Comparing CyberKnife & Radiation
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Radiation therapy has been used for decades to treat prostate cancer with varying degrees of success. The goal of any radiation therapy for prostate cancer is to deliver the appropriate dose of radiation needed to treat the cancer, while protecting the surrounding tissues and structures, and minimizing patient discomfort and side effects.
Today, CyberKnife Robotic-assisted Radiosurgery - a robotic radiation therapy that sounds like surgery, but isn't - is offering new hope for patients with localized prostate cancer. It also can be used to treat prostate cancer that has recurred after a radical prostatectomy or previous treatment. CyberKnife has been used to treat prostate cancer for about six years. Studies are showing CyberKnife treatment produces exceptionally good PSA responses with a low incidence of side effects.
External Beam Radiotherapy (EBRT)
External Beam Radiotherapy (EBRT) has been used for decades to treat localized prostate cancer. EBRT delivers five to seven beams of radiation from a source outside the body. The accuracy and orientation of the beams of radiation are limited by the equipment.
To compensate for patient and prostate movement and the inaccuracy of the EBRT equipment, the physician must target the prostate and a rim of surrounding normal tissue. This means portions of the nearby rectum and bladder receive a full dose of radiation. To prevent injury to these tissues, radiation is delivered in many small, daily doses over a period of eight to nine weeks.
Older EBRT techniques delivered 65-70 Gy (units of radiation), which is now considered inadequate. Modern advancements, such as intensity-modulated radiotherapy (IMRT) and image-guidance radiotherapy (IGRT) have made it possible to increase external beam radiation doses. While these higher doses improved cancer control rates, they remained inferior to the cure rates with brachytherapy.
This graph shows the proportion of patients free from a recurrence of prostate cancer in the years following various doses of external beam radiotherapy. If an EBRT treatment is curative, the downward trend should level off and become a horizontal line (which corresponds with the cure rate).
As shown, prostate cancer treated with a higher dose (80 Gy or greater) takes longer to recur, but there is no clear leveling off. This suggests even the highest doses of external beam radiotherapy may be insufficient to cure prostate cancer. (Data from Fox-Chase Cancer Center. Nadir+2 definition for biochemical failure used. Adapted from Eade IJROBP 68(3), 682-89.)
Low-Dose Rate Brachytherapy (seed implant)
Brachytherapy is a type of radiation therapy for prostate cancer. Unlike external beam radiation therapy that targets the cancer from a radiation source outside the body, with brachytherapy radiation is delivered from a source inside the body.
This is a side view of a LDR implant procedure. An ultrasound probe placed into the rectum guides needles into the prostate, where radioactive seeds” are deposited.
Over the next several weeks, the "seeds" slowly deliver radiation to the cancer. Eventually only non-radioactive "seeds" remain in the body.
This X-ray shows the radioactive "seeds" throughout the prostate.
A higher dose of radiation can be delivered with seed implants than with external beam radiation. Cancer control rates are therefore higher with seed implants than with external beam radiation. Patients who have brachytherapy have cure rates similar to radical prostatectomy while avoiding major surgery.
Results have shown that with a sufficient dose of radiation, seed implants can cure prostate cancer. Patients who have brachytherapy have cure rates similar to radical prostatectomy while avoiding major surgery.
Advantages of LDR brachytherapy include:
- Treatment often requires only a single outpatient procedure
- Cure rates superior to external beam, and similar to radical prostatectomy
Disadvantages of LDR brachytherapy include:
- Required anesthesia
- Discomfort in the area where the needles were inserted for a few days after the procedure
- Temporary urinary discomfort and/or urgency following the procedure
- Urinary frequency and reduced stream, which may be permanent
- Urinary obstruction, which might require a catheter to drain the bladder
- Impotence rates five years after treatment are similar to radical prostatectomy
- Patients are radioactive for several weeks to months
High-Dose Rate Brachytherapy
High-dose rate brachytherapy is another method of giving internal radiation to the prostate.
As with LDR implants, the patient is placed under general anesthesia or a spinal block that numbs the lower part of the body. An ultrasound probe is placed in the rectum, which guides the placement of 15-20 needles through the perineum (floor of the pelvis) into the prostate. The needles are then replaced with catheters which are held in place by a template that is sutured to the perineum.
After the catheters are in place, a CT scan determines the prostate location. A customized radiation plan is then developed for each patient. By adjusting the time the radiation source is programmed to be at each position in the catheters, the physician can compensate for possible errors in catheter positioning. The customized plan maximizes the radiation dose delivered to the prostate, while minimizing the dose to surrounding organs.
A shielded after-loading device moves the high-intensity radiation source through the catheters into the prostate according to the plan. Radiation is then delivered to pre-programmed positions in the prostate in four to six large doses. This usually requires two separate implant procedures.
In most cancers, giving many small doses of radiation over a longer period of time helps avoid damage to normal tissues. In prostate cancer, however, prolonging treatment doesn't appear to offer any advantage over high doses of radiation delivered in a shorter period of time. Studies have shown that HDR brachytherapy alone (without additional external beam radiation therapy) has a five-year cancer relapse rate that is equal to or better than low-dose rate (LDR) brachytherapy (seed implant). Additionally, there are fewer acute and late side effects than with LDR brachytherapy. One study shows potency is also better preserved with HDR implants.
Advantages of HDR brachytherapy over LDR brachytherapy (seed implant) include:
- Fewer side effects after the catheters are removed
- Possible lower incidence of impotence
- Cure rates equal to or better than LDR implants
Disadvantages of HDR brachutherapy include:
- Patient must lie on his back in the hospital for one to two days with catheters in the prostate and a Foley catheter in the bladder
- Pain and discomfort while catheters are in place and also when they are removed without anesthesia