Localization of the prostatic apex for radiation therapy using implanted markers

3D-Conformal Radiation Therapy in Prostate Cancer. Technical Considerations after 5 Years of Experience and 334 Patients Treated at the Istituto Europeo Di Oncologia of Milan, Italy

Aims and Background

To report the technique of 3D-conformal radiation therapy (3D-CRT) currently used at our Institute for the treatment of prostate cancer with a curative intent. A critical review of the technical aspects of the technique is provided.

Methods and Study Design

Between December 1995 and October 2000, 334 patients with biopsy-proven adenocarcinoma of the prostate were treated with 3D-CRT. All patients were treated in a prone position with 15 MV X-ray beams and a 6-field technique for all but 20 patients, who were treated with a 3-field technique. Patients were simulated with the rectum and bladder empty. To ensure reproducible positioning, custom-made polyurethane foam or thermoplastic casts were produced for each patient. Subsequently, consecutive CT scan slices were obtained. The clinical target volume and critical organs (rectum and bladder) were identified on each CT slice. The beam's eye view technique was used to spatially display these structures, and the treatment portals were manually shaped based on the images obtained. The beam apertures were initially realized by conventional Cerrobend blocks (48 patients), which were replaced in October 1997 by a computer-driven multi-leaf collimator. The total target dose prescribed at the ICRU point is 76 Gy, delivered in 38 fractions and 54 days. The seminal vesicles are excluded at 70 Gy. Dose-volume histograms were obtained for all patients. If more than 30% of the bladder and/or more than 20% of the rectum receive >95% of the prescribed total dose, the treatment plan is judged as unsatisfactory and is adjusted. The dose-volume histogram can be improved by changing the beam's arrangement and/or weights or by introducing or modifying the wedge filters.

Conclusions

3D-CRT in prostate cancer patients is a highly sophisticated and time-consuming method of dose delivery. Important technical issues remain to be clarified.

Urethrogram-Directed Stereotactic Body Radiation Therapy for Clinically Localized Prostate Cancer in Patients with Contraindications to Magnetic Resonance Imaging

Purpose

Magnetic resonance imaging (MRI)-directed stereotactic body radiation therapy (SBRT) has been established as a safe and effective treatment for prostate cancer. For patients with contraindications to MRI, CT-urethrogram is an alternative imaging approach to identify the location of the prostatic apex to guide treatment. This study sought to evaluate the safety of urethrogram-directed SBRT for prostate cancer.

Methods

Between February 2009 and January 2014, 31 men with clinically localized prostate cancer were treated definitively with urethrogram-directed SBRT with or without supplemental intensity-modulated radiation therapy (IMRT) at Georgetown University Hospital. SBRT was delivered either as a primary treatment of 35–36.25 Gy in five fractions or as a boost of 19.5 Gy in three fractions followed by supplemental conventionally fractionated IMRT (45–50.4 Gy). Toxicities were recorded and scored using the Common Terminology Criteria for Adverse Events version 4.0 (CTCAE v.4.0).

Results

The median patient age was 70 years with a median prostate volume of 38 cc. The median follow-up was 3.7 years. The patients were elderly (Median age = 70), and comorbidities were common (Carlson comorbidity index ≥2 in 36%). Seventy-one percent of patients utilized alpha agonists prior to treatment, and 9.7% had prior procedures for benign prostatic hyperplasia. The 3-year actuarial incidence rates of ≥Grade 3 GU toxicity and ≥Grade 2 GI toxicity were 3.2 and 9.7%, respectively, and there were no Grade 4 or 5 toxicities.

Conclusion

Magnetic resonance imaging is the preferred imaging modality to guide prostate SBRT treatment. However, urethrogram-directed SBRT is a safe alternative for the treatment of patients with prostate cancer who are unable to undergo MRI.

Future directions from past experience: a century of prostate radiotherapy

Prostate cancer is the most commonly diagnosed noncutaneous malignancy in men, yet 100 years ago it was considered a rare disease. Over the past century, radiation therapy has evolved from a radium source placed in the urethra to today's advanced proton therapy delivered by only a few specialized centers. As techniques in radiation have evolved, the treatment of localized prostate cancer has become one of the most debated topics in oncology. Today, patients with prostate cancer must often make a difficult decision between multiple treatment modalities, each with the risk of permanent sequelae, without robust randomized data to compare every treatment option. Meanwhile, opinions of urologists and radiation oncologists about the risks and benefits involved with each modality vary widely. Further complicating the issue is rapidly advancing technology which often outpaces clinical data. This article represents a complete description of the evolution of prostate cancer radiation therapy with the goal of illuminating the historical basis for current challenges facing oncologists and their patients.

