No greater technologic advances have been seen anywhere in medicine than in the fields of diagnostic imaging and radiation therapy. The two have gone hand in hand in defining the tumor and providing better treatment planning and delivery for prostate cancer patients. Technology in both areas has simultaneously progressed, and the authors have beautifully illustrated the potential for innovative interaction.
From the radiation oncologist’s standpoint, when distant disease has been ruled out, treatment choices and applications hinge on defining the tumor volume. Neither computed tomography (CT) nor pelvic magnetic resonance imaging (MRI) has been very helpful in delineating the extent of prostate cancer within the gland or in assessing the possibility of extracapsular extension and seminal vesicle invasion. As the authors note, the addition of the endorectal coil component and fast spin echo imaging has increased the accuracy of MRI in detecting local extension outside the prostate to approximately 80%. Although the negative predictive value of MRI is 80% to 96% for seminal vesicle disease, it is significantly less for extracapsular extension, ranging from 67% to 90%.[1-5] This is important for our urology colleagues, as they try to determine preoperatively whether prostatectomy has a high probability of eradicating all local disease.
As a procedure to outline tumor within the gland, MRI is not highly sensitive (78%) and even less specific (55%), owing to the high false-positive rate. The advantage of combining the sensitivity of MRI with the specificity of magnetic resonance spectroscopic imaging (MRSI) is a total specificity of 91% in detecting cancer located in a certain area of the prostate gland if both are positive. As the authors suggest, this may be very helpful in directing biopsies that have been negative despite a rising prostate-specific antigen (PSA) level. Another advantage for radiation oncologists is the ability to more accurately visualize the dominant or bulky disease locations.
Clinical Advantages of Improved Imaging
The tremendous progress made in developing treatment equipment that can preferentially direct more dose to the site of greatest tumor burden allows us to take advantage of the described imaging techniques. Intensity-modulated radiation therapy enables us to apply an accurate dose gradient differentially, according to the various amounts of tumor within the prostate depicted by the combined MRI/MRSI study. It appears to make sense that more dose to the sites where radiation is most needed may lead to more local cures. The authors seem to have been applying this technique consistently, although outcome studies have not matured.
Data are accruing, however, on the effect of increasing radiation doses to the prostate in general. A randomized study from M. D. Anderson Cancer Center recently reported an advantage for patients receiving a 78-Gy dose vs a 70-Gy dose. Other institutions likewise have reported promising results with dose escalation.[7,8] To date, it appears that intermediate-risk patients (T2, Gleason score £ 7, PSA 10 to 20 ng/mL) may benefit most; MRI/MRSI would likely delineate these prostatic lesions for treatment planning and differential dose delivery.
Follow-up after irradiation can be problematic, since the first sign of recurrence is usually an elevated PSA without the appearance of a local or distant clinical site. PSA levels can fluctuate, producing a sawtooth curve instead of a steady, gradual rise. There is also the PSA "bounce" phenomenona one-time rise, followed by a subsequent decline to baseline. Any adverse changes in PSA generally cause great alarm to patients, of course.