4

For brachytherapy, there are now 10- to 15-years of published data that support the excellent treatment outcomes that are at least comparable to those of RP or EBRT (Table 4.1) (5−9). Some of its advantages can include a more rapid postoperative recovery time when compared to RP and a shortened overall treatment time when compared to EBRT. Brachytherapy also has the advantage of being a much less expensive form of radiation with a cost to Medicare of $17,076 per patient as compared to $31,574 for intensity modulated radiation therapy (IMRT) (10).

Relatively recent important advances in imaging and procedural technologies have also enabled prostate brachytherapy to evolve further into a minimally invasive, state-of-the-art, computerized planning software-aided treatment approach. Since its inception as a freehand open laparotomy procedure in the 1960s at Memorial Sloan-Kettering Cancer Center (MSKCC), permanent interstitial prostate brachytherapy has benefited from the development of transrectal ultrasonography guidance approach, template-guided closed transperineal technique, and sophisticated commercialized planning software technology for CT-based and MRI-assisted postoperative dosimetry.

Table 4.1  Data supporting the treatment outcomes

           Absolute and relative contraindications: Current ABS guidelines list as absolute contraindications limited life expectancy, unacceptable operative risks, distant metastases, absence of rectum precluding trans rectal-ultrasound (TRUS) guidance, large trans-urethral resection of the prostate (TRUS) defects precluding good dosimetry, and ataxia-telangectasia (13).

           Obese patients: Obesity is known to be an adverse prognostic factor after both RP (possibly due to greater difficulty visualizing the operative field [33,34]), and after EBRT (possibly due to greater organ motion leading to geographic misses, or daily changes in position of an abdominal pannus changing the shape of the dose distribution [35,36]). However, after brachytherapy, obesity does not appear to be prognostic, perhaps because direct implantation of the seeds into the prostate under ultrasound guidance eliminates the problem of geographic miss (37,38). Therefore, brachytherapy may be the best choice of therapies for obese patients when choosing among surgery, EBRT, and brachytherapy.

Impotence

ULTRASOUND-GUIDED PERMANENT SEED IMPLANTATION OF THE PROSTATE

If a patient meets appropriate criteria for an LDR prostate implant, the size of the prostate and the geometry of the prostate in relationship to the pubic arch will need to be determined. In our practice of intraoperatively planned implants, prostate volume studies are accomplished in one of two ways, either with a TRUS or by performing a CT scan of the prostate and pubic arch in a modified dorsal lithotomy position. Both are accurate in determining the number of seeds needed for intraoperatively planned implants as well as for determining the potential for pubic arch interference at the time of implant. Nomograms exist to estimate the number of seeds needed for an implant based on isotope activity and prostate volume. Centers are encouraged to construct their own nomograms with information from their actual implant techniques to determine the activity and number of seeds needed for their particular implant style (Figure 4.1) (66).

When preplanning techniques are utilized for the creation of the seed and needle positions prior to the day of the implant, a TRUS volume study is performed and serves multiple purposes. In addition to determining the size and shape of the prostate gland, the geometry of the prostate to the pubic arch is also assessed (67). The ultrasound images are then imported into the treatment planning system to generate a plan with appropriate needle and isotope placement within the prostate. With this approach careful measurements are recorded to help ensure that the patient will have a reproducible position at the time of implant. When intraoperative planning techniques are utilized for implant, consideration to the treatment planning system is necessary. Commercially available systems can differ between how images are acquired for volume study, either transverse or sagittal. On most ultrasound probes the sagittal crystal is proximal to the transverse crystal, which has implications of the needed depth of insertion to visualize the prostate, seminal vesicles, and bladder.

Figure 4.1 Nomogram of implanted activity (mCi) versus implant volume (cc) based on ­departmental data for ¹²⁵I.