Sector analysis of 125I permanent prostate brachytherapy provides a rapid and effective method of evaluating and comparing pre- and post-implant dosimetry

PURPOSE

To evaluate a sector analysis program in the assessment and comparison of pre- and post-implant dosimetric parameters during the development of an (125)I permanent prostate brachytherapy service.

METHODS AND MATERIALS

A total of 50 consecutive men being treated with permanent prostate brachytherapy had dose-volume analysis in 12 sectors of their pre-implant ultrasound (USpre) and post-implant CT (CTpost) studies. Individual sectors were created by dividing prostate into three equal lengths, namely base, midgland, and apex. Each of these volumes was then divided into four axial sectors. Dosimetric parameters were compared in adjoining sectors within each study and between studies.

RESULTS

There were statistically significant differences between individual sectors on USpre and CTpost volumes with CTpost higher than USpre (p=0.001). Statistically significant differences were found in corresponding sectors on USpre and CTpost for all dosimetric parameters. The dosimetric parameters were significantly lower on CTpost in the anterior base and midgland (p=0.001) and significantly higher at the posterior apex and midgland (p=0.05). Dose homogeneity was demonstrated in adjoining sectors in all USpre and most adjoining sectors on CTpost.

CONCLUSIONS

Sector analysis allows rapid assessment of USpre and CTpost dosimetry. It offers a scientific method of identifying areas of increased and reduced dosing on CTpost when compared with USpre, providing a learning tool to refine dosimetric analysis and highlight sectors where implant quality could be improved.

Gold marker displacement due to needle insertion during HDR-brachytherapy for treatment of prostate cancer: a prospective cone beam computed tomography and kilovoltage on-board imaging (kV-OBI) study

PURPOSE

To evaluate gold marker displacement due to needle insertion during HDR-brachytherapy for therapy of prostate cancer.

PATIENTS AND METHODS

18 patients entered into this prospective evaluation. Three gold markers were implanted into the prostate during the first HDR-brachytherapy procedure after the irradiation was administered. Three days after marker implantation all patients had a CT-scan for planning purpose of the percutaneous irradiation. Marker localization was defined on the digitally-reconstructed-radiographs (DRR) for daily (VMAT technique) or weekly (IMRT) set-up error correction. Percutaneous therapy started one week after first HDR-brachytherapy. After the second HDR-brachytherapy, two weeks after first HDR-brachtherapy, a cone-beam CT-scan was done to evaluate marker displacement due to needle insertion. In case of marker displacement, the actual positions of the gold markers were adjusted on the DRR.

RESULTS

The value of the gold marker displacement due to the second HDR-brachytherapy was analyzed in all patients and for each gold marker by comparison of the marker positions in the prostate after soft tissue registration of the prostate of the CT-scans prior the first and second HDR-brachytherapy. The maximum deviation was 5 mm, 7 mm and 12 mm for the anterior-posterior, lateral and superior-inferior direction. At least one marker in each patient showed a significant displacement and therefore new marker positions were adjusted on the DRRs for the ongoing percutaneous therapy.

CONCLUSIONS

Needle insertion in the prostate due to HDR-brachytherapy can lead to gold marker displacements. Therefore, it is necessary to verify the actual position of markers after the second HDR-brachytherapy. In case of significant deviations, a new DRR with the adjusted marker positions should be generated for precise positioning during the ongoing percutaneous irradiation.

