Scholarly article on topic 'A phantom study to assess accuracy of needle identification in real-time planning of ultrasound-guided high-dose-rate prostate implants'

A phantom study to assess accuracy of needle identification in real-time planning of ultrasound-guided high-dose-rate prostate implants Academic research paper on "Clinical medicine"

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Abstract of research paper on Clinical medicine, author of scientific article — Matthew Schmid, Juanita M. Crook, Deidre Batchelar, Cynthia Araujo, David Petrik, et al.

Abstract Purpose High-dose-rate brachytherapy of the prostate is commonly performed using transrectal ultrasound (US) guidance, with CT imaging used for needle reconstruction and treatment planning. Transrectal ultrasound images can, however, be used for the entire process, allowing treatment without changes in the patient position. This study assesses needle reconstruction accuracy using US images. Methods and Materials Prostate phantoms were implanted with 10–18 needles. Three-dimensional US images were acquired, and needles were reconstructed using specialized software. A CT scan was also obtained. The image sets were registered and needle reconstruction errors were assessed. A dose plan was obtained using the US images and the dwell times were transferred to the CT reconstruction to obtain the true “delivered dose,” which was evaluated using standard dosimetric parameters. Results Two sources of error were identified. First, reconstruction based on the bright echoes in the US images introduces a systematic error because these echoes correspond to the proximal wall of the needle, and not the center of the needle channel. If left uncorrected, this shift can lead to an underestimate of urethral doses. Second, incorrect needle tip identification can occur in the cranial–caudal direction. Errors up to 5.8mm were observed. A measurement of needle lengths protruding beyond the template can be used to compensate for this. Conclusions Factors limiting the accuracy of US-based needle reconstruction have been identified. Once recognized, these errors can be corrected for, resulting in accurate implant geometry. This facilitates a treatment technique combining excellent anatomic definition, minimal prostate motion, and accurate dose planning and delivery.

Academic research paper on topic "A phantom study to assess accuracy of needle identification in real-time planning of ultrasound-guided high-dose-rate prostate implants"

Brachytherapy 12 (2013) 56-64

A phantom study to assess accuracy of needle identification in real-time planning of ultrasound-guided high-dose-rate prostate implants

Matthew Schmid1, Juanita M. Crook2 *, Deidre Batchelar1, Cynthia Araujo1, David Petrik2,

2 2 David Kim , Ross Halperin

1 Department of Radiation Physics, British Columbia Cancer Agency, Center for the Southern Interior, Kelowna, BC, Canada 2Department of Radiation Oncology, British Columbia Cancer Agency, Center for the Southern Interior, Kelowna, BC, Canada

ABSTRACT PURPOSE: High-dose-rate brachytherapy of the prostate is commonly performed using transrectal ultrasound (US) guidance, with CT imaging used for needle reconstruction and treatment planning. Transrectal ultrasound images can, however, be used for the entire process, allowing treatment without changes in the patient position. This study assesses needle reconstruction accuracy using US images.

METHODS AND MATERIALS: Prostate phantoms were implanted with 10-18 needles. Three-dimensional US images were acquired, and needles were reconstructed using specialized software. A CT scan was also obtained. The image sets were registered and needle reconstruction errors were assessed. A dose plan was obtained using the US images and the dwell times were transferred to the CT reconstruction to obtain the true ''delivered dose,'' which was evaluated using standard dosimetric parameters.

RESULTS: Two sources of error were identified. First, reconstruction based on the bright echoes in the US images introduces a systematic error because these echoes correspond to the proximal wall of the needle, and not the center of the needle channel. If left uncorrected, this shift can lead to an underestimate of urethral doses. Second, incorrect needle tip identification can occur in the cranial-caudal direction. Errors up to 5.8 mm were observed. A measurement of needle lengths protruding beyond the template can be used to compensate for this.

CONCLUSIONS: Factors limiting the accuracy of US-based needle reconstruction have been identified. Once recognized, these errors can be corrected for, resulting in accurate implant geometry. This facilitates a treatment technique combining excellent anatomic definition, minimal prostate motion, and accurate dose planning and delivery. © 2013 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.

