The Renaissance of Radiotherapy's Oldest Workhorse
In the world of modern radiation oncology, where cutting-edge technologies like proton therapy and MR-LINACs dominate headlines, an unexpected contender remains indispensable: kilovoltage X-ray therapy (kV therapy). Born from Wilhelm Röntgen's monumental discovery of X-rays in 1895, kV therapy represents both the earliest form of radiotherapy and a surprisingly relevant tool in contemporary cancer treatment 1 . Despite predictions that advanced technologies would render it obsolete, kV therapy continues to offer unique advantages for certain cancers, particularly superficial malignancies like basal cell carcinoma and keloid scars 5 . This article explores how this century-old technology continues to earn its place in modern radiotherapy departments through precision, practicality, and ongoing innovation.
The therapeutic use of X-rays began remarkably soon after their discovery. By July 1896—mere months after Röntgen's announcement—Victor Despeignes was already attempting to treat stomach cancer with X-rays 1 . These early applications established the foundation of radiation oncology, though they lacked the sophistication and safety measures of modern practice.
The mid-20th century witnessed a significant shift with the development of megavoltage (MV) therapy equipment. Linear accelerators capable of producing higher-energy radiation beams offered deeper penetration into tissue and skin-sparing effects that kV systems couldn't match.
Despite these advancements, kV therapy never disappeared completely. As radiation oncologists gained more experience treating various cancers, they recognized that different clinical scenarios required different radiation energies.
Kilovoltage X-ray beams have a distinct physical property that makes them ideal for treating surface lesions: the maximum dose occurs very close to the surface 1 . This enables precise targeting of skin cancers while minimizing radiation exposure to deeper structures.
The simplicity of kV treatments provides both clinical and practical benefits. Unlike advanced MV techniques that require complex planning, kV therapy often involves direct application with minimal setup.
The rapid dose fall-off means that radiation is concentrated in the target area with limited penetration beyond. This becomes a significant advantage when treating superficial targets near critical structures 9 .
kV therapy systems offer economic advantages over more advanced technologies. They require less shielding, have a smaller physical footprint, and come at a lower cost 2 .
One of the most exciting recent developments is Converging Lens Radiotherapy (CLRT), which uses specialized optics to create a focused, nearly monoenergetic X-ray beam at approximately 60 keV 2 .
The CLRT system combines a conventional kV X-ray tube with a proprietary focusing lens that creates a convergent, hollow beam pattern. Early studies have demonstrated superior organ-at-risk sparing in certain cases 2 .
CLRT enables kV energy to be focused on deep-seated tumors—traditionally impossible with conventional kV systems.
Another significant innovation is Kilovoltage Intrafraction Monitoring (KIM), which uses the kV imaging system already mounted on modern linear accelerators for real-time tumor tracking during treatment 3 .
The first clinical treatment using KIM occurred in September 2014 for a prostate cancer patient. During delivery, the system detected a 3 mm posterior displacement of the prostate 3 .
Modern approaches have revolutionized kV therapy through Monte Carlo (MC) simulations, which explicitly model radiation transport through complex geometries and materials 4 .
These models show excellent agreement with measurements (within 2-4% for depth doses and profiles) and enable accurate 3D dose calculations that previously weren't possible 4 .
| Aspect | Traditional Approach | Modern Innovations |
|---|---|---|
| Dose Calculation | Manual point doses in water | Monte Carlo simulations in patient CT data |
| Targeting | Fixed beams | Robotic positioning with 6 degrees of freedom |
| Imaging Guidance | None or limited | Real-time intrafraction monitoring |
| Field Shaping | Standard applicators | Custom 3D-printed bolus and shields |
| Depth Penetration | Limited to superficial targets | Converging lenses for deeper focus |
A seminal study published in Medical Dosimetry directly compared kilovoltage X-ray and electron beam dose distributions for radiotherapy of the sternum 9 . The research team created treatment plans using both:
Dose distributions were calculated using four different algorithms with Monte Carlo simulation serving as the gold standard 9 .