Image fusion techniques in permanent seed implantation

Over the last twenty years major software and hardware developments in brachytherapy treatment planning, intraoperative navigation and dose delivery have been made. Image-guided brachytherapy has emerged as the ultimate conformal radiation therapy, allowing precise dose deposition on small volumes under direct image visualization. In this process imaging plays a central role and novel imaging techniques are being developed (PET, MRI-MRS and power Doppler US imaging are among them), creating a new paradigm (dose-guided brachytherapy), where imaging is used to map the exact coordinates of the tumour cells, and to guide applicator insertion to the correct position. Each of these modalities has limitations providing all of the physical and geometric information required for the brachytherapy workflow. Therefore, image fusion can be used as a solution in order to take full advantage of the information from each modality in treatment planning, intraoperative navigation, dose delivery, verification and follow-up of interstitial irradiation. Image fusion, understood as the visualization of any morphological volume (i.e. US, CT, MRI) together with an additional second morphological volume (i.e. CT, MRI) or functional dataset (functional MRI, SPECT, PET), is a well known method for treatment planning, verification and follow-up of interstitial irradiation. The term image fusion is used when multiple patient image datasets are registered and overlaid or merged to provide additional information. Fused images may be created from multiple images from the same imaging modality taken at different moments (multi-temporal approach), or by combining information from multiple modalities. Quality means that the fused images should provide additional information to the brachytherapy process (diagnosis and staging, treatment planning, intraoperative imaging, treatment delivery and follow-up) that cannot be obtained in other ways. In this review I will focus on the role of image fusion for permanent seed implantation.

Variations in inter-observer contouring and its impact on dosimetric and radiobiological parameters for intensity-modulated radiotherapy planning in treatment of localised prostate cancer

Inter-observer variations in contouring and their impacts on dosimetric and radiobiological parameters in intensity-modulated radiotherapy (IMRT) treatment for localised prostate cancer patients were investigated. Four observers delineated the gross tumour volume (GTV) (prostate and seminal vesicles), bladder and rectum for nine patients. Contouring done by radiologist was considered as gold standard for comparison purposes and for IMRT plan optimisation. Maximum average variations in contoured prostate, bladder and rectum volumes were 3% (SD = 8.4), 2.5% (SD = 4.12) and 13.2% (SD = 6.77), respectively. The average conformity index for standard contouring set (observer A) was 0.85 (SD = 0.028) and statistically significant differences were observed for observers A–B (p = 0.008), A–C (p = 0.006) and A–D (p = 0.011). Average values of normal tissue complication probability for bladder and rectum for observer A were 0.361% (SD = 0.036) and 1.59% (SD = 0.14). Maximum average tumour control probability was 99.94% (SD = 0.035) and statistically significant difference was observed for observers A–B (p = 0.037) and observers A–C (p = 0.01). Inter-observer contouring variations have significant impact on dosimetric and radiobiological outcome in IMRT treatment planning. So accurate contouring of tumour and normal organs is a fundamental prerequisite to make good correlation between calculated and clinical observed results.

Anatomic-based three-dimensional planning precludes use of catheter-delivered contrast for treatment of prostate cancer

PURPOSE

Retrograde urethrography is a standard method to identify the prostatic apex during planning for prostate cancer radiotherapy. This is an invasive and uncomfortable procedure. With modern three-dimensional computed tomography planning, we explored whether retrograde urethrography was still necessary to accurately identify the prostatic apex.

METHODS AND MATERIALS

Fifteen patients underwent computed tomography simulation with and without bladder, urethral, and rectal contrast. The prostatic base and apex were identified on both scans, using contrast and anatomy, respectively. The anatomic location of the prostatic apex as defined by these methods was confirmed in another 57 patients with postbrachytherapy imaging.

RESULTS

The prostatic base and apex were within a mean of 3.8 mm between the two scans. In every case, the beak of the retrograde urethrogram abutted the line drawn parallel to, and bisecting, the pubic bone on the lateral films. With these anatomic relationships defined, in the postbrachytherapy patients, the distance from the prostatic apex to the point at which the urethra traversed the pelvic floor was an average of 11.7 mm. On lateral films, we found that the urethra exited the pelvis an average of 16.6 mm below the posterior-most fusion of the pubic symphysis. On axial images, this occurred at a mean separation of the ischia of about 25 mm.

CONCLUSION

With a knowledge of the anatomic relationships and modern three-dimensional computed tomography planning equipment, the prostatic apex can be easily and consistently identified, obviating the need to subject patients to retrograde urethrography.