Keywords: Prostate cancer; Prostate brachytherapy; HDR; US-based planning

Introduction

High-dose-rate brachytherapy (HDR-BT) of the prostate involves the placement of a number of hollow needles into

Received 22 December 2011; received in revised form 13 February 2012; accepted 13 March 2012.

Conflicts of interest: None.

Financial disclosure: This project was funded by internal operating funds from the British Columbia Cancer Agency, with the knowledge and support of Varian Medical Systems.

* Corresponding author. Department of Radiation Physics, British Columbia Cancer Agency, Center for the Southern Interior, 399 Royal Avenue, Kelowna, BC, Canada V1Y5L3. Tel.: +1-250-712-3979; fax: +1-250-712-3911.

E-mail address: jcrook@bccancer.bc.ca (J.M. Crook).

the prostate through which an HDR radioactive source can be introduced using an afterloading device. Before delivery of the treatment, needle placement with respect to the prostate and organs at risk (OARs) must be determined and, based on this, a suitable dose plan must be generated.

Typically, prostate HDR-BT begins with the insertion of needles into the prostate under transrectal ultrasound (TRUS) guidance with the patient in the dorsal lithotomy position. There are advantages to using TRUS for this, most notably that the prostate and urethra are well visualized in ultrasound (US) images making development of appropriate implant geometry relatively straightforward. Additionally, needle placement can be followed in real time during insertion, which allows for adjustment of subsequent

1538-4721/$ - see front matter © 2013 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.10167j.brachy.2012.03.002

needle positions to compensate for any nonideal needle placement.

Following needle implantation, the most common practice is to send the patient for a CT scan. Typically, this requires lowering the patient's legs and transferring the patient onto and off of both a stretcher and a CT scanner table. After acquisition of the CT images, the target and OAR are contoured, the implant geometry is reconstructed, and a dose plan based on the CT images is produced. When the reconstruction and planning are complete, the treatment may be delivered.

CT is known to be geometrically accurate and is an excellent imaging modality for identifying the needle locations. However, the change in position of the patient's legs, the movement of the patient, and the delay between imaging and treatment are all known to produce changes to the needle positions and/or implant geometry (1—8). This is problematic because any such changes will result in differences between the planned dose and the dose that is actually delivered to the prostate and to the adjacent organs. When multiple fractions are delivered based on a single plan, which is often the case with CT-based planning but is not done with the one-step US-based procedure investigated here, the problem of needle migration is of even greater concern.

An alternate approach to prostate HDR-BT is to use TRUS imaging both to guide the implantation of needles and for treatment planning. In this process, implantation of the needles, three-dimensional (3D) imaging, dose planning, and treatment are integrated into a single process that does not require any change in patient position or movement of the patient. This approach solves many problems related to patient and needle motion, but does present other challenges. Although the prostate is generally much better delineated on TRUS compared with CT, TRUS images are not as geometrically accurate, and ultrasonic shadows produced by posterior needles often obscure the exact needle placement of more anterior needles. To realize the potential gains of this approach, the effects of these limitations on needle reconstruction must be understood. Highly accurate treatment plans can only be achieved through accurate reconstruction of the implant geometry.

The purpose of this study is to evaluate the accuracy of the implant reconstructions based on TRUS images using Vitesse software (Varian Medical Systems, Palo Alto, CA).

Methods and materials

Specialized prostate US phantoms (model 053MM; Computerized Imaging Reference Systems Inc., Norfolk, VA) were used for this study. These phantoms incorporate internal structures (prostate, urethra, seminal vesicles, and two nodules) that are clearly visible in both US and CT images. A transverse TRUS image of one of the phantoms and its corresponding CT image are shown in Figs. 1a and 1b, respectively. The central structure is the urethra. The structure on the left side of the images is a simulated nodule. These nodules proved useful in registering the images, but are otherwise not relevant to this study.

Six phantoms were implanted under US guidance using a standard technique for TRUS-based implants. The number of needles implanted in each phantom varied from 10 to 18. In each phantom, the prostate was visualized on TRUS (Flex Focus; B&K Medical Systems, Peabody, MA) at a midgland position, and the needles were implanted using a standard implant template. The needles were first advanced to the midgland position under TRUS guidance in the transverse mode. After all needles had been advanced to this position, the longitudinal transducer was selected and the needles were advanced one at a time to the base of the prostate. The positions of the needle tips in the cranial—caudal direction were tracked in the live image during this process, and their final positions were determined during this step. This last step is always carried out from anterior to posterior so that the needles do not fall into the shadow of more posterior needles as they are advanced. The needles used in this study (Varian Medical Systems) were plastic with a diameter of 2 mm.

