Imaging of Prostate Cancer for Radiation Treatment Planning & Follow-Up
Nandita deSouza1
1Institute for Cancer Research, London, United Kingdom

Synopsis

Localised prostate cancer is treated with prostatectomy, external beam radiotherapy with or without androgen deprivation therapy (ADT), brachytherapy and in selected cases, active surveillance. Because of its superior soft-tissue contrast, MRI plays an important role not only in staging disease, but in planning radiation-based treatment and in assessing recurrence following treatment.

TARGET AUDIENCE

Radiologists with an interest in prostate cancer

OBJECTIVES

To understand why, when and how to image prostate tumors when planning radiotherapy and for follow-up.

Introduction

Localised prostate cancer is treated with prostatectomy, external beam radiotherapy with or without androgen deprivation therapy (ADT), brachytherapy and in selected cases, active surveillance. External beam radiotherapy is associated with long term disease control in most men and mortality from prostate cancer is very low (1, 2). MRI is used to plan treatment, to aid tumor and/or organ-at-risk (OAR) delineation when delivering radiation to the prostate. However, for image-guidance during treatment delivery, MRI has not yet replaced CT, because it has not been possible until very recently to integrate MR technology with linear accelerators. More recently, the considerable hurdles preventing successful delivery of charged particles in the vicinity of a magnetic field have been overcome so that radiation therapy delivery is enabled under MRI guidance.
Because of its superior soft-tissue contrast, MRI also plays an important role in assessing recurrence following treatment.

MRI for planning External Beam Radiotherapy

CT remains the primary technique in radiotherapy planning and delivery.
The current role of MRI is in aiding delineation of target volumes (prostate, intraprostatic lesions and, occasionally, lymph nodes) (3) as an adjunct to the planning CT to improve the accuracy of their delineation (4). The soft tissue contrast of MRI is superior to CT for localisation of the prostatic apex (5) and visualisation of the bladder interface. Target volumes using an MRI have consistently been demonstrated to be around 30% smaller than CT-derived volumes (6). Additionally, inter-observer variability of the target volumes is lower with MRI (7, 8), whereas with CT alone the largest variation exists around the apex, base and seminal vesicles (9). Smaller target volumes lead to a reduction in treatment toxicity (3,10).
Unfortunately, MRI is not usually acquired in the treatment position, so that the geometric accuracy of the organs varies from that at radiotherapy set-up. MRI can be co-registered with the CT to assist planning, either by matching gold seeds implanted in the prostate (11) or by soft tissue matching of the prostate outline. Alternatively, MRI can be used as a visual aid to guide delineation on the planning CT, without image co-registration.

Brachytherapy

Low dose rate (LDR) permanent seed brachytherapy and high dose rate (HDR) are effective and established treatments for localised prostate cancer (12). Here, transrectal ultrasound is primarily used for defining the target volume, planning and guidance. Routine implementation of MRI in the brachytherapy workflow has been slow.
MR can aid in the placement of brachytherapy sources to enable maximal radiation dose to the peripheral zone while avoiding peri-prostatic toxicity to the rectum and urethra (13, 14). The urethra is defined by the visible catheter which remains in place throughout the procedure. Although, this is an adequate approximation of the urethra during high dose-rate (HDR) brachytherapy where the treatment is delivered with the catheter in situ, it is not with low dose-rate (LDR) brachytherapy as the urethral shape and position change when the catheter is removed, and exposure continues (15). Real time MRI guidance has been developed for the placement of sources but this is resource intensive (16).
The use of MRI-CT image fusion for post-needle implant dosimetry for HDR brachytherapy has been encouraged by the American Brachytherapy Society (ABS) to improve the reproducibility of contouring and the reliability of dosimetry (17). MR can be used to outline and plan biological sub-volumes within the prostate (18). MRI in post-implant LDR assessment is superior to CT in visualisation of the prostate gland; however, the seeds are not easily visualised. MRI-CT fusion improves seed visualisation but is not routinely implemented. MRI guided brachytherapy has been shown to improve visibility and accuracy of dose administration and has the potential to improve outcomes (16).

Use of MRI in identifying recurrence

MR imaging does not play a routine role in surveillance following radiation treatment. Prostate cancer recurrence is usually detected biochemically by a rise in successive PSA readings. MRI, in patients in whom persistent or recurrent tumor is suspected, is used to assess the patient’s suitability for local salvage surgery or stereotactic body radiotherapy. Due to lower T2-W signal intensity post-radiation therapy, diffusion-weighted and particularly dynamic contrast-enhanced sequences are useful for identifying recurrent disease (19, 20). MRI is also useful for assessing local recurrence post prostatectomy. The most common sites for recurrence are the vesicouretral anastomosis (approximately 70%) and the retrovesical region around the seminal vesicle bed (approximately 30%) (21, 22).

Acknowledgements

No acknowledgement found.

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Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)