Volume Study

Both preplanned and intraoperative planning will require an assessment of the prostate in relationship to the pubic arch and to organs at risk (OAR) including the urethra, bladder, and rectum. When a TRUS volume study is performed outside of the operating room (OR), the patient is asked to have a comfortably full bladder and is placed in the dorsal lithotomy (treatment) position. Depending on the commercial treatment system being utilized, images are acquired either transversely or from a sagittal reconstruction. Described first is a technique utilized by transverse-based planning systems. The rectal ultrasound mount is secured to the examination table or the floor and the rectal ultrasound transducer is secured onto the stepper (Figure 4.2). The angle of the mount and transducer is recorded. A cover is secured over the transducer and the apparatus is then lubricated with ultrasound jelly and gently inserted into the rectum. The best visualization of the prostate comes when there is no air interface between the ultrasound transducer and the rectal wall. Once the ultrasound transducer is in position, the bladder, prostate, seminal vesicles, and rectum can be identified. The technique developed at the Seattle Prostate Institute (67) establishes the base of the prostate first, which can be found at the proximal end of the prostate interface with the bladder. This interface can be best confirmed on a sagittal view (Figure 4.3). Once the base of the prostate is identified, the prostate is contoured in 5 mm increments from the base to the apex. The physician can then define a margin around the prostate as a target to be implanted, typically 3 to 5 mm in all directions except posterior where no margin is planned. Once the prostate gland has been defined, the number of contours should be compared with the length of the prostate measured on the sagittal views to ensure consistency. The volume of the gland should also be recorded. To determine pubic arch interference, a tracing of the pubic arch (Figure 4.4) is superimposed over each defined slice of target volume. If there is no pubic arch interference and the prostate volume is within the defined limits, the contours and images can be transferred to a treatment-planning system for prostate brachytherapy. If the prostate gland is large or there is arch interference secondary to gland size, neo-adjuvant androgen suppression can be used to shrink the gland, with a rule of thumb of 3 to 4 months of a luteinizing hormone releasing hormone (LHRH) agonist resulting in appropriately a one-third reduction in prostate size (68,69). However, the risk of urinary symptoms that are associated with a large gland do not necessarily decrease when the gland size is decreased with androgens (70). If the clinician is concerned about the subjective urinary symptoms reported by a patient, then formalized urinary dynamics and post void residual volumes should be considered.

Figure 4.2 Typical transrectal ultrasound (TRUS) volume study setup.

In our current practice of interactive intraoperative planning utilizing sagittal image guidance of needle positioning and placement, which is described in detail subsequently, volume studies are performed with either TRUS or with CT scan performed in the modified lithotomy position. When utilizing a TRUS sagittal volume study the technique is similar to transverse volume study as described earlier but the process is automated. Instead of stepping the probe out of the rectum from base to apex as described previously, the probe is inserted into the rectum allowing sagittal viewing of the prostate, the entire seminal vesicle, and the proximal prostate–bladder interface. The probe is then automatically rotated in a 140º sweep scanning the entire prostate. The prostate is reconstructed in the transverse, coronal, and sagittal planes for treatment planning. A CT-based technique can also be used to delineate the prostate volume in a modified dorsal lithotomy position (Figure 4.5), which also allows for three-dimensional (3D) contouring of the prostate and pubic arch. A 3D reconstruction of the prostate and the pubic arch is easily accomplished in most external beam treatment planning systems in commercial use today. Geometric assessment of the prostate to potential pubic arch interference is performed (Figure 4.6A without pubic arch interference and Figure 4.6B with pubic arch interference). When CT scans are used for our volume studies, a TRUS-based volume study is performed intraoperatively for treatment planning.

Figure 4.6 (A) Geometric assessment of the prostate to pubic arch using CT scan and 3D rendering with external beam planning system (eclipse). Images demonstrate no prostate–pubic arch interference. (B) Geometric assessment of the prostate to pubic arch using CT scan and 3D rendering with external beam planning system (eclipse). Images demonstrate prostate–pubic arch interference. 3D, three dimensional.