After the completion of the implant, 3D US images of the phantoms were acquired using the Vitesse (Varian) software program. This software makes two modes available for 3D reconstruction. In Twister (Varian Medical Systems) mode, the probe is rotated about its long axis as images are acquired using the longitudinal transducer. The rotational

Fig. 1. (a) Midprostate transverse view of the phantom obtained using ultrasound and (b) the corresponding view obtained using CT.

position of the probe is determined by an encoder incorporated into the TRUS probe holder (CIVCO EXII; Civco Medical Solutions, Kalona, IA). A 3D image is then reconstructed from the multiple longitudinal images. A more conventional transverse mode is also available, in which the probe is translated in the cranial/caudal direction as images are acquired using the transverse transducer. In this case, the linear position of the probe is determined by a second encoder on the probe holder. Although image sets were acquired using both of these modes, this work focuses on the results obtained using the conventional linear acquisition.

The 3D images acquired suffer from a number of limitations inherent in US imaging, namely poor delineation of the needles, spatial inaccuracies, and shadowing. To deal with these limitations, special tools incorporated into the Vitesse (Varian) software program are used to reconstruct the needle paths. This is of special relevance because these

tools define exactly how the individual needles are placed with respect to the images.

The Vitesse (Varian) software is designed to facilitate tracking the bright flashes in the TRUS image. This tool works well even when tracking curved needles. When a needle has been tracked properly, the display will show a straight line in the needle path images, labeled ''Path Image 1'' and ''Path Image 2'' as shown in the two bottom right panes of Fig. 2. The software then places the center of the needle along the identified path, which corresponds to the bright flash in the image. This technique was used to define the needle paths in the US images for all phantoms.

After the US imaging was complete, the phantoms were taken to a CT scanner and imaged with high resolution (slice thickness: 0.625 mm). The spatial accuracy and the clearly visible needle channels make accurate needle reconstruction possible. In this study, the CT image set is taken as the gold standard, that is, differences in geometry

Fig. 2. A view of the Vitesse (Varian) screen showing the path images used to track the needle positions in the lower right panel.

between the CT and TRUS data sets are assumed to be inaccuracies in the US data.

The US image set, along with the reconstructed needle paths, were then transferred to a dose calculation program (BrachyVision; Varian Medical Systems).

The prostate, urethra, and a surrogate for the rectum were contoured in the US image set and an optimized dose distribution was produced. Active dwell positions were defined in each needle within a margin around the prostate. The margins used were 7 mm superior, 5 mm inferior, lateral and anterior, and 0 mm posterior. The objectives were to cover the prostate with a dose of 1000 cGy, while limiting the dose to the urethra and the rectum. The urethral constraint was a maximum dose of 1150 cGy. The rectal constraint was that no more than 1 cc should receive a dose higher than 750 cGy.

The CT data set was also imported into BrachyVision and the TRUS image set was then registered to the CT data set based on the anatomic structures in the phantoms. The prostate volume was contoured in the CT data set to aid in this registration and to assess the consistency of the contouring. The comparison between the CT and US prostate volumes is shown in Table 1. The differences between the reconstructed dwell locations in the US data set and the corresponding positions in the CT data set were tabulated. The dwell locations (and corresponding dwell times) in the US plan were then moved to their correct locations as determined in the CT images to produce a representation of the true delivered dose. The results were evaluated using a number of dosimetric parameters, including D90 (the

minimum dose received by 90% of the prostate volume), V100 and V150 (percentage of the prostate volume enclosed by the 100% and 150% isodose), and the doses to the urethra and rectal surrogate.

