Single institution experience of MRI-guided radiotherapy for thoracic tumors and clinical characteristics impacting treatment duty cycle.

Introduction: MRI-guided radiotherapy (MRgRT) allows for direct motion management and real-time radiation treatment plan adaptation. We report our institutional experience using low strength 0.35T MRgRT for thoracic malignancies, and evaluate changes in treatment duty cycle between first and final MRgRT fractions. Methods: All patients with intrathoracic tumors treated with MRgRT were included. The primary reason for MRgRT (adjacent organ at risk [OAR] vs. motion management [MM] vs. other) was recorded. Tumor location was classified as central (within 2cm of tracheobronchial tree) vs. non-central, and further classified by the Expanded HILUS grouping. Gross tumor volume (GTV) motion, planning target volume expansions, dose/fractionation, treatment plan time, and total delivery time were extracted from the treatment planning system. Treatment plan time was defined as the time for beam delivery, including multileaf collimator (MLC) motion, and gantry rotation. Treatment delivery time was defined as the time from beam on to completion of treatment, including treatment plan time and patient respiratory breath holds. Duty cycle was calculated as treatment plan time/treatment delivery time. Duty cycles were compared between first and final fraction using a two-sample t-test. Results: Twenty-seven patients with thoracic tumors (16 non-small cell lung cancer and 11 thoracic metastases) were treated with MRgRT between 12/2021 and 06/2023. Fifteen patients received MRgRT due to OAR and 11 patients received MRgRT for motion management. 11 patients had central tumors and all were treated with MRgRT due to OAR risk. The median dose/fractionation was 50 Gy/5 fractions. For patients treated due to OAR (n=15), 80% had at least 1 adapted fraction during their course of radiotherapy. There was no plan adaptation for patients treated due to motion management (n=11). Mean GTV motion was significantly higher for patients treated due to motion management compared to OAR (16.1mm vs. 6.5mm, p=0.011). Mean duty cycle for fraction 1 was 54.2% compared to 62.1% with final fraction (p=0.004). Mean fraction 1 duty cycle was higher for patients treated due to OAR compared to patients treated for MM (61% vs. 45.0%, p=0.012). Discussion: Duty cycle improved from first fraction to final fraction possibly due to patient familiarity with treatment. Duty cycle was improved for patients treated due to OAR risk, likely due to more central location and thus decreased target motion.

Gamma Knife radiosurgery of saccular aneurysms in a rabbit model.

OBJECTIVEIntracranial aneurysms are vascular abnormalities associated with neurological morbidity and mortality due to risk of rupture. In addition, many aneurysm treatments have associated risk profiles that can preclude the prophylactic treatment of asymptomatic lesions. Gamma Knife radiosurgery (GKRS) is a standard treatment for trigeminal neuralgia, tumors, and arteriovenous malformations. Aneurysms associated with arteriovenous malformations have been noted to resolve after treatment of the malformation. The aim of this study was to determine the efficacy of GKRS treatment in a saccular aneurysm animal model.METHODSAneurysms were surgically produced using an elastase-induced aneurysm model in the right common carotid artery of 10 New Zealand white rabbits. Following initial observation for 4 years, each rabbit aneurysm was treated with a conformal GKRS isodose of 25 Gy to the 50% margin. Longitudinal MRI studies obtained over 2 years and terminal measures obtained at multiple time points were used to track aneurysm size and shape index modifications.RESULTSAneurysms did not rupture or involute during the observation period. Whole aneurysm and blood volume averages decreased with a linear trend, at rates of 1.7% and 1.6% per month, respectively, over 24 months. Aneurysm wall percent volume increased linearly at a rate of 0.3% per month, indicating a relative thickening of the aneurysm wall during occlusion. Nonsphericity of the average volume, aspect ratio, and isoperimetric ratio of whole aneurysm volume all remained constant. Histopathological samples demonstrated progressive reduction in aneurysm size and wall thickening, with subintimal fibrosis. Consistent shape indices demonstrate stable aneurysm patency and maintenance of minimal rupture risk following treatment.CONCLUSIONSThe data indicate that GKRS targeted to saccular aneurysms is associated with histopathological changes and linear reduction of aneurysm size over time. The results suggest that GKRS may be a viable, minimally invasive treatment option for intracranial aneurysm obliteration.

A review of nonstandardized applicators digitization in Nucletron™ HDR procedures.

