Licensing of the Orthanc ecosystem


The objectives of the Orthanc ecosystem is to share technical knowledge about DICOM, to build a consistent platform for developing medical imaging software, and to foster scientific collaborations in medical imaging by subscribing to the open-science paradigm. To this end, Orthanc is provided as free and open-source software to the benefit of the worldwide community of medical imaging.

In order to support this objective of global knowledge sharing, the Orthanc project enforces reciprocity. If someone finds Orthanc useful to her academic work or to her business, the community of medical imaging should gain an advantage from this use by enlarging the knowledge base. This virtuous circle guarantees the fact that Orthanc will be developed in a sustainable way in the long-term, to the benefit of all stakeholders. Predatory behaviors should be prevented, while preserving the freedoms of the users of Orthanc, including the commercial uses.

According to this philosophy, the University Hospital of Liège decided to release the Orthanc ecosystem under the GPLv3+ license in 2012. The GPL is a strong copyleft license that is recognized worldwide, and that is designed to enforce reciprocity.

As Orthanc is lightweight and designed for Web applications and for sharing medical images over Internet, it has been quickly deployed on cloud platforms in order to host large amount of data. Orthanc considers this use as very legitimate, for instance for scientific purpose (think of open-data databases) or for societal needs (think of teleradiology platforms in developing countries). Unfortunately, the GPL does not protect from predatory commercial behaviors over cloud platforms because of the so-called “ASP loophole”, that does not enforce modified versions of a free and open-source software running on a server to be given back to the community.

For this reason, the plugins that provide scalability-related or cloud-related features (for instance the PostgreSQL and Web viewer plugins that are necessary for Web applications distributed at a large scale) were released under the stronger AGPLv3+ licence. This license protects the community of medical imaging by ensuring that the features included in Orthanc instances running in remote servers are publicly available as well.


Over the years, it was observed that people fear the use of GPL and AGPL licenses, that are wrongly considered as preventing commercial uses. This is most often a wrong assumption, given that the Orthanc server is a standalone executable, not a software library.

The following table provides a simple summary of the most common situations, and indicates whether the use is accepted (“Yes”), prevented (“No”), or restricted (“Dual licensing”):

  Mode of distribution of the third-party system, or of the third-party plugin/script
Usage of the Orthanc ecosystem Permissive (MIT, BSD, Apache...) GPLv3 AGPLv3 Internal use Proprietary software distributed to clients Proprietary cloud platform
Using Orthanc as such, even if some AGPL-licensed plugin is installed N/A N/A N/A Yes Yes Yes
Calling Orthanc from a third-party application (using REST API or DICOM protocol), even if some AGPL-licensed plugin is installed Yes Yes Yes Yes Yes Yes
Creating an Orthanc plugin or a Lua script, that is hosted by an Orthanc server where no AGPL-licensed plugin is installed No Yes Yes Yes Dual licensing Yes
Creating an Orthanc plugin or a Lua script, that is hosted by an Orthanc server where some AGPL-licensed plugin is installed No Yes Yes Yes Dual licensing Dual licensing
Using a modified version of the GPL-licensed code of Orthanc, or using a modified version of some GPL-licensed plugin, or reusing their original code in a third-party system No Yes Yes Yes Dual licensing Yes
Using a modified version of some AGPL-licensed plugin, or reusing its original code in a third-party system No No Yes Yes Dual licensing Dual licensing
Using the Stone of Orthanc library No No Yes Yes Dual licensing Dual licensing


