Advances in conformal radiotherapy: using Monte Carlo Code to design new IMRT and IORT accelerators and interpret CT numbers

The introductory chapter of this monograph, which follows this Preface, provides an overview of radiotherapy and treatment planning. The main chapters that follow describe in detail three significant aspects of radiotherapy on which the author has focused her research efforts. Chapter 2 presents studies the author worked on at the German National Cancer Institute (DKFZ) in Heidelberg. These studies applied the Monte Carlo technique to investigate the feasibility of performing Intensity Modulated Radiotherapy (IMRT) by scanning with a narrow photon beam. This approach represents an alternative to techniques that generate beam modulation by absorption, such as MLC, individually-manufactured compensators, and special tomotherapy modulators. The technical realization of this concept required investigation of the influence of various design parameters on the final small photon beam. The photon beam to be scanned should have a diameter of approximately 5 mm at Source Surface Distance (SSD) distance, and the penumbra should be as small as possible. We proposed a draft for this system based on the PRIMUS 6MV DKFZ accelerator and investigated new geometry of the source-target-collimator system. We assessed the influence of different collimator parameters, different target construction and various incident electron beam characteristics. Based on this work, it was possible to define adequate parameters for the target-collimator system and the scanning electron beam for new a IMRT system. Examples of the intensity modulated field produced by the resulting photon beam are shown. In Chapter 3, attention is turned to recent and ongoing work on a new mobile electron accelerator for Intraoperative Radiotherapy (IORT) at the Polish National Centre for Nuclear Research (NCBJ) in Świerk. Based on Monte Carlo calculations, we have designed, verified and optimized an electron beam forming system for IORT that uses two different docking systems for applicators. When developed, the accelerator will deliver electron beams in an energy range of 4 – 12 MeV. It will use thin-walled metal applicators with diameters ranging from 3 – 12 cm, possibly larger at lower energies, which can be attached to a universal therapeutic head. The treatment head uses a fixed system of collimators and scattering foils that is independent of beam energy and applicator diameter. Dose distribution in the patient plane, inside and outside operating room, meets all regulatory requirements for radiation protection. A prototype will now be constructed and tested in the laboratory. Chapter 4 describes work to improve treatment planning for hadron therapy with protons or heavy ions, which also took place at DKFZ in Heidelberg. Treatment planning is a complex process and the need to develop new strategies to reduce uncertainties in such planning remains an ongoing challenge to physicists. To calculate ion range in tissue, medical physicists who prepare treatment plans apply an empirical correlation between measured Carbon ranges and X-ray computed tomography (CT) numbers, the value of which depends on the parameters used during measurement. We undertook a systematic study of the effect of various measurement parameters on CT-numbers. Monte Carlo simulations were used to model a complete CT machine and phantom with tissue substitute and receive projections. These results were then processed using a reconstruction algorithm that converted them to CT-numbers calculated in Hounsfield units (HU). We also systematically investigated deviations in CT-numbers that result from different voltage settings of the X-ray tube, composition of substitutes, and changes in the diameter and material of the phantom that is used for CT measurements. Subsequently we translated these into range uncertainties using CT data from an actual patient with a chondrosarcoma at the base of the skull. The studies described in this monograph all required teamwork. The IMRT study in Chapter 2 was performed in the laboratory of Prof Gunther Hartmann at DKFZ, who is an extremely creative medical physicist with three decades of experience in conformal radiotherapy, as well as a wonderful teacher, role model and friend. The author worked (and continues to work) on the IORT study described in Chapter 3 with a fine team of NCBJ colleagues in the Division of Accelerator Physics & Technology, including Dr Eugeniusz Pławski, Dr Przemek Adrich, Dr Adam Wasilewski, and our Division head, Dr Sławomir Wronka. Special recognition also goes to Prof Grrzegorz Wrochna, the Director NCBJ, for diverting his eyes from the cosmos for a time, taking note of the significance of radiotherapy accelerators here on Earth and greatly encouraging our work in IORT. We have also been advised by two recognized international experts in IORT, our friends Dr Peter Biggs of the Massachusetts General Hospital and Dr Frank Hensley of the University Clinic in Heidelberg. The study in Chapter 4 was performed together with Dr Sima Qamhiyeh, a very ambitious young physicist who received her PhD at Heidelberg. Our chief was Dr Oliver Jäkel, head of Medical Physics in the Heidelberg Ion-beam Therapy (HIT) Center, father of numerous new ventures in hadrontherapy, as well as the father of three fantastic children. This work would not have happened at all had I not been guided into radiotherapy by a number of fine mentors, including Prof Barbara Gwiazdowska, Prof Jerzy Tołwiński, Prof Gerhard Kraft, Prof Wolfgang Schlegel and Prof Jean Chavoudra, and helped along the way by many colleagues from Oncological Centers across Europe, all of whom believe, as I do, that radiotherapy is a challenging field that offers great opportunities for physicists. Nor would this monograph have been possible without the ongoing support of Prof Stanisław Kuliński and Prof Ryszard Romaniuk, who convinced me to write it and kept encouraging me to complete it. Finally, I want to thank my husband and editor, Dr Kenneth Rabin, for his understanding of the nuances of science writing in English, his frank advice, consistent support, and almost endless patience.