¹²⁵I and ¹⁰³Pd are the most commonly used isotopes in LDR brachytherapy for prostate cancer and both have demonstrated excellent long-term clinical outcomes in terms of efficacy and safety. Cesium-131 (¹³¹Cs), which was introduced in 2004, is a more recent option for LDR implant. Historically, ¹²⁵I was the first isotope used with modern TRUS-guided implants, although all isotopes are capable of providing excellent dosimetry. Each isotope requires a thorough understanding of the characteristics of the isotope and how planning and final dosimetry may be influenced by implant style. Currently ¹²⁵I is the most frequently implanted isotope. Randomized trials have not found differences in biochemical-free survival between ¹²⁵I and ¹⁰³Pd (71). There are several centers that use ¹⁰³Pd and ¹³¹Cs effectively and some brachytherapists use different isotopes based on clinical situations. It has been suggested that ¹⁰³Pd is more effective against dedifferentiated tumors than ¹²⁵I (72). Through extrapolation, a similar assertion could be made for ¹³¹Cs; however, no differences in clinical outcomes for patients with prostate cancer treated with either ¹⁰³Pd or ¹²⁵I have been found (73,74). The ABS (13) does not recommend one specific isotope over another, although the longest toxicity data exist for both ¹²⁵I and ¹⁰³Pd. The dosimetric properties of these three isotopes are compared in Table 4.2.

Table 4.2  ¹²⁵I, ¹⁰³Pd, and ¹³¹Cs properties

Isotopes are available as either loose seeds or stranded seeds (Figure 4.7). Commercial systems exist to link seeds together using real time data created from intraoperative planning, as we currently use in our practice (Figure 4.8). Loose seeds are associated with a greater likelihood of seed migration (75,76). However, with very few exceptions seed migration in most cases is not clinically worrisome (77−80). The incidence of seed migration is decreased with stranded seeds as compared with that of loose seeds (risk ratio [RR]: 3.08 vs 6.97), with the greatest difference being in migration to the lung and perineum (81). Dosimetrically, the two appear to be very similar. It is also possible to perform implants simultaneously utilizing both loose and stranded seeds with excellent dosimetry and clinical outcomes. Interpretation of the literature requires care, as implant technique can influence dosimetry as much as implanting loose or stranded seeds. A prospective trial demonstrated that although 15% of strands can shift 5 mm or more within the gland over 4 weeks, the impact on dosimetry was negligible (82). Knowledge of the anatomy of the prostate and surrounding tissues helps to minimize seed migration. Migration pathways of seeds are typically associated with the deep dorsal vein and neurovascular bundle. The venous drainage of the prostate starts with the deep dorsal vein. The vein has three major branches, the superficial branch and the right and left branch. The superficial branch lies on top of the prostate and bladder neck and drains into the dorsal venous complex. Care should be taken to delineate the prostate from the venous structures so as not to place seeds in this area that have the potential to migrate. Furthermore, the neurovascular bundle typically lies at 5 and 7 o’clock positions and is another potential pathway for seed migration. Duplex ultrasound allows the clinician to identify blood flow and thus allow the identification of these structures when not grossly apparent.

Activity

The average activity of ¹²⁵I seed used in prostate cancer is 0.41 mCi (range: 0.16–1 mCi) and 1.32 mCi (range: 0.50–1.90 mCi) for ¹⁰³Pd (83). Historically, the Seattle group used low-activity ¹²⁵I seeds (0.35 mCi) to improve dose homogeneity within the prostate as well as decrease the dose to the rectum and urethra (84,85). A dose study performed by D’Souza et al compared dose homogeneity among different seed strengths (0.35, 0.44, and 0.66 mCi). They found the 0.44 mCi-activity seed to have the best dose distribution (86). Narayana et al (87) performed a randomized trial between high-activity ¹²⁵I seeds (0.60 mci) and low-activity ¹²⁵I seeds (0.31 mCi) and found excellent dosimetry with both high and low activities. The implant technique is likely just as important as the activity of seed implanted. With the use of higher activity seeds, fewer seeds are required to cover the target volume, especially in peripheral loading techniques. The placement of each seed becomes more important where deviations in planned placement from actual placement have the potential to increase toxicity to normal structures or to under dose the target (88). This has become apparent in our clinical practice. When using intraoperative planning, it is possible to devise plans with intended coverage dependent on one or two well-placed seeds, which is magnified with increasing isotope activity. The overall number of seeds has a direct impact on the number of needles used during the procedure. Eapen et al (42) have reported that needle trauma to the prostate contributes to acute urinary toxicity. One could argue that the placement of more needles well on the first pass may in theory be less traumatic than placing fewer needles multiple times. Therefore, we try to take great care to ensure that the needle is on the correct trajectory before puncturing the prostate.