Results

Images from the CT data set for one of the implants are shown in Fig. 3. Note that the solid plastic tips of the needles are clearly visible and that the air channel inside each needle is very well defined. Reconstruction of the air spaces is what determines the location of the source dwell positions, and it is apparent that the needle reconstruction can be carried out accurately using these images. By way of contrast, Fig. 4 shows the same views in the US image. Although some of the needles are well visualized in the US image, others are not. In this extreme case (which was the worst in the study), the needle highlighted in the transverse view is almost completely obscured in the US image. Although needle shadowing occurs frequently to some extent, the tools incorporated in the Vitesse (Varian) software, and in particular the path images tool, allow accurate needle tracking even in cases where a large part of the track is obscured. This image is taken from a phantom, which was implanted with 16 needles. In general, this problem of ''needle shadowing'' becomes markedly worse as the number of needles in the implant increases.

Figure 5 shows the result of registering the US image to the CT image. It is immediately apparent that the bright

Table 1

The effect of needle reconstruction accuracy on the dose delivered to the prostate and organs at risk as evaluated by standard dosimetric parameters

Phantom

number 1 2 3 4 5 6

Number

of needles 16 14 10 18 12 16 Average Average

US CT US CT US CT US CT US CT US CT US CT difference

Prostate 62.4 61.8 59.6 62.2 61.4 60.3 62.2 63.1 64.1 64.1 63.1 62.2 62.1 62.3 0.2

volume

V100% 92.9 89.4 95.8 94.2 93.5 90.0 96.1 94.2 92.9 91.6 95.8 93.1 94.5 92.1 -2.4

D100% 62.0 62.0 74.3 70.6 67.1 58.3 74.6 74.0 66.9 66.7 72.1 72.7 69.5 67.4 -2.1

D90% 104.3 99.4 106.4 104.4 105.1 100.1 108.8 106.4 105.0 102.5 105.6 102.7 105.9 102.6 -3.3

V150% 26.7 22.9 26.6 22.3 26.4 23.2 29.8 27.8 30.2 28.8 25.3 24.0 27.5 24.8 -2.7

V125% 55.2 47.8 55.1 51.0 52.6 46.9 60.5 57.1 57.1 53.3 55.9 51.8 56.1 51.3 -4.8

V200% 7.1 6.1 8.4 6.0 9.6 8.4 8.1 7.4 11.1 10.3 7.6 6.6 8.7 7.5 -1.2

Rectum max 80.2 70.1 75.3 71.7 78.9 70.8 80.0 74.6 80.5 75.5 77.1 74.4 78.7 72.9 -5.8

Rectum D1cc 66.4 60.1 68.0 64.4 69.4 64.3 69.5 64.7 67.8 63.1 68.1 64.2 68.2 63.5 - 4.7

Rectum V70% 3.1 0.0 3.7 0.3 5.6 0.1 5.8 1.1 3.7 0.8 4.1 1.0 4.3 0.6 -3.7

Urethra max 112.8 116.9 112.9 116.7 114.9 113.3 116.5 121.7 122.3 132.3 115.8 117.0 115.9 119.7 3.8

Urethra D1cc 106.5 103.0 106.9 106.1 109.1 107.0 110.7 109.7 110.7 109.2 108.4 107.8 108.7 107.1 -1.6

Urethra V115% 0.0 0.1 0.0 0.1 0.0 0.0 0.3 0.3 0.3 1.0 0.2 0.9 0.1 0.4 0.3

US = ultrasound; V100, (V125, V150, V200) = % of the prostate volume receiving 100%, (125%, 150%, 200%) of the prescription dose or greater; D90 (D100) = isodose as a % of the prescription dose, enclosing 90% (100%) of the prostate volume; rectum max = maximum dose received by the rectum as a % of the prescription dose; rectum D1cc = Dose to 1 cc of the rectal wall as % of prescription dose; rectum V70% = volume of the rectal wall in cubic centimeter receiving a minimum of 70% of prescription dose; urethra max = maximum dose received by the urethra as a % of the prescription dose; urethra D1cc = Dose to 1 cc of the urethra as % of prescription dose; urethra V115% = volume of the urethra in cubic centimeter receiving a minimum of 70% of prescription dose.

Fig. 3. Orthogonal views of an implanted phantom obtained using CT. The air spaces in the needles are very well defined. All needles are clearly visible.

flashes in the US images do not correspond to the centers of the needles, but rather to the wall of the needle proximal to the US transducer. Because the Vitesse (Varian) software is designed to track the bright flashes, there will be an obvious systematic error in the reconstruction of the implant. If the relationship between the US flash and the needle location as described above is understood, the needle locations can be adjusted accurately in the transverse views.

