Abstract
Purpose: To evaluate dosimetric impact of air cavities and their corresponding electron density correction for 0.35 tesla (T) Magnetic Resonance-guided Online Adaptive Radiation Therapy (MRgART) of prostate bed patients. Methods: Three 0.35 T MRgRT plans (anterior–posterior (AP) beam, AP–PA beams, and clinical intensity modulated radiation therapy (IMRT)) were generated on a prostate bed patient’s (Patient A) planning computed tomography (CT) with artificial rectal air cavities of various sizes (0–3 cm, 0.5 cm increments). Furthermore, two 0.35 T MRgART plans (‘Deformed’ and ‘Override’) were generated on a prostate bed patient’s (Patient B) daily magnetic resonance image (MRI) with artificial rectal air cavities of various sizes (0–3 cm, 0.5 cm increments) and on five prostate bed patient’s (Patient 1–5) daily MRIs (2 MRIs: Fraction A and B) with real air cavities. For each MRgART plan, daily MRI electron density map was obtained by deformable registration with simulation CT. In the ‘Deformed’ plan, a clinical IMRT plan is calculated on the daily MRI with electron density map obtained from deformable registration only. In the ‘Override’ plan, daily MRI and simulation CT air cavities are manually corrected and bulk assigned air and water density on the registered electron density map, respectively. Afterwards, the clinical IMRT plan is calculated. Results: For the MRgRT plans, AP and AP–PA plans’ rectum/rectal wall max dose increased with increasing air cavity size, where the 3 cm air cavity resulted in a 20%/17% and 13%/13% increase, relative to no air cavity, respectively. Clinical IMRT plan was robust to air cavity size, where dose change remained less than 1%. For the MRgART plans, daily MRI electron density maps, obtained from deformable registration with simulation CT, was unable to accurately produce electron densities reflecting the air cavities. However, for the artificial daily MRI air cavities, dosimetric change between ‘Deformed’ and ‘Override’ plan was small (<4%). Similarly, for the real daily MRI air cavities, clinical constraint changes between ‘Deformed’ and ‘Override’ plan was negligible and did not lead to change in clinical decision for adaptive planning except for two fractions. In these fractions, the ‘Override’ plan indicated that the bladder max dose and rectum V35.7 exceeded the constraint, while the ‘Deformed’ plan showed acceptable dose, although the absolute difference was only 0.3 Gy and 0.03 cc, respectively. Conclusion: Clinical 0.35 T IMRT prostate bed plans are dosimetrically robust to air cavities. MRgART air cavity electron density correction shows clinically insignificant change and is not warranted on low-field systems.
Highlights
MR-guided radiation therapy (MRgRT) systems combine a magnetic resonance imaging (MRI) scanner with a linear accelerator (LINAC) radiation therapy system
On-board MRI allows for accurate daily patient setup, which can be further utilized for MR-guided online adaptive radiation therapy (MRgART), where treatment plans can be modified based on the patient’s daily anatomy [2]
For the AP and AP–PA plan, hot and cold spots, within the rectum, significantly increased in magnitude and size, relative to no air cavity, with increasing air cavity size due to the electron return effect (ERE), where the 3 cm air cavity resulted in a 20%/17% and 13%/13% increase to the rectum/rectal wall max dose, respectively
Summary
MR-guided radiation therapy (MRgRT) systems combine a magnetic resonance imaging (MRI) scanner with a linear accelerator (LINAC) radiation therapy system. The integrated MRI scanner allows for radiation-free on-board imaging with superior soft tissue contrast as opposed to conventionally used x-ray-based on-board imaging. On-board MRI allows for accurate daily patient setup, which can be further utilized for MR-guided online adaptive radiation therapy (MRgART), where treatment plans can be modified based on the patient’s daily anatomy [2]. The effects of the magnetic field on the radiation beam have been a primary concern for patients being treated on MRgRT systems. Most notably, traveling electrons, generated within the body by the irradiating beam, are redirected by the Lorentz’s force from a perpendicular magnetic field. At tissue-to-air-interfaces, some electrons are subjected to the electron return effect (ERE), in which the Lorentz force redirects the electrons back upstream to the tissue, resulting in increased tissue dose deposition and potentially treatment hotspots. The ERE creates treatment cold spots at latter air-tissue interfaces due to less tissue dose deposition downstream [4,6,7]
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