70 MeV delivered the best balance between target coverage and tissue sparing
Proton therapy for Hodgkin’s lymphoma has long been attractive because the Bragg peak allows most of the dose to stop inside the tumor instead of spilling into nearby normal tissue. In young patients, that matters twice: first because the disease is often curable, and second because long-term toxicity can shape the rest of life. This study used a GATE-based Monte Carlo framework to ask a practical planning question rather than a theoretical one—what proton energy best fits a simplified mediastinal-type target, and how much dose reaches nearby organs when that beam is applied in a standardized body model?
The target geometry was deliberately simple, using a spherical lesion 37 mm in diameter placed at depths ranging from 5 to 42 mm in a homogeneous water phantom. Energies from 60 to 90 MeV were tested, and the plan quality was judged using a weighted Target Coverage & Sparing metric that combined dose to the target with sparing of tissue beyond it. Among the energies examined, 70 MeV produced the best result for the modeled depth range, with a TCS value of 0.876 and 93.8% target coverage.
That finding is not a minor technical detail. A proton beam can look elegant on paper and still fail to cover a target adequately if the energy is mismatched to depth, especially when the target sits within a narrow thoracic window where millimeters matter. Here, the 70 MeV configuration offered the most favorable compromise between fully covering the lesion and limiting distal dose deposition. For a treatment modality built on precision, the energy choice becomes part of the prescription itself.
Out-of-field dose fell steeply with distance from the target
The second half of the analysis moved beyond the idealized water phantom and into a voxelized CIRS ATOM anthropomorphic model to estimate absorbed dose to organs at risk. The pattern was straightforward but clinically important: organ doses declined approximately exponentially as the distance from the target increased. The left lung received the highest relative organ dose, at 39.9 mGy per Gray of physical dose to the target, while the thyroid received 0.079 mGy per Gray.
That steep gradient is exactly what clinicians hope to see from proton therapy, yet the numbers also show that out-of-field exposure does not disappear. Nearby organs remain part of the dosimetric equation, especially in Hodgkin lymphoma, where treatment fields often sit near the chest, neck, and upper mediastinum. The left lung result stands out because it reflects how a thoracic target can still expose adjacent pulmonary tissue despite a tightly conformal beam. By contrast, the very low thyroid dose illustrates how quickly dose falls away as anatomical distance increases.
The study also reported small statistical uncertainty, staying below 2% in the target volume and below 3.2% in organs at risk. Those low uncertainties strengthen confidence in the simulation outputs and make the comparison between energies more stable. In Monte Carlo work, precision matters because plan ranking can change when the calculations are noisy or underpowered. Here, the dose estimates were consistent enough to support an energy-selection framework rather than a loose qualitative impression.
Why this matters for Hodgkin lymphoma planning
Hodgkin lymphoma is one of the cancers where long-term toxicity prevention can matter as much as tumor control. Survivors may live for decades, so even modest reductions in radiation to the heart, lungs, breast tissue, and thyroid can carry outsized value. Proton therapy is attractive in this setting because it can reduce unnecessary integral dose, but the benefit is not automatic. Beam energy, depth, target size, and nearby anatomy all influence the final distribution.
This work offers a standardized dosimetric reference point for that problem. By using a simplified geometry and a common anthropomorphic phantom, it creates a reproducible platform for comparing treatment concepts before patient-specific optimization begins. That is useful for physics commissioning, benchmarking across centers, and preliminary estimation of secondary cancer risk in a controlled setting. It does not replace patient planning, but it provides a baseline against which more individualized simulations can be measured.
The biological logic behind the result is familiar. When the beam energy is matched to the target depth, the dose peaks where it is needed and falls off sharply afterward. If the energy is too low, the target is undercovered; if it is too high, the distal falloff may shift beyond the lesion and increase exposure to healthy tissue. In a disease treated in young patients, that tradeoff is not abstract. It can shape late cardiac, pulmonary, endocrine, and second-malignancy risks years after treatment ends.
A modeling framework, not a clinical endpoint
The study remains a simulation-based dosimetric analysis, and its conclusions sit inside that boundary. The geometry was simplified to a spherical target in a homogeneous medium before extension into a standardized phantom, so the results cannot be read as a patient-specific treatment recommendation. Real Hodgkin lymphoma cases vary by age, body habitus, tumor shape, prior treatment, and the complex motion of thoracic structures. Those variables can shift the actual dose distribution in ways that a simplified model cannot capture.
Even so, the structure of the analysis is clinically useful because it separates two decisions that are often blended together in routine discussion: energy selection and organ sparing. The 70 MeV beam was the strongest performer for the modeled depth range, and the organ-dose profile showed the expected sharp decline with distance. For medical physicists and radiation oncologists, those findings support continued use of standardized Monte Carlo evaluation when comparing proton plans for Hodgkin lymphoma.
The next step is straightforward in concept, but demanding in practice: validate this framework with patient-specific anatomy and motion-aware planning. Until then, the most defensible interpretation is limited to the geometry tested here, where a 70 MeV proton beam achieved the best balance of coverage and sparing in a controlled model of Hodgkin’s lymphoma treatment.