The ABS recommends prescription doses of 145 Gy and 125 Gy for ¹²⁵I and ¹⁰³Pd, respectively, in monotherapeutic brachytherapy. In the setting of utilizing seeds as a boost in conjunction with 41.4 to 50.4 Gy of EBRT, the recommended prescription doses are 108 to 110 Gy with ¹²⁵I and 90 to 100 Gy with ¹⁰³Pd (13). Typically, these doses are prescribed to the minimum peripheral dose (MPD; Figure 4.9). The MPD is considered to be the maximum dose that covers 100% of the target volume. This dose is dependent on the position of the seeds within the prostate and the dose may vary by up to 25%. Despite this, usually 90% of the target volume will receive the prescription dose (89). The mean peripheral dose is the average of the dose at the surface of the target volume and varies less with seed position. Some centers will therefore prescribe the mean peripheral dose.

Many variants of loading techniques exist in treatment planning for transperineal implantation of the prostate. Over the years, techniques have evolved to provide the prostate with curative doses of radiation while decreasing the radiation dose to surrounding normal tissues of the urethra and rectum. One of the earliest techniques, uniform loading, utilized low-energy seeds placed at fixed distances from each other throughout the entire prostate (Figure 4.10). This technique gave a high dose of radiation to the length of the prostatic urethra, as the urethra was not intentionally spared. To spare the urethra while maintaining coverage to the target, degrees of peripheral loading were developed including modified uniform loading, nonuniform loading, and peripheral loading. These techniques shared the principle of decreasing the number of seeds centrally, which had the greatest impact on urethral doses. In modified uniform loading, two thirds of the planned seeds occupy the posterior border of the prostate and the base. Nonuniform loading avoids loading seeds in the central aspect of needles inserted close to the urethra, and peripheral loading (Figure 4.11) uses higher-activity seeds with the posterior implant plane 5 mm anterior to the posterior border of the prostate (90). Attention to the OAR, as well as dose to the prostate, have become more important as greater attention is focused on decreasing the toxicities of implants. Breakthroughs in computing power have brought inverse planning algorithms to the OR where doses to the OAR and prostate are inversely planned with commercially available treatment planning systems. As such techniques exist to carve out the V125 isodose line to the urethra with asymmetrical loading patterns and inverse planning (Figure 4.12).

Figure 4.11 Peripheral loading of an ¹²⁵I implant.

Figure 4.12 Inversely planned intraoperative ¹²⁵I implant. Notice V125 isodose line sparring the urethra.

Our intraoperative inversely planned technique are described in detail as follows. This approach allows for a greater freedom of needle and isotope placement within the gland during an intraoperative implant. The brachytherapist is able to modify the dose constraints to the target and OAR, which build on principles of peripheral implants, but with greater abilities for dose shaping of the target volume and collateral tissues. Also strict limits to where seeds can be placed and doses to the OAR can be easily incorporated. In this way, the freedom of inverse planning can be utilized while still maintaining rules previously discussed with peripheral loading techniques.

