Particle radiotherapy is a relatively recent development in the field of radiation therapy, whereby protons or carbon ions are accelerated to very high speeds and then fired at a tumor with extreme precision. Siemens produces systems for radiation therapy with both types of particle. "Some types of tumor are resistant to radiation, but the cells in them can still be destroyed using carbon ions," says Rietzel to explain the difference between the two types of therapy. Inside the tumor, the particles cause irreparable damage to the malignant cells. The initial experience in this field was gathered at the Helmholtz Center for Heavy Ion Research (GSI), where Rietzel worked after completing his studies. In the period since 1997, more than 400 patients have been successfully treated with carbon ions. Thanks to a technology transfer agreement, Siemens has greatly profited from this know-how and is now providing the Heidelberg Ion Therapy Center (HIT), which is due to start treating patients in spring 2009, with the necessary radiotherapy equipment. Meanwhile, two further particle radiotherapy centers from Siemens are now being set up in cooperation with the respective German university clinics in Marburg and Kiel.

Rietzel is currently working on a project to optimize the process of calculating and controlling the radiation beam down to the exact millimeter. This involves the use of a grid-scanning technique developed by GSI, whereby the tumor is scanned layer by layer with a fine particle beam, thus enabling the requisite radiation to be dispensed with extreme precision. This ensures that the beam exactly irradiates the tumor without damaging any of the surrounding healthy tissue. "It's difficult to achieve the necessary precision when the location of the tumor moves as a result of the patient breathing, especially in the case of lung cancer," explains Rietzel. He therefore set to work on techniques to eliminate dosage errors in such situations and, together with colleagues from GSI, came up with several solutions. One option here is to interrupt the beam so that the tumor is only irradiated when the patient breathes in or out. "But some patients find this very difficult," he explains. He therefore developed a special technique which repeatedly irradiates the tumor in a quick succession of bursts and thus balances out any resulting errors in dosage. The most precise method, however, is to make the beam follow the tumor as it moves. This involves adjusting the direction of the beam to the actual position of the tumor, more or less in real time. "We've already successfully tested this method," says Rietzel.

Rietzel has also helped to enhance treatment planning, which is the basis of any successful course of radiotherapy for cancer patients. Of great benefit here was his time at the Harvard Medical School in Massachusetts General Hospital, where he spent two years gathering practical experience in the field of radiotherapy before joining Siemens four years ago. In this short period he has been responsible for 61 inventions, 17 of which have resulted in patents.

As a rule, a treatment plan is only drawn up once before the start of therapy. However, a patient's anatomy may change between treatment sessions, thus rendering the original plan imprecise. Rietzel has therefore developed methods to adapt the therapy parameters on the basis of suitable – generally, CT – scanning techniques. The challenge now is to refine these methods and techniques for use with particle radiotherapy. In addition, Rietzel has come up with numerous innovations designed to improve the speed and accuracy of the particle beam. "A key task here is to reduce the duration of exposure so that more patients can be treated," Rietzel explains.