4D Image Verification

Diplomarbeit in Computer Science

submitted

by

Christian Schaller

Started: 15.07.2006
Finished: 10.01.2007

Abstract

Far reaching developments and technical advances took place within the field of radiotherapy in the last years. Radiotherapy within the chest and abdomen area is especially important in the field of radiotherapy. Within this regions, organ and tumor positions are significantly affected by patient respiration. The tumor motion, caused due to respiration is compensated by extending the treated area. This extension covers all possible positions of the tumor and therefore also includes healthy tissue.

Several clinical studies provide evidence of a survival advantage for higher dose levels. To spare a maximum of healthy tissue physicians use ’gated radiotherapy’. Common recent approaches for gated radiotherapy are based on the observation of a surrogate. This either can be an implanted fiducial marker or an external signal, which is trying to capture the patients’ respiration.

Within this thesis principles and methods of ’gated radiotherapy’ are described. Additionally an overview of recent patents and products related to radiotherapy are presented and advantages and disadvantages of both common approaches are discussed. This discussion leads to a new developed method, which is introduced. The method joins advantages of both known methods but disregards their disadvantages. The developed algorithm is using image guided methods and methods of medical image processing. A mapping between a 4D-CT planning volume and a most recent acquired fluoroscopic sequence of the same patient is calculated before treatment. Using this mapping and an external breathing signal the physician can define gating intervals and treat the patient in certain breathing phases.

The developed algorithm is included in an existing prototype developed by Siemens Corporate Research (SCR) in Princeton, NJ, USA. Using this prototype, the application of the method is shown. Furthermore another prototype to acquire respiration synchronized fluoroscopic sequences is developed. Both applications are introduced within this thesis.

Contents

1 Introduction ... 1

2 RelatedWork and Patents ...  5

3 Managing Respiratory Motion in Radiation Therapy  ... 9
3.1 Introduction ...  9
3.2 Treatment Planning  ... 12
3.3 Motion-encompassing methods  ...  13
3.3.1 Slow CT scanning ... 14
3.3.2 Inhalation and exhalation breath-hold CT  ... 14
3.3.3 Four-dimensional CT/respiration-correlated CT  ... 14
3.4 Respiratory Gating Methods and Procedures ...  15
3.4.1 Internal Gating ...  15
3.4.2 External Gating  ...  17
3.5 Clinical Procedure ...  18

4 Image Sequence Synchronization  ... 23
4.1 Introduction  ... 23
4.2 Proposed Method  ... 24
4.3 Similarity Matrix  ...  26
4.4 Preprocessing  ...  28
4.4.1 Gain Removal  ...  28
4.4.2 Wild Card  ... 30
4.4.3 Transition matrix  ... 34
4.5 Model-based Dynamic Programming  ... 36

5 Clinical Prototype  ... 43
5.1 Introduction  ...  43
5.2 Clinical Systems  ... 44
5.2.1 SOMATOM Sensation Open  ...  44
5.2.2 ONCOR Linear Accelerator  ...  45
5.2.3 Respiratory Gating System  ... 46
5.3 Clinical Applications  ...  48
5.3.1 AcquireIt  ...  48
5.3.2 RTReg4D  ... 50

6 Results  ... 55
6.1 Introduction  ...  55
6.2 Level I: Synthetic Data ... 57
6.3 Level II: Phantom Data  ... 58

7 Discussion and Future Work  ... 69
7.1 Introduction  ...  69
7.2 Wild Card Detection Improvement  ... 69
7.3 Model Improvement  ...  70
7.4 Cone Beam Acquisition Integration  ... 70
7.5 On-the-fly Expansion  ...  71
7.6 Registration Improvement  ... 72
7.7 Parallelization ... 72

8 Summary  ... 73

List of Figures  ... 77
List of Tables  ... 81
Bibliography  ... 83

Chapter 1

Introduction

Lung cancer is still the most fatal type of cancer, even though the cases of death have been declined in the last years. Regarding to the American Cancer Society (ACS), lung cancer was the reason for almost 29% of all cancer deaths in the United States in 2005. There were 172,570 new estimated cases diagnosted with an estimated 163,510 deaths. Although one can recognize a continuous downward movement of deaths caused by lung cancer in the past 16 years, it is important to improve early detection and treatment continously (Figure 1.1).

Besides radiation therapy lung cancer is also treated with surgery, chemotherapy, and targeted biological therapies, depending on the type and stage of the cancer. Regarding to the ACS, the 1- year relative survival rate for lung cancer was 42% in 2000, however the 5-year relative survival rate for all stages combined was only 15% [Ame05].

Receiving radiation treatment therapy especially in the thorax and abdomen area causes a major health risk for patients. As the tumor is moving due to intrafraction organ motion which is mainly caused by patient respiration, physicians have to treat healthy tissue as well. Facing this movement, physicians are inevitable stuck in their decision about a proper treatment for the patient. On the one hand to be able to deliver x-rays throughout the whole treatment physicians have to increase the treated region by the whole range, where the tumor potentially could be. On the other hand there is clinical evidence of survival advantage for higher dose levels [Oku95], [Per86] [Mac05]. Concerning these two conflictive issues, physicians are biased. Increasing the dose level would accord a better chance of survival, but regarding the extended treatment region, a lot of healthy tissue would be exposed to this high dose, too. Certainly, this is very bad for the patient.

An optimal designed system should provide the radiation oncologist both opportunities. The oncologist still should be able to use a higher dose for treatment, but at the same time minimal

Figure 1.1: Age-Adjusted Cancer Death Rates, US, 1930 - 2002; Source: US Mortality Public Use Data Tapes 1960 - 2002, US mortality Volumes 1930-1959, National Center for Health Statistics, Centers for Disease Control and Prevention, 2004

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healthy tissue should be harmed. Therefore a system should both consider patient respiration and enable a significant decrease of the treated area.

About 15 years ago, physicians came up with the idea of using gated radiotherapy to face this problem [Oha89]. The general idea of gated radiotherapy is to reduce the incidence and severity of normal tissue complications and to increase local control through dose escalation. In order to obtain these issues, a range of values within the treatment beam is turned on has to be specified. Therefore the localization of the tumor has to be known first. This either can be achieved by tracking an implanted fiducial marker or by assuming a correlation between an external surrogate placed on the patient chest for example. Hence, gated radiotherapy is divided into two groups (Figure 1.2):

• internal gating
• external gating

Both methods are based on the observation of a surrogate, where the correlation between an internal surrogate is considered to be more accurate than a correlation between an external surrogate and the real tumor position. Considering these two methods, there are various problems and each method has its own advantages and disadvantages, respectively (Table 1).

Figure 1.2: [1]: Internal Gating, a fiducial marker is implanted in the tumor (green) and used as a surrogate; [2]: External Gating, an external surrogate is used, assuming a correlation between the tumor and the surrogate 

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Internal gating requires additional dose for the patient because image acquisition is necessary to locate the implanted marker properly. This additional dose can be more than what is clinically acceptable for patients with many treatment fractions or a long treatment time of a single fraction. Furthermore internal gating is difficult for thoracic tumors or even not possible because fiducial markers cannot be inserted and there is a risk of pneumothorax.

In opposite external gating is non-invasive but it is quite inaccurate because there often is a bad correlation between the external surrogate and the real tumor position. Consolidating, current state of the art techniques are not satisfying enough in a clinical environment.

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