Under spinal or general anesthesia, the patient is positioned in the lithotomy position utilized at the time of the initial volume study to ensure the same couch angle and ultrasound probe angle. Care is needed to accurately duplicate the position of the prostate for which the treatment plan was designed. A Foley catheter or a gel–air mixture–filled catheter is then placed within the urethra. With newer generation ultrasounds, a Foley catheter may prove sufficient to identify the urethra without the need for a gel–air mixture. The applicator template is secured to the TRUS apparatus. Figure 4.13 depicts the classical manually loaded technique. The image plane is confirmed by scaling through the length of the prostate with the ultrasound stepper to ensure that the images correspond to those taken at the time of the volume study. The reference plane 0.0 (base of the prostate) is located (67). The base location becomes the zero plane on the ultrasound stepper. The physician is guided by a preprinted legend of the needle location, plane, and number of seeds per needle (92). Preloaded needles are kept within a shielded vault (Figure 4.14) until the physician is ready to insert the individual needle. Each hollow bore needle has a sharp beveled edge and a central stylet (Figure 4.15). When the needles are preloaded, the seeds sit within the hollow needle with a bone wax plug at the tip preventing them from dislodging. The central stylet is posterior to the seeds and can be secured in position with a gasket. When removing the needles from the vault, it is important to hold the needle horizontally with the bevel up to avoid dislodging the seeds. Following the preplan legend, the needle is inserted into the correct template location and gentle pressure will advance the needle through the perineal skin and into the prostate. The ultrasound images in both the axial and sagittal views guide the placement of the needle. Rotation of the needle, if it is in the correct plane of view, shows the beveled edge, sometimes known as the hamburger sign (Figure 4.16). To “drop” the seeds within the prostate, the central stylet is held securely in place and the outer needle is slowly pulled back along the stylet. If the stylet is advanced into the needle, it will push the seeds out of the needle and clump them within the prostate. As such it is important to “sow the seeds” as the quality of the implant will be dependent on depositing the seeds as the needle is retracted rather than pushing the seeds into the prostate. Holding the stylet firmly without motion and retracting the needle to the stylet will result in the proper deposition of the seeds within the prostate. Figure 4.17 shows an example of the intended linear alignment and spacing of seeds.

Some brachytherapists use stabilizing needles to minimize prostate motion during the implant (Figure 4.18) (93,94). Whether stabilizing needles are used or not, careful attention to the location of the base of the prostate is necessary throughout the entire implant to ensure that the isotopes are deposited as they were planned. Several techniques can be found in the literature. Of interest is the Seattle technique of a two-stage needle system to minimize prostate movement, improve needle loading, and decrease surgical time. This technique involves the insertion of sleeves to the base of the prostate, allowing needles with spacers equivalent to the retraction plane at the tip end to be inserted after confirmation of sleeve placement has been made (95). Several preplanning techniques exist and the right one is the one the brachytherpaist feels comfortable with and has spent time learning to execute well.

A plain film is routinely taken on completion of the implant to evaluate symmetry and compare seed placement to the preplan. Seeds that have migrated to the perineum will also be visible. Bladder irrigation and cystoscopy can be performed to evacuate migrated seeds to the bladder as well as eliminate potential blood clots in the bladder. Cystoscopy has essentially been eliminated in our intraoperative technique, as ­high-quality ultrasound in combination with sagittal image guided seed placement, makes violation of the bladder unlikely.

In the OR, an ultrasound system with a biplanar axial/sagittal transducer is used to acquire the ultrasound images of the prostate, seminal vesicle, dorsal venous plexus, urethra, rectum, bladder, bladder neck, neurovascular bundle, and external urinary sphincter. A Foley catheter is placed to facilitate localizing the urethra as it courses through the prostate. The probe is mounted to a stepper (Figure 4.19), which is rotated along its axis for longitudinal image acquisition through the Oncentra Prostate™ (Nucletron, an Elekta company [Elekta AB, Stockholm, Sweden]) treatment planning software-controlled motor. During image acquisition, the software captures live images every 0.5º as the motor rotates the probes along a 140º sweep of the prostate and OAR. These sagittally acquired images are the input for a 3D image reconstruction, which provides axial, sagittal, and coronal planes of the acquired volume (Figure 4.20). An 18G template is mounted on the stepper for needle insertion, previously calibrated to the digital template in the planning software for geometrical integrity. Axial images are reconstructed from the 140º sweep at 1.0 mm increments. At a minimum, the prostate, urethra, and rectum are contoured on these axial images while checking the sagittal and coronal images for 3D accuracy.

Figure 4.19 Ultrasound probe attached to motor used for sagittal image acquisition for 3D ­reconstruction of the prostate and OAR. OAR, organs at risk.