The Monte Carlo simulations revealed significant differences between the two modalities:
The study used data from a 48-year-old woman with metastatic breast cancer in the sternum to compare kV and electron beam approaches.
| Metric | Algorithm | kV X-ray Beam | Electron Beam |
|---|---|---|---|
| Target Min Dose | Monte Carlo | 94% | 74% |
| Target Avg Dose | Monte Carlo | 103% | 97% |
| Target Max Dose | Monte Carlo | 108% | 107% |
| Lung Max Dose | Monte Carlo | 94% | 84% |
| Lung Avg Dose | Monte Carlo | 31% | 14% |
This experiment was methodologically significant because it:
The findings reinforced that no single beam energy is ideal for all clinical scenarios and that treatment selection must consider specific patient anatomy and clinical priorities 9 .
of radiotherapy centres have at least one kV treatment unit 5
patients treated annually with kV therapy in the UK alone 5
The most common application is for basal cell carcinoma, which accounts for 44% of all kV treatments 5 . Other frequent applications include:
| Tool/Technology | Function | Application Example |
|---|---|---|
| Monte Carlo Simulations | Accurately models radiation transport through complex geometries | Predicting dose distributions in heterogeneous tissues 4 |
| SpekPy Software | Calculates X-ray spectra and beam qualities | Determining half-value layers (HVLs) for beam characterization 7 |
| Piranha MULTI Meter | Measures HVL and other beam parameters | Quality assurance and beam calibration 7 |
| GATE/Geant4 | Monte Carlo simulation platforms | Modeling radiation transport and interactions 7 |
| EBT3 GafChromic Film | High-resolution dose measurement | Validating calculated dose distributions 4 |
| 3D Printing | Creates patient-specific applicators and bolus | Customizing treatment delivery for individual anatomy 8 |
| Robotic Positioning Systems | Precisely aligns radiation beams | Enabling non-coplanar treatment approaches 2 |
These tools have collectively addressed historical limitations in kV therapy, particularly regarding dose calculation accuracy and treatment customization. For example, Monte Carlo simulations now allow researchers to accurately model the effects of tissue inhomogeneities and custom shielding 4 .
Recent research has explored connections between radiation delivery techniques and immune responses. Spatially fractionated radiotherapy techniques may potentially stimulate enhanced immune recognition of cancer cells 6 .
Three-dimensional printing technology enables creation of patient-specific applicators, shields, and bolus materials that optimize dose distributions for individual anatomy 8 .
Miniaturized kV sources that function like traditional brachytherapy sources represent a growing application of kV technology 5 . These systems offer the dosimetric advantages of kV energy with applicator-based delivery.
The integration of real-time imaging with kV treatment delivery opens possibilities for adaptive approaches. Systems like Kilovoltage Intrafraction Monitoring (KIM) demonstrate that kV imaging can provide sufficient data for motion tracking 3 .
Kilovoltage therapy exemplifies how medical technologies can evolve rather than become obsolete. From its origins as radiotherapy's earliest tool, kV therapy has continuously adapted to maintain relevance in modern practice. Its persistence stems from unique physical properties that remain valuable for specific clinical scenarios, particularly superficial tumors where rapid dose fall-off is advantageous rather than limiting.
Recent technological innovations have addressed historical limitations and expanded potential applications. Converging lenses enable focusing on deeper targets, Monte Carlo dose calculation improves accuracy in heterogeneous tissues, and real-time imaging supports precision delivery 2 3 4 . These advances complement kV therapy's inherent advantages: simplicity, cost-effectiveness, and clinical efficacy for appropriate cases.
Kilovoltage therapy remains, indeed, well and truly alive and needed in modern radiotherapy centres.
As radiation oncology continues advancing toward increasingly personalized care, kV therapy remains well-positioned to address specific needs that higher-energy approaches cannot optimally fulfill. Its enduring presence in modern radiotherapy centers reflects a fundamental principle of medical technology: sometimes the optimal tool isn't the most technologically complex, but the most physically and clinically appropriate for the task at hand.