Image guidance in radiation therapy for better cure of cancer

Abstract

The key goal and main challenge of radiation therapy is the elimination of tumors without any concurring damages of the surrounding healthy tissues and organs. Radiation doses required to achieve sufficient cancer-cell kill exceed in most clinical situations the dose that can be tolerated by the healthy tissues, especially when large parts of the affected organ are irradiated. High-precision radiation oncology aims at optimizing tumor coverage, while sparing normal tissues. Medical imaging during the preparation phase, as well as in the treatment room for localization of the tumor and directing the beam, referred to as image-guided radiotherapy (IGRT), is the cornerstone of precision radiation oncology. Sophisticated high-resolution real-time IGRT using X-rays, computer tomography, magnetic resonance imaging, or ultrasound, enables delivery of high radiation doses to tumors without significant damage of healthy organs. IGRT is the most convincing success story of radiation oncology over the last decades, and it remains a major driving force of innovation, contributing to the development of personalized oncology, for example, through the use of real-time imaging biomarkers for individualized dose delivery.

Keywords: MR-linac; adaptive radiotherapy; brachytherapy; cone-beam CT; image guidance; molecular imaging; radiation; stereotactic radiotherapy.

Fig. 1

(A) Initial CBCT prototype by Jaffray et al. (B) Modern integrated CBCT system. (C) Software system for image reconstruction and analysis illustrating a workflow‐based design.

Fig. 2

ViewRay system consists of 0.35T split magnet MRI system (A), with a rotating gantry housing a 6 MV linac (B), equipped with 138‐leaf double‐focused double‐stacked multileaf collimator (C).

Fig. 3

(A) Schematic design and (B) Elekta Unity system.

Fig. 4

PSMA‐PET‐based focal boosting in prostate cancer. (A) Axial PSMA‐PET‐CT slice showing the contours of the prostate (red), GTV (cyan), rectum (brown), and the 50 Gy isodose (5 fractions of 10 Gy; marine blue). (B) Corresponding CT slice with color wash isodose curve showing conformal dose shaping to the prostate (clinical tumor volume) treated to 35 Gy in five fractions of 7 Gy and intraprostatic tumor (GTV) with sparing of the rectum and urethra. The intraprostatic lesion (cT1c, Gleason 3 + 4 = 7, iPSA = 16.6 ng·mL⁻¹) is located in the left transition zone. The patient participated into the multicenter prospective phase II hypo‐FLAME study (NCT02853110, ClinicalTrials.gov).

Fig. 5

Development of image guidance in SBRT. (A) External stereotactic coordinates of the stereotactic body frame, (B) in‐room CT, (C) integrated CBCT, (D) integrated MRI imaging.

Fig. 6

Median volumes and mean doses (D90 for adaptive CTV‐T_HR, D98 for adaptive CTV‐T_IR and GTV‐T_res) for cervix cancer (unpublished data from the EMBRACE I and II studies). In EMBRACE II, the median volume of GTV‐T_init is 55 cm³ (time of diagnosis). The extent of the GTV‐T_init is reflected in the adaptive CTV‐T_IR (time of brachytherapy), and this region received a median near‐minimum dose of 62 Gy in EMBRACE I. In good‐responding tumors, the dose at the border of the GTV‐T_init is 60–70 Gy, while in poor‐responding tumors, this region may receive doses similar to the adaptive CTV‐T_HR (e.g., around 80 Gy). Figure is modified from [88] Fig. 6.

Fig. 7

(A) Schematic representation of different putative adaptation measures, subsequent adaptation measures, and resulting therapeutic consequences. (B) Illustration of a ART workflow: Before the start of treatment, an optimal treatment plan is generated. During treatment, systematic feedback measurements about tumor response are taken into account in order to adapt RT in terms of field size, target, or radiation dose level to yield most optimal therapeutic results.