The exact location of each needle tip in the cranial—caudal direction must also be determined if the needle position is to be accurately reconstructed. For needles that are well visualized in the US image, this is not a problem. For needles that are obscured, however, it can be very difficult.

Figure 6 shows the distribution of the displacements (millimeter) of the first dwell positions in the US images from their correct positions as determined from the CT images for all the needles in all six phantoms. These displacements were calculated in a cylindrical coordinate system. The radial component is measured radially outward from the probe, the angular component represents a rotation in the transverse plane, and the third component is in the cranial/caudal direction. The systematic error caused by defining the needle paths along the flash in the US images is again readily apparent. This is evidenced by the fact that the displacement distribution for the radial direction is not centered about zero. Naively, one would

Fig. 4. The ultrasound view showing the same orthogonal planes shown in Fig. 3. The highlighted needle is completely obscured by some of the posterior needles.

expect the displacement to be approximately equal to the radius of the needles (in our case 1.0 mm). In fact, the average error in this direction was 1.0 mm. The errors in the angular component are distributed relatively evenly about zero, as are the errors in the cranial—caudal direction.

These measured displacements are based solely on the Vitesse (Varian) reconstructions of the needle paths. For cases where a needle falls in the shadow of a lower needle, the path reconstruction can be very unreliable. Because the needles all curve to some extent, it is unlikely that one needle will be obscured along its entire length. This usually allows for a reasonably accurate reconstruction of its radial and angular position, at least at a number of points along its

length. If however, the needle is obscured at its tip, there is no other information available in the image that can be used to position its tip properly in the cranial—caudal direction. This can result in relatively large errors in this direction, which accounts for the outliers in the histogram.

Ultimately, the importance of needle reconstruction accuracy lies in the effect on the dose delivered to the target and the OARs. A number of dosimetric parameters were used to evaluate this and these are summarized in Table 1.

The target doses in the US-based plan generally show only small differences relative to those determined based on the CT needle reconstruction. The doses to the OARs, however, showed some larger changes. These can be attributed almost entirely to the systematic error in the radial

Fig. 5. A transverse view of the registered CT and ultrasound (US) images. Note that the centers of the needles lie above the bright flashes in the US image.

direction. In the optimized dose distributions, the isodose line corresponding to the maximum allowed urethral dose generally conforms very closely to the urethral structure. These dose distributions were, however, determined based on incorrect needle positions. When the distributions are transferred to the CT-determined needle positions, which represent the dose that would be delivered, the distributions are shifted, moving the high-dose region into the urethra. This is illustrated in Fig. 7, where Fig. 7a shows the dose planned on the basis of the US images, and Fig. 7b shows the dose that would be delivered based on the CT images. The largest change in the urethral maximum dose was an increase of 10%, with the average change being 3.8% of the prescribed dose.

Displacement Histograms

n ■ ■

„ n ■ _l nln _

-«■5 -4 -3 -2 -1 0 1 2 3 4 5 6 Displacement (mm)

■ r «Thêta □ Cranial/Caudal

Fig. 6. Histograms of the measured displacements between the ultrasound-reconstructed needle tip positions and the CT-reconstructed positions in the radial, angular, and cranial—caudal directions. Note that the distribution of the radial displacements is not centered on zero.

The changes in the doses to the rectum are negative in all cases, meaning the rectal dose is lower than the dose predicted by the US reconstruction. In this case, correcting for the systematic error in the radial direction moves the dose cloud away from the rectum.