The major errors in HDR procedures were failures to enter the correct treatment distance, which could be caused by either entering wrong transmission lengths or imprecisely digitizing the dwelling positions. Most of those errors were not easily avoidable by enhancing the HDR management level because they were caused by implementations of nonstandardized applicators utilizing transmission tubes of different lengths in standard HDR procedures. We performed this comprehensive study to include all possible situations with different nonstandardized applicators that frequently occurred in HDR procedures, provide corresponding situations with standard applicator as comparisons, list all possible errors and in planning, clarify the confusions in offsets setting, and provide mathematical and quantitative solutions for each given scenarios. Training on HDR procedures with nonstandardized applicators are normally not included in most residential program for medical physics, thus this study could be meaningful in both clinical and educational purpose. At precision of 1 mm, our study could be used as the essential and practical reference for finding the correct treatment length as well as locating the accurate dwelling positions in any HDR procedure with nonstandardized applicators.

Comparison of conventional to intensity modulated radiation therapy for abdominal neuroblastoma.

OBJECTIVE: To compare three different techniques of irradiating abdominal neuroblastoma. PATIENTS AND METHODS: Six children with a median age of 4.1 years underwent radiotherapy (RT) to the primary site as part of treatment for high-risk neuroblastoma. Four had midline disease while two had well-lateralized lesions. Three different RT techniques were compared. Technique A used parallel-opposed AP/PA fields prescribed to the midplane of the patient. For Techniques B and C, intensity modulated radiation therapy (IMRT) plans were developed using inverse treatment planning with a sliding window or dynamic multileaf collimator approach, seven coplanar beams, and a 0.25 x 0.5 cm minimum beam resolution. The clinical target volume (CTV) included the tumor present prior to second look surgery but after induction chemotherapy with a 1.5 cm margin. The planning target volume (PTV) was the CTV with a 0.5 cm margin. The CTV was planned to receive 100% of the prescribed dose. For Technique C, the vertebral bodies adjacent to the tumor were included in the PTV to minimize heterogeneity of dose. Six MV photons were used for all techniques. Bilateral kidneys, liver, spine, spleen, stomach and bilateral iliac crests were contoured. RESULTS: Dose to the PTV and CTV were not significantly different using the three techniques. In comparison to Technique A, Techniques B and C delivered a lower mean dose to the bilateral kidneys in the four children with midline tumors but not the two children with a lateralized tumor where the contralateral kidney received a higher mean dose. Dose to the spine was less homogeneous with Technique B compared to Techniques A and C. The spleen, liver and stomach mean doses were higher using Techniques B and C compared to Technique A. CONCLUSION: Although Technique C was the best method of RT delivery in midline tumors with respect to kidney doses, this was at a cost of a higher mean dose to the liver, stomach, and spleen. This, together with the theoretical increase in secondary malignancies, should be considered when treating a child with IMRT techniques. IMRT was not found to be better than the conventional AP/PA field for lateralized tumors.

Measurements of secondary neutron dose from 15 MV and 18 MV IMRT.

Secondary neutron dose-equivalents were determined for conventional and intensity modulated radiation therapy (IMRT) prostate treatments for 15 and 18 MV X-ray beams. Conventional and IMRT treatment plans were generated to deliver 45 Gy to the prostate, seminal vessicles and external and internal iliac lymph nodes. Neutron spectra were determined by unfolding measurements from a TLD-based Bonner sphere system. Treatments using 18 MV IMRT and conventional plans result in neutron ambient dose-equivalents of 687 and 112 mSv, respectively. Delivery of the 15 MV IMRT and conventional plans results in neutron ambient dose-equivalents of 327 and 52 mSv, respectively. The data illustrate that using lower photon energies for IMRT reduces the secondary neutron dose, while still achieving comparable treatment volume coverage and sparing critical normal tissue.

Investigation of secondary neutron dose for 18 MV dynamic MLC IMRT delivery.

Secondary neutron doses from the delivery of 18 MV conventional and intensity modulated radiation therapy (IMRT) treatment plans were compared. IMRT was delivered using dynamic multileaf collimation (MLC). Additional measurements were made with static MLC using a primary collimated field size of 10 x 10 cm2 and MLC field sizes of 0 x 0, 5 x 5, and 10 x 10 cm2. Neutron spectra were measured and effective doses calculated. The IMRT treatment resulted in a higher neutron fluence and higher dose equivalent. These increases were approximately the ratio of the monitor units. The static MLC measurements were compared to Monte Carlo calculations. The actual component dimensions and materials for the Varian Clinac 2100/2300C including the MLC were modeled with MCNPX to compute the neutron fluence due to neutron production in and around the treatment head. There is excellent agreement between the calculated and measured neutron fluence for the collimated field size of 10 x 10 cm2 with the 0 x 0 cm2 MLC field. Most of the neutrons at the detector location for this geometry are directly from the accelerator head with a small contribution from room scatter. Future studies are needed to investigate the effect of different beam energies used in IMRT incorporating the effects of scattered photon dose as well as secondary neutron dose.