Cone-beam CT using a mobile C-arm: a registration solution for IGRT with an optical tracking system

A method for registering images acquired from a prototype flat panel mobile C-arm, capable of kilovoltage (kV) cone-beam computed tomography (CT), to a linear accelerator (LINAC) isocenter is presented. A calibration procedure is performed which involves locating reflective markers placed on the C-arm and a phantom in two coordinate systems. A commercial optical tracking system locates the markers relative to the LINAC isocenter (room coordinates). The cone-beam imaging capabilities of the C-arm provide the location of the markers on the calibration phantom in image coordinates. A singular value decomposition (SVD) algorithm is used to determine the relationship between the C-arm, image coordinates and room coordinates. Once the calibration is completed, the position of the C-arm at any arbitrary location is accurately determined from the tracking system. A final transformation is calculated capable of mapping voxels in the reconstructed image set to their corresponding position in room coordinates. An evaluation to determine the accuracy of this method was performed by locating markers on a phantom. The position of the phantom markers in room coordinates was obtained directly using the optical tracking system and compared with that using the described method above. A mean absolute distance of 1.4+/-0.5 was observed for a completely transformed image set. This is comparable to that of systems routinely used for image-guided radiation therapy (IGRT).

A review of prostate motion with considerations for the treatment of prostate cancer

The motion of the prostate gland can influence the efficacy of radiation therapy. This article examines the literature concerning prostate gland motion with considerations for the treatment of cancer. The objectives of this review include providing radiation oncologists, medical physicists, and dosimetrists with data to assist in determining the best treatment adaptation for individual patients. The prostate gland is not a static structure, but rather a dynamic structure and this should be a consideration in the treatment protocol. The treatment planning personnel must add a margin to the clinical treatment volume (CTV) radiation field to account for prostate motion and patient setup errors resulting in a planning treatment volume (PTV). The movement of the prostate in a radiation field with a small margin to protect the anterior rectum may allow the posterior aspect of the gland to escape the prescribed dose. Thus, an understanding of potential prostate movements in radiation therapy is critical to achieve tumor control and minimize radiation complications in patients.

Ultrasound-Based Localization

Ultrasound is a noninvasive, relatively easy, rapid, and real-time imaging technique for organ targeting for radiotherapy. Its application has been developed to a greater extent in prostate cancer than in other sites in which it has been shown to improve the accuracy of daily treatment delivery. With the move toward dose escalation and the need to maximally spare the adjacent critical structures through more conformal therapy and smaller field margins, an innovative technique for accurate and reproducible tumor targeting is mandatory. Basic ultrasound principles and organ location lend themselves well to the application of this modality in prostate cancer. Promising results using daily ultrasound-guided B-mode acquisition and targeting for patients with upper abdominal tumors suggest an area for additional trials and study. For breast cancer radiotherapy, ultrasound serves to define involved primary and nodal sites, especially in patients in whom surgical evaluation will not be the first therapeutic step.

Vessel-sparing prostate radiotherapy: Dose limitation to critical erectile vascular structures (internal pudendal artery and corpus cavernosum) defined by MRI

PURPOSE

Most evidence suggests that impotence after prostate radiation therapy has a vascular etiology. The corpus cavernosum (CC) and the internal pudendal artery (IPA) are the critical vascular structures related to erectile function. This study suggests that it is feasible to markedly decrease radiation dose to the CC and the IPA and directly determine the impact of dose limitation on potency.

METHODS AND MATERIALS

Twenty-five patients (10 external beam, 15 brachytherapy) underwent MRI/CT-based treatment planning for prostate cancer. In addition, 10 patients entered on the vessel-sparing protocol underwent a time-of-flight MRI angiography sequence to define the IPA. The distance from the MRI-defined prostate apex to the penile bulb (PB), CC, and IPA was measured and compared to the distance from the CT-defined apex. Doses (D5 and D50) to the PB, CC, and IPA were determined for an 80 Gy external beam course. In 5 patients, CT plans were generated and compared to MRI-based plans.

RESULTS

The combination of coronal, sagittal, and axial MRI data sets allowed superior definition of the prostate apex and its relationship to critical vascular structures. The apex to PB distance averaged 1.45 cm (0.36 standard deviation) with a range of 0.7 cm to 2.1 cm. Peak dose (D5) to the proximal CC in the MRI-planned 80 Gy course was 26 (9) Gy (0.36 of CT-planned dose), and peak dose to the IPA was 39 (13) Gy (0.61 of CT-planned dose).