Discussion

Until the recent introduction of TRUS-based planning for prostate HDR-BT, the major drawback of this modality has been the need for a multistep procedure involving:

1. TRUS-guided needle insertion under anesthesia in the dorsal lithotomy position

2. Transportation of the patient to a CT scanner for CT imaging in the supine position and CT-based planning

3. Transportation of the patient back to the treatment delivery suite

4. Reverification of needle position and adjustment of the cranial—caudal depth of insertion

The multistep nature of CT-planned prostate HDR-BT prolongs the process; limits the number of cases that can be done in a day; adds discomfort and inconvenience for the patient; and, most importantly, introduces an unacceptable source of error owing to needle retraction in the caudal direction away from the base of the prostate. Mean displacements have been reported of 3—11 mm with a range up to 28 mm (1—3,6—8). It is felt that any displacement greater than 3 mm should be corrected (3). Inaccuracies are inherent in the readjustment of the depth of insertion several hours postimplantation with the patient awake (1,3—6). TRUS-based planning allows both the procedure and treatment to be performed in a single location and under anesthesia, eliminating both the risk of needle displacement during patient transfer, and associated patient discomfort while being transferred and repositioned with the needles in place. However, to rely on TRUS-based planning, one must be confident that the TRUS-identified needle positions represent the actual 3D coordinates of that needle relative to the prostate and adjacent normal structures.

Implant reconstruction involves identification of the needle tips and the needle path accurately. "Tip location'' positions the needle in the cranial—caudal direction and thus determines the location of the source dwell positions relative to the anatomy in the treatment plan. Others have studied the accuracy of needle tip identification (9) and have found the locations of needle tips in general to be accurate; however, their study was idealized in that they were identifying individual needle tips inserted one at a time into a water bath. In practice, the challenge is to identify needle locations in a geometric arrangement of multiple needles. Needle tip location in this phantom study was determined to have median difference of 0.5 mm (range, —5.8—3.4 mm) compared with the CT-based tip location.

Fig. 7. (a) The optimized dose distribution based on the ultrasound reconstruction. The yellow line represents the 115% isodose line. (b) The corrected dose distribution based on the CT reconstruction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The median difference is reassuringly small and most differences were less than 2.0 mm (Fig. 6); however, the magnitude of the outlying discrepancies is clearly unacceptable. Although misidentifying the tip of a small number of needles per implant did not have as large a negative impact on overall dosimetry as the systematic shift in needle channel positions did, this error will result in local dosi-metric changes that may be important if the planned dose cloud surrounding the needle is actually closer to OAR structures or farther from target structures.

There are a number of strategies that can be used to mitigate this problem. As difficulty in identifying needle tip location is increased when the needle under consideration falls in the shadow of a more posterior needle, one possibility is to track and identify the tips of the more anterior needles first. This can be accomplished by observing the

tips using longitudinal US images as the needle is advanced to its final position. The Vitesse (Varian) software has tools that aid in doing this and allow one to lock down the tip position of each needle as it is identified. Care must be exercised, however, to ensure that the needles do not move once the tip has been identified.

Measuring the lengths of the needles that protrude from the implant template can provide a check that the needles have not moved and ameliorate difficult needle tip identifications. Knowing this length, it would be possible, using knowledge of the physical location of the TRUS transducer with respect to some external mark on the probe, to determine exactly where the needle tip was with respect to the plane of the TRUS transducer. In practice, however, it is more practical to use the measured lengths of the protruding needles to determine the tip locations relative

Table 2

Comparison of standard dosimetric parameters from CT-based reconstruction and US-based reconstruction after application of systematic correction of 1 mm in radial direction

Phantom

number 1 2 3 4 5 6

Number

of needles 16 14 10 18 12 16 Average Average difference

US CT US CT US CT US CT US CT US CT US CT

Prostate 62.4 61.8 59.6 62.2 61.4 60.3 62.2 63.1 64.1 64.1 63.1 62.2 62.1 62.3 0.2

volume

^100% 92.0 89.4 94.0 94.2 91.8 90.0 94.1 94.2 91.3 91.6 93.3 93.2 92.8 92.1 -0.7

D100% 62.5 62.0 74.7 70.6 65.3 58.3 76.2 74.0 66.8 66.7 71.0 72.7 69.4 67.4 -2.0

D90% 102.6 99.4 104.2 104.4 102.8 100.1 106.1 106.4 101.9 102.5 103.4 103.9 103.5 102.8 -0.7

V150% 23.9 22.9 23.1 22.3 24.1 23.2 26.9 27.8 28.0 28.8 22.8 24.2 24.8 24.9 0.1

V125% 51.2 47.8 50.6 51.0 48.1 46.9 55.4 57.1 52.6 53.3 51.1 52.6 51.5 51.5 0.0

V200% 6.1 6.1 6.3 6.0 9.2 8.4 7.3 7.4 10.1 10.3 5.1 6.5 7.4 7.5 0.1

Rectum max 71.7 70.1 69.7 71.7 71.5 70.8 72.7 74.6 73.3 75.5 70.7 73.9 71.6 72.8 1.2