  • The wording “third-party system” is very broad, as it encompasses many possibilities. It can for instance be a Web application, a heavyweight desktop application, an automated script, or more generally any system that takes advantage of Orthanc as a service in its global architecture.
  • If your use case falls in a “Dual licensing” cell, please get in touch with Osimis, the commercial partner of the Orthanc project that is the only entity entitled to grant a license exception to your company for the Orthanc core and its associated official plugins.
  • If you reuse code from Orthanc or one of its associated plugins, you must mention the copyright of the Orthanc project.
  • An Orthanc plugin cannot be licensed under a permissive license (MIT, BSD, Apache...) because it cannot run independently of the Orthanc SDK, which implies that the plugin and the Orthanc core form a single combined program, which in turn means that the plugin should be licensed under GPLv3 by copyleft contamination. Check out the license compatibility matrix on Wikipedia. Here is the corresponding entry about this topic in the GPL FAQ: “If the main program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single combined program, which must be treated as an extension of both the main program and the plug-ins. [...] If the main program and the plugins are a single combined program then this means you must license the plug-in under the GPL or a GPL-compatible free software license and distribute it with source code in a GPL-compliant way.”
  • If you deal with medical applications in Europe, note that Osimis sells CE-approved versions of a Web viewer plugin.
  • You are kindly invited to cite the reference paper about Orthanc in your scientific work.
  • This is our own simplified, technical interpretation of the GPLv3+ and AGPLv3+ in the very specific context of Orthanc. It is not intended to be a complete guide to copyleft licensing. Please get in touch with the Free Software Foundation for more legal information.

Contributing to the code of Orthanc

Contributed vs. internal code

It is important to make the distinction between contributed code and internal code:

  • Contributed code refers to source code that takes advantage of Orthanc and/or that extends Orthanc, such as new plugins, Lua scripts, or any higher-level application that uses the REST API of Orthanc. This code can live outside of the official source repositories of the Orthanc ecosystem. External contributors can distribute such contributed code on whatever platform they prefer, in a way that is fully uncoupled from the Orthanc project, and keep the intellectual property of their developments. Such contributors are however kindly invited to index their contributions in the dedicated repository on GitHub, and contributed plugins should also be indexed in the Orthanc Book.
  • Internal code refers to source code that only makes sense if embedded within the Orthanc core or within one of the official plugins. This includes new features and bugfixes. The way to contribute to the internal code of the Orthanc ecosystem is described in the sections below.

Important: You should always favor the creation of a new plugin over modifications to the internal code (see below)!

Contributor License Agreement

It is necessary for the Orthanc project to make sure that the internal code of Orthanc can be interfaced with proprietary systems, as those are still unfortunately everywhere in the healthcare market. This forces us to require all the intellectual property over the source code of Orthanc to be centralized, with the University Hospital of Liège together with the Osimis company acting as the official guardians of the whole Orthanc ecosystem. This centralization also enables the dual licensing scheme described above, which in turn allows Osimis to collect money from the industry in order to fund further free and open-source development of the Orthanc ecosystem to the benefit of the worldwide community of medical imaging, according to a virtuous cycle.

As a consequence, before any code can be accepted into the official repositories of Orthanc, the individual code contributors must sign a Contributor License Agreement (CLA). Here is the procedure:

  1. Download the individual CLA (ICLA) form from the Orthanc homepage.
  2. Print the document, then write down your signed initials on pages 1 and 2, and sign page 3.
  3. Return a scanned copy of the document to e-mail
  4. Wait for confirmation from the Osimis company.

Important: This form is only valid for individual contributors acting as physical persons. If your company wishes to become contributor as a juridical person, please request a Corporate CLA at the same e-mail address:

Submitting code

Once the CLA onboarding process has succeeded, use Mercurial to fork the official repository of interest from BitBucket. Here are the location of those repositories:

Once you have finished modifying the code in your forked repository, issue a pull request.

Some words of warning:

  • Please stick to the coding style of Orthanc.
  • It is your responsibility to make sure that you have the intellectual property over all the source code you commit into Orthanc.
  • All the contributions will be carefully reviewed. Some contributions may be modified, yet even rejected. A rejection might for instance occur if your contribution does not match the Orthanc roadmap, does not meet our high-quality code standards, or breaks backward compatibility. Please be sure that we warmly welcome and appreciate your contributions, but be aware of the fact that we are quite strict, and that the review process might take time. This is why the recommended way of contributing to Orthanc is always by creating contributed plugins.
  • Your pull requests should be kept as small as possible, and should be focused on one very specific issue or feature. Large architectural changes are reserved for the core development team of Orthanc, as we must follow our long-term roadmap.
  • Unit testing is mandatory. Integration tests should be submitted to the dedicated repository.
  • In the case of a doubt wrt. a potential contribution, please discuss it on the Orthanc Users discussion group before starting the actual development.