CONCLUSION

The distance between the prostate apex and critical vascular structures is highly variable. Current empiric rules for CT contouring (apex 1.5 cm above PB) overestimate or underestimate the distance between the prostate apex and critical vascular structures. When defined by MRI T2 and MRI angiogram with CT registration, limitation of dose to critical erectile structures is possible, with a more significant gain than has been previously reported using dose limitation by commonly applied intensity modulated radiation therapy studies based on CT imaging. These techniques make "vessel-sparing" prostate radiotherapy feasible.

MR and CT image fusion for postimplant analysis in permanent prostate seed implants

PURPOSE

To compare the outcome of two different image-based postimplant dosimetry methods in permanent seed implantation.

METHODS AND MATERIALS

Between October 1999 and October 2002, 150 patients with low-risk prostate carcinoma were treated with (125)I and (103)Pd in our institution. A CT-MRI image fusion protocol was used in 21 consecutive patients treated with exclusive brachytherapy. The accuracy and reproducibility of the method was calculated, and then the CT-based dosimetry was compared with the CT-MRI-based dosimetry using the dose-volume histogram (DVH) related parameters recommended by the American Brachytherapy Society and the American Association of Physicists in Medicine.

RESULTS

Our method for CT-MRI image fusion was accurate and reproducible (median shift <1 mm). Differences in prostate volume were found, depending on the image modality used. Quality assurance DVH-related parameters strongly depended on the image modality (CT vs. CT-MRI): V(100) = 82% vs. 88%, p < 0.05. D(90) = 96% vs. 115%, p < 0.05. Those results depend on the institutional implant technique and reflect the importance of lowering inter- and intraobserver discrepancies when outlining prostate and organs at risk for postimplant dosimetry.

CONCLUSIONS

Computed tomography-MRI fused images allow accurate determination of prostate size, significantly improving the dosimetric evaluation based on DVH analysis. This provides a consistent method to judge a prostate seed implant's quality.

Online image-guided intensity-modulated radiotherapy for prostate cancer: How much improvement can we expect? A theoretical assessment of clinical benefits and potential dose escalation by improving precision and accuracy of radiation delivery

PURPOSE

To quantify the theoretical benefit, in terms of improvement in precision and accuracy of treatment delivery and in dose increase, of using online image-guided intensity-modulated radiotherapy (IG-IMRT) performed with onboard cone-beam computed tomography (CT), in an ideal setting of no intrafraction motion/deformation, in the treatment of prostate cancer.

METHODS AND MATERIALS

Twenty-two prostate cancer patients treated with conventional radiotherapy underwent multiple serial CT scans (median 18 scans per patient) during their treatment. We assumed that these data sets were equivalent to image sets obtainable by an onboard cone-beam CT. Each patient treatment was simulated with conventional IMRT and online IG-IMRT separately. The conventional IMRT plan was generated on the basis of pretreatment CT, with a clinical target volume to planning target volume (CTV-to-PTV) margin of 1 cm, and the online IG-IMRT plan was created before each treatment fraction on the basis of the CT scan of the day, without CTV-to-PTV margin. The inverse planning process was similar for both conventional IMRT and online IG-IMRT. Treatment dose for each organ of interest was quantified, including patient daily setup error and internal organ motion/deformation. We used generalized equivalent uniform dose (EUD) to compare the two approaches. The generalized EUD (percentage) of each organ of interest was scaled relative to the prescription dose at treatment isocenter for evaluation and comparison. On the basis of bladder wall and rectal wall EUD, a dose-escalation coefficient was calculated, representing the potential increment of the treatment dose achievable with online IG-IMRT as compared with conventional IMRT.

RESULTS

With respect to radiosensitive tumor, the average EUD for the target (prostate plus seminal vesicles) was 96.8% for conventional IMRT and 98.9% for online IG-IMRT, with standard deviations (SDs) of 5.6% and 0.7%, respectively (p < 0.0001). The average EUDs of bladder wall and rectal wall for conventional IMRT vs. online IG-IMRT were 70.1% vs. 47.3%, and 79.4% vs. 72.2%, respectively. On average, a target dose increase of 13% (SD = 9.7%) can be achieved with online IG-IMRT based on rectal wall EUDs and 53.3% (SD = 15.3%) based on bladder wall EUDs. However, the variation (SD = 9.7%) is fairly large among patients; 27% of patients had only minimal benefit (<5% of dose increment) from online IG-IMRT, and 32% had significant benefit (>15%-41% of dose increment).

