning, and finally RT delivery. The time frame from consultation to RT can be up to 2 weeks, overly lengthy for severely symptomatic patients. We explored the viability of using diagnostic CT (dCT) imaging, usually available to radiation oncologists at the time of consultation, for RT planning - with the ultimate goal of removing the simulation step to reduce the interval from consultation to RT. In this initial phase of the project, we set out to compare differences in key dosimetric parameters from dCT- and pCT- generated RT plans. Methods: We retrospectively reviewed patients (n=28) treated for bone metastases at a single academic institution from 2020-2023. We utilized rigid image registration to transfer physician contours from pCT images to dCT images to account for changes in patient positioning. Beam arrangements, energies, and administered monitor units were also transferred from the pCT to the dCT for accurate dose calculation. We calculated RT dose coverage of each target metastasis using “V100”, or the percentage of the target lesion receiving 100% of the RT prescription dose. Correspondingly, the “V95” and the maximum point dose (“hotspot”) in patients were also calculated. Results: Out of 28 patients, 24 were treated to single and 4 to multiple (2-4) bone metastases. We excluded patients who had no dCT (n=7) or whose body positioning on the dCT (e.g. arms) created obstructions for beam arrangements (n=2). Eventually, 24 clinical plans made from 19 patients’ pCTs were migrated to their dCTs to generate comparable dosimetric plans. For all 24 dCT-based plans (12 spines and 12 extremities), the mean full dose coverage V100 decreased by 7.14% ± 15.89%. Overall, the plans had 91.14% ± 18.6% for V95 to PTV. Hotspots rose by 2.75% ± 3.61%. Two cases were observed to have full-dose-coverage drop over 40%: one was caused by patient’s outline change induced by different couches used for diagnosis and simulation, and the other one had an initial coverage of 57.6% for V100 in the original clinical plan. The remaining plans had a drop of 3.14% ± 8.37% on V100, and 96.1% ± 8.2% for V95. Conclusions: It was feasible to generate radiation treatment plans for bone metastases with good tumor coverage using dCT imaging. Patient selection plays a crucial role in dCT-based treatment planning, as the dose distribution is highly sensitive to anatomical variations resulting from patient positioning. This treatment planning approach circumvents the simulation step and would reduce patients' wait time for RT, thereby enabling more rapid access to RT for patients suffering with symptoms from bone metastases.