1. Figure 3 Target volume definitions
Figure 3 | Target volume definitions. a | Anteroposterior image of the prostate and seminal vesicles (left). The principal organs at risk include the urethra, bladder, and rectum. The gross tumour volume (GTV; red) is the gross demonstrable extent and location of the malignant growth (right). The clinical target volume (CTV; light blue) encompasses the GTV as well as areas at risk for subclinical cancer involvement. The planning target volume (PTV; purple) encompasses the CTV plus an additional margin to account for patient movement, set‑up error, and organ movement. b | For prostate cancer treated with external beam radiotherapy (EBRT), the PTV is typically CTV + 0.5–1.0 cm101,102. The PTV expansion is necessary because the prostate can move owing to nearby organ changes, such as rectal distention, and because the patient might be positioned differently on the treatment table. Movements can be translational, rotational, and deformational. Note that the CTV (blue volume) stays inside of the PTV (purple volume). EBRT (right; red) covers the PTV by passing through the soft tissues of the pelvis. c | For brachytherapy, the expansion from a CTV to make a PTV can typically be minimized because the set‑up error is almost nonexistent10,11. In the case of low-dose-rate brachytherapy (LDR‑BT), once the needles are deposited, the dose cloud (green cloud) would cover the CTV, even if the patient or their organs were to move. In the case of high-dose-rate brachytherapy (HDR‑BT), the needles (typically ~12) anchor the prostate while the 192Ir source moves to the dwell positions, shown as circles inside the needles, for prespecified dwell times to deliver the prescribed dose (blue cloud).
Zaorsky, N. G. et al. (2017) The evolution of brachytherapy for prostate cancer
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