CONCLUSIONS

The ideal maximum dose increment achievable with online IG-IMRT is, on average, 13% with respect to the dose-limiting organ of rectum. However, there is a large interpatient variation, ranging <5%-41%. The results can be applied to calibrate other practical online image-guided techniques for prostate cancer radiotherapy, when intratreatment organ motion/deformation and machine delivery accuracy are considered.

A method to compare supra-pubic ultrasound and CT images of the prostate: Technique and early clinical results

We describe a unique method that allows the comparison of spatially registered ultrasound (SRUS) images and computed tomography-derived contours (CTDCs) that were acquired with a minimal time lapse. As such, we have a tool that will provide validation of the spatial accuracy of the US system and that will allow comparison of anatomical boundaries derived via the two different imaging modalities. We describe the method by which the commercial US system is mechanically registered to a CT simulator and a unique data processing procedure. This data processing procedure circumvents the standard data acquisition and manual contouring sequence, thus reducing the time lapse from CT to US image acquisition to 10 minutes on average. Verification using a phantom demonstrated the method to be spatially accurate to within +/- 1 mm in the anterior-posterior (AP) and lateral directions and +/- 3 mm in the inferior-superior (IS) direction. Early clinical results gathered on 8 patients demonstrated alignment between the US and the CTDCs to be 0 mm in the AP and lateral directions and 2 mm in the IS direction, on average. The technique was used to compare the appearance of the prostate using US and CT imaging. The lateral dimension of the prostate indicated by the CTDCs was larger than that indicated by US imaging in all cases and on average by 0.9 cm. The height of the prostate in the AP direction was larger on average by 0.3 cm using CTDCs than US, and was larger by 5 mm or more in 3 out of 7 cases. The role of uncertainties in the determination of the CTDCs is examined as a possible cause and implications for treatment planning are described.

A comparison of CT scan to transrectal ultrasound-measured prostate volume in untreated prostate cancer

PURPOSE

To compare CT and transrectal ultrasound (TRUS)-measured prostate volumes in patients with untreated prostate cancer.

METHODS AND MATERIALS

Between 1995 and 1999, 48 consecutive patients at the Portland Veterans Affairs Medical Center were treated with external beam radiotherapy. In 36 of these patients, TRUS and CT measurements of the prostate volume were obtained before treatment and <6 months apart. The TRUS volume was calculated using the prolate ellipsoid formula. The CT volume was calculated from the contours of the prostate drawn by one physician, who was unaware of the TRUS volume calculation, on axial CT images.

RESULTS

The TRUS and CT prostate volume measurements correlated strongly (Pearson's correlation coefficient = 0.925, 95% confidence interval 0.856-0.961, p < 0.0001). The CT volume was consistently larger than the TRUS volume by a factor of approximately 1.5. In men with a TRUS prostate volume less than the median (<28 cm(3)), the CT/TRUS volume ratio was 1.7, and it was 1.4 for men whose volume was greater than the median. The CT volumes were correlated similarly with the TRUS volumes regardless of the CT slice interval.

CONCLUSION

A strong correlation was found between CT scan and TRUS measurement of the prostate volume; however, CT consistently overestimated the prostate volume by approximately 50% compared with TRUS.

Marker seed migration in prostate localization

PURPOSE

Marker seed location was analyzed to test the hypothesis that there is no intraseed migration within the prostate, a premise fundamental to the technique of marker seed localization of this organ. Despite increasing interest in the use of implanted seeds as fiducial markers for gland location, there are few data available with which to evaluate the validity of this technique, particularly over the entire course of external beam radiation therapy.

METHODS AND MATERIALS

Between May 2001 and December 2001, after obtaining fully informed written consent, 9 patients with early stage prostate cancer were enrolled on an institutionally reviewed protocol. Patients had four to five marker seeds implanted into the prostate under transrectal ultrasound guidance before definitive radiotherapy. The porous gold seeds were each 1.2 x 2.0 mm in dimension. Seed locations from orthogonal radiographs based on the initial simulation and weekly orthogonal films were digitized using a CMS Focus planning system, thereby facilitating the determination of intraseed spacing. The digitization of the isocenter from each orthogonal pair of radiographs was used to determine digitizing error for seed localization. Pubic symphysis, bilateral femoral heads, and isocenter were also digitized and will be analyzed at a later date.

RESULTS

Overall, the average migration of all the seeds in the