Image-Guided Radiotherapy-On Center Stage

Abstract

Linear accelerator-based radiotherapy, an actor on the therapeutic anticancer stage for the past 50 years, acts indiscriminately of cancer cell types and cell surface markers. Both cancer and normal cells sustain potentially lethal DNA damage, with normal cells often repairing radiation-induced damage quickly after exposure [2]. Today, local control of cancer may happen through use of radiation treatment portals depositing radiation dose that use narrow margins around a cancer target. This is done all in an effort to spare healthy normal tissues from intolerant radiation dose. To provide tight radiation dose margins requires high mechanical accuracy of linear accelerators and even better accuracy in patient positioning. There is an unmet need for radiotherapy technology that assists in accurate patient positioning prior to radiation delivery. Up-to-now, this need has been met by image-guided radiotherapy. With new celebrity-like buzz surrounding emerging novel radiotherapy platforms, radiation oncologists and medical physicists must screen each radiotherapy platform’s overall technologic portfolio. Critical to a platform’s performance is its linear accelerator treatment isocenter. The treatment isocenter is the ideal isocenter point about which all linear accelerator motions occur. Also, the treatment isocenter references all images used in image-guided radiotherapy. Perhaps most compelling, it is the position that informs algorithms calculating radiation dose prescription and resultant radiation dosimetry. While the treatment isocenter is optimal, in actual radiation delivery, a radiation isocenter serves as the focal point for linear accelerator movements. Rather than being a point in space like the treatment isocenter, the radiation isocenter is thought of as a small sphere in space. The radiation isocenter deviates, on a small scale in all directions, from the treatment isocenter based on positional uncertainties of the mechanical gantry, collimator, and couch rotation [3]. The coincidence of the treatment isocenter and radiation isocenter in clinical practice is limited ideally to ± one millimeter for stereotactic radiosurgery and ± two millimeters for intensity modulated radiation therapy [3]. Novel radiotherapy platforms delivering conventional or ablative (>7 Gy per fraction) radiation by integrated robotics and finetuned microleaf collimators must be held to even greater accuracy in radiation isocenter alignment. Imagine image-guided radiotherapy platforms that allow for intrafraction and interfraction compensation for cancer target movement as a consequence of physiologic change in organ size, shape, and motions due to respiration, heartbeat, swallowing, and differential rectal and bladder fillings.