Rectum D1cc 61.7 60.1 63.0 64.4 64.3 64.3 64.2 64.7 62.8 63.1 63.0 63.5 63.2 63.4 0.2

Rectum V70% 0.2 0.0 0.0 0.3 0.1 0.1 0.7 1.1 0.7 0.8 0.0 0.5 0.3 0.5 0.2

Urethra max 113.3 116.9 113.1 116.7 112.4 113.3 118.6 121.7 132.0 132.3 116.7 118.5 117.7 119.9 2.2

Urethra D1cc 104.9 103.0 105.5 106.1 106.7 107.0 108.4 109.7 108.1 109.2 107.2 108.3 106.8 107.2 0.4

Urethra V115% 0.1 0.1 0.0 0.1 0.0 0.0 0.2 0.3 0.7 1.0 0.4 1.2 0.2 0.5 0.3

US = ultrasound; V100, (V125, V150, V200) = % of the prostate volume receiving 100%, (125%, 150%, 200%) of the prescription dose or greater; D90 (D100) = isodose as a % of the prescription dose, enclosing 90% (100%) of the prostate volume; rectum max = maximum dose received by the rectum as a % of the prescription dose; rectum D1cc = Dose to 1 cc of the rectal wall as % of prescription dose; rectum V70% = volume of the rectal wall in cubic centimeter receiving a minimum of 70% of prescription dose; urethra max = maximum dose received by the urethra as a % of the prescription dose; urethra D1cc = Dose to 1 cc of the urethra as % of prescription dose; urethra V115% = volume of the urethra in cubic centimeter receiving a minimum of 70% of prescription dose.

to one of the lower needles that are well visualized in the image. This technique was applied to the needles in the phantom study, and this reduced the maximum error in the cranial—caudal direction from 5.8 to 1.9 mm. Using both strategies in combination to improve needle tip identification provides a robust needle tip identification and quality assurance process.

Regarding needle path reconstruction, the registration of the TRUS images with CT has revealed that the dominant discrepancy when using the Vitesse (Varian) software is a systematic error in determining the radial position of the needle. This results in the needle channel being reconstructed 1.0 mm closer to the probe than its actual location as determined by CT imaging. Because this was a consistent phenomenon, prior knowledge of this discrepancy between TRUS- and CT-based needle reconstruction allows one to make a straightforward systematic correction to compensate for it. Table 2 shows the changes in dosimetric parameters between the US-based reconstruction with a systematic correction of 1.0 mm applied in the radial direction and the CT-based reconstruction. Making the correction in the radial direction significantly reduces the discrepancies between the two data sets. After correction, the largest residual error was in the maximum urethral dose, which is the parameter most sensitive to needle positioning. The greatest increase in the maximum urethral dose was reduced to 3.7% and the average difference was reduced to 2.2% (of prescription dose). The differences in the rectal doses between the corrected US data and the CT data were very small.

Conclusions

One-step TRUS-based planning represents a significant advance in the delivery of prostate HDR-BT, making the procedure more efficient in resource utilization as well as more convenient and comfortable for the patient. This approach also increases dose delivery accuracy as the lack of patient repositioning between implantation and treatment delivery removes the threat of needle migration. The improved accuracy of dose delivery of a one-step TRUS-based procedure brings the ultimate goal of dose escalation to dominant intraprostatic nodules closer to reality (10—12).

Achievement of these advantages does, however, depend on accurate reconstruction of the implant geometry. This study demonstrates two potential sources of error in needle path reconstruction: uncertainty in the identification of

needle tips owing to US artifacts and a systematic shift in the reconstructed position of the needle channels owing to the way in which the Vitesse (Varian) software is used to track needle paths. Knowledge of these errors has, however, allowed us to develop strategies to minimize, in the case of needle tip misidentification, or eliminate, in the case of the systematic shift in needle positions, their impact on overall implant quality.

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