Implant Techniques

In our institutions, we favor two interactive inversely planned approaches, which differ in whether isotopes are placed loosely or if they are custom linked in the OR. The first approach has been termed sagittally annealed vector evaluation (SAVE). In this technique, inversely planned simulated annealing (IPSA) is utilized to devise an intraoperative plan allowing for custom linked seed trains to be created in the OR and placed under image guidance with the needle vector overlaid on the live sagittal ultrasound image for precise placement of the linked seed train. This technique allows the brachytherapist the opportunity to link seeds together when extracapsular and seminal vesicle implants are desired. Our second approach is termed high dose rate emulated low dose rate prostate (HELP) brachytherapy (96), as needles are first placed and then the actual needle position is optimized utilizing IPSA to find the best isotope distribution within the already placed needle, much like a catheter when HDR implants are provided. This technique can be widely used during the implant of loose seeds, although the technique is often utilized in settings of prior radiation, TURPs, potential pubic arch interference, and times when mitigating factors make the knowledge of actual needle placement prior to isotope optimization beneficial.

In the traditional intraoperative planning approach, after the contours are completed, a treatment plan is generated in two steps. First, an inverse plan based on dose criteria to the contoured structures (prostate, urethra, and rectum) is generated using the simulated annealing optimization engine available in the planning software, which optimizes the needle and seed locations. The plan is then manually modified by the physicist and radiation oncologist to fully satisfy the required dose distribution goals (Figure 4.21). The metrics used for planning and evaluation are D90 > 140 Gy, V100% > 90%, V150% < 70%, and V200% < 30% for the prostate. The urethra goals are V150% = 0 cc and V125% < 1 cc. The rectal goal is V100% < 1 cc.

One additional degree of freedom for the planning process is offered by the use of linked seeds: the seeds and spacers are connected to each other during the assembly process and provide a semirigid linear structure (Figure 4.7). This enables all or part of the seed–spacer assembly outside of the prostate itself with reduced risk of seed migration. The option to place seeds more peripherally can permit better urethral sparing during the planning process (Figure 4.12).

After the plan is approved by the radiation oncologist, the seed–spacer trains are assembled by the physicist using the QUICKLINK (Bard Inc., Covington, GA) and inserted into needles, which are in turn housed in a shielded enclosure (Figures 4.8 and 4.14). Needles are inserted one by one under ultrasound guidance using the sagittal view. The software, under the direction of the physicist, helps guide the physician’s needle insertion by providing an outline of the planned needle location overlaid on the live ultrasound image (Figure 4.22). Once a needle is placed, seeds and spacers are manually deposited into the prostate and the needle is removed. The actual delivered needle location is updated in the treatment planning software after insertion. This sequential process of needle insertion and seed–spacer delivery is repeated for each needle until all planned needles are delivered. A new ultrasound image is then acquired on which the contours, seeds, and dose are overlaid. After verifying that the contours represent planned anatomy and the seeds are consistent with planned needle tracts, the dose is carefully reviewed to assess if the planning goals have been met. If there are areas of undercoverage, additional seeds can be inserted to fully achieve the dosimetric goals. Figure 4.23 shows the layout and working areas of the brachytherapy team within the OR.

In comparison, the HELP technique (96) differs in a few critical steps from the technique described earlier. After the radiation oncologist completes the contours, an ideal empty needle distribution is generated by the physician and physicist, which is similar to what might be done in an HDR procedure. The main criteria for the distribution are peripheral locations, adequate spacing, and enough potential seed locations for an optimum dose distribution. The needles are inserted by the radiation oncologist under sagittal ultrasound guidance and with the outline of the intended needle location overlaid on the live image. The actual needle position is not necessarily required to agree with the exact intended location. Rather, the software is simply updated to reflect the actual position of the empty needle (Figure 4.24). During this process the empty needle plan can be modified to account for the actual needle position and correct any imbalances in positioning. A second ultrasound scan is acquired with all needles inserted, on which the original prostate, urethra, and rectum contours are overlaid and modified by the radiation oncologist as needed. Just as in the standard technique, the treatment plan is generated in two steps; an inverse plan is generated using the simulated annealing IPSA engine and then modified manually. The only difference between the two techniques is that the seed distribution is optimized based on the actual needle locations already within the prostate (Figure 4.25). This treatment plan, with updated contours and needle position, provides the basis for the dosimetry on the day of treatment “d0 dosimetry” for this technique. After the radiation oncologist approves the treatment plan, the automated seed–spacer delivery takes place. A seed afterloader (SeedSelectron^TM, Nucletron, an Elekta company, Elekta AB, Stockholm, Sweden) is used to build the seed–spacer sequence specific to each planned needle (Figure 4.26). The seeds and spacers, housed in cartridges, are placed inside the needle and pushed to the planned location by a drive wire. The afterloader also retracts the needle through a retraction mechanism independent of the drive wire, leaving the seeds and spacers (which are not linked to each other) inside the patient. A detailed description of the SeedSelectron is published elsewhere (97). This sequential process of needle insertion and seed–spacer delivery is repeated for each needle until all planned needles are delivered. In programs without a SeedSelectron the use of a Mick applicator (Mick Radionuclear Instruments, New York, NY) to deposit loose seeds can also be considered.

Dosimetric Constraints

Posttreatment planning is used to confirm the dose delivered to the prostate as well as evaluate the dosimetry to the OAR. Postplan dosimetry can be performed immediately after the procedure, 1 day postoperatively, or up to 1 month from the date of implant. The ABS recommends that all implants be evaluated with CT-based dosimetry. The feasibility of ultrasound and MRI-based assessment is also being explored. The optimal time for postimplant dosimetry is isotope dependent based on the influence of half-life and expected postimplant edema (98). For ¹²⁵I and ¹⁰³Pd, assessment is typically performed at 30 ± 7 days and 16 ± 4 days, respectively. For ¹²⁵I, many brachytherapist wait for 4 weeks to allow for maximal resolution of inflammation and edema associated with the implant. However, it can be done earlier, with the realization that the final dosimetry would likely improve at a later date when swelling resolves. Postimplant dosimetry with ultrasound is difficult because the artifact from the seeds causes image degradation; however, the ultrasound should be included in conjunction with the CT scan for dosimetric assessment of the implant. The ultrasound not only confirms what was the target at the time of the implant, it also helps to verify the length of the prostate to help define the apex of the prostate, which at times can be difficult on CT. MRI can be used as an excellent way to define prostatic anatomy; however, void artifacts from the seeds (Figure 4.27) make seed identification and subsequent dosimetry difficult. Newer MRI protocols allow for greater seed suppression, although voids are still apparent. CT scans allow visualization of the permanent seeds; however, the prostate anatomy is difficult to define due to seed artifact (99) (Figure 4.28). Image fusion between CT scan and MRI used to evaluate the prostate can also be applied for dosimetry (100) and evaluation of prostatic swelling (101). In our practice we often pair a CT scan and MRI scan to allow for precise isotope location on the CT scan and clear prostate anatomy on the MRI scan (Figure 4.29). Data from the ultrasound at the time of implant help to further refine the clinical assessment of the implant. Catheterization at the time of CT allows for better visualization of the urethra.

Dosimetric data depend on the contours drawn to represent the postimplant prostate and OAR. To evaluate coverage of the target, the dose to 90% of prostate (D90) is calculated. Implants with a D90 > 90% of the prescribed dose or alternatively 140 Gy for ¹²⁵I (102) have been shown to have better PSA relapse-free outcomes (103). Interobserver variation in contouring the prostate based on CT images varies greatly; therefore, it is important to have a standard methodology for contouring the prostate. With CT-based dosimetry, the temptation to circle seeds can be great. To help minimize this tendency, utilizing data from the TRUS volume study and/or an MRI scan can greatly improve the quality of implant assessment. The ABS recommends the reporting of target D90, D80, D100, V80, V90, V100, V150, and V200, as well as rectal and urethral doses (13).

Figure 4.27 Seed void artifacts seen on MRI scan.

LDR CASES

---

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related posts:

Gynecologic Brachytherapy Technical Aspects of Brachytherapy Soft Tissue Sarcoma Brachytherapy Gastrointestinal Brachytherapy Head and Neck Brachytherapy Thoracic Brachytherapy

Stay updated, free articles. Join our Telegram channel

Join