Abstract
This work investigated the possibility of using proton beam for total body irradiation (TBI). We hypothesized the broad‐slow‐rising entrance dose from a monoenergetic proton beam can deliver a uniform dose to patient with varied thickness. Comparing to photon‐based TBI, it would not require any patient‐specific compensator or beam spoiler. The hypothesis was first tested by simulating 250 MeV, 275 MeV, and 300 MeV protons irradiating a wedge‐shaped water phantom in a paired opposing arrangement using Monte Carlo (MC) method. To allow ±7.5% dose variation, the maximum water equivalent thickness (WET) of a treatable patient separation was 29 cm for 250 MeV proton, and >40 cm for 275 MeV and 300 MeV proton. The compared 6 MV photon can only treat patients with up to 15.5 cm water‐equivalent separation. In the second step, we simulated the dose deposition from the same beams on a patient's whole‐body CT scan. The maximum patient separation in WET was 23 cm. The calculated whole‐body dose variations were ±8.9%,±9.0%, ±9.6%, and ±14% for 250 MeV proton, 275 MeV proton, 300 MeV proton, and 6 MV photon. At last, we tested the current machine capability to deliver a monoenergetic proton beam with a large uniform field. Experiments were performed on a compact double scattering single‐gantry proton system. With its C‐shaped gantry design, the source‐to‐surface distance (SSD) reached 7 m. The measured dose deposition curve had 22 cm relatively flat entrance region. The full width half maximum field size was measured 105 cm. The current scatter filter had to be redesigned to produce a uniform intensity at such treatment distance. In conclusion, this work demonstrated the possibility of using proton beam for TBI. The current commercially available proton machines would soon be ready for such task.PACS number(s): 87.53.Bn, 87.55.K‐, 87.55.‐x, 87.56.‐v
Highlights
Total body irradiation (TBI) is commonly used as part of the bone marrow transplant preparative regimen for leukemia, aplastic anemia, or lymphoma treatment
For 250 MeV proton beams, the very high dose regions at the thick corners of the water phantom were produced by the Bragg peak
For 275 MeV and 300 MeV protons, Bragg peaks were not observed within the water-equivalent phantom when the separation exceeding 40 g/cm2
Summary
Total body irradiation (TBI) is commonly used as part of the bone marrow transplant preparative regimen for leukemia, aplastic anemia, or lymphoma treatment. To achieve a uniform dose across patient’s body with varied thickness, the ideal radiation beam should have a large field size, uniform intensity, as well as uniform dose deposition across varied depths. The current standard TBI treatment utilizes the linac-based megavoltage photon beam. By placing the patient at extended source-to-surface distance (SSD) with maximum field opening, a large beam of relatively uniform fluence can be achieved from a modern linac. A common TBI setup uses two paired-opposing beams with beam spoiler.[3] The beam spoiler placed in front of the patient during treatment accommodates the dose buildup region to avoid underdosing patients’ skin. Patient-specific compensators have to be designed to compensate for the body thickness variation. Overall, ± 10% dose variation can be achieved with careful planning and delivery.[4]
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