Mohamathu Rafic K1,2and B Paul Ravindran1
1Christian Medical College, Vellore, Tamil Nadu, India
2The Tamil Nadu Dr MGR Medical University, Chennai, Tamil Nadu, India.
ABSTRACT
Clinical studies have emphasized the significance of image guided adaptive radiotherapy (IGART) to enhance the therapeutic ratio. In general, acquisition of repeat CT (re-CT) is the key requisite to implement IGART in the routine clinical setting. Several attempts were made to address the practical concerns and challenges associated with re-CT based IGART, majority of them employed daily cone beam CT (CBCT) image informations either directly or indirectly for customization of treatment plans within the planned course of treatment. However, there are no standard and comprehensive guidelines dealing with effective utilization of this in-room imaging for IGART. In most of clinical scenarios, longitudinal field-of-view is found to be clinically inadequate for localizing the large and complex targets (with regional nodes) encountered in high precision radiotherapy. More specifically, there is no protocol in the literature for localization of the entire craniospinal axis and adaptive plan evaluation methods with CBCT. In addition, inaccuracies in Hounsfield units (HU) in CBCT image restrict its potential use in IGART. Therefore, a need arises to formulate comprehensive IGART approaches and guidelines with in-room CBCT to reduce the uncertainties related to treatment geometry and patient deformation.
The research presented in this thesis describes the potential use of the CBCT-based IGART in several aspects, including specific planning guidelines with custom-developed software, coded in MatLab. The protocol acquires two or more CBCT images with a linear translation of treatment couch in the patient plane, allowing 1 cm penumbral overlap (i.e. cone beam abutment) and fused as a single DICOM data set (CBCTeLFOV) for extended localization. In order to improve the dose calculation accuracy with CBCT, optimum HU correction (in the high density regions) and HU mapping (from deformed initial CT) methods were also coded in our software. Possible misalignment arising out of uncertainties in treatment setup, table co-ordinates, and minor difference in reconstruction diameter are effectively managed using rigid registration and mutual intensity metric based misalignment management algorithm. New quality assurance approaches are described for comprehensive validation of CBCTeLFOV image sets with combined geometry of Catphan 504 and 604 phantoms. Few case studies are used to illustrate the CBCTeLFOV-based IGART workflow in terms of dosimetric and clinical perspective. Moreover, dynamic (rotational) dosimetry specific to CBCT imaging and new practical beam quality index for kV cone beams are also described.
CBCT images yielded incorrect HU values, especially in the regions of high density under high scatter conditions. The dosimetric difference between conventional CT and CBCT was found to be more if CBCT images are directly utilized for dose calculation. CBCT image sets acquired and processed according our protocol found to provide accurate HU values and could therefore be used for dose calculation, including for extended localization of large target volumes. Dosimetric evaluation of the protocol demonstrated that the dual- and multi-scan CBCT fusion and HU modification strategies are reliable and more practical. The comparable dosimetric outcome observed between conventional CT and CBCTeLFOV validates the use of the latter as a viable alternative for IGART. The standardization of online imaging protocols and incorporation of CBCT imaging dose in the therapeutic plan would facilitate frequent CBCTeLFOV guided adaptive radiotherapy while circumventing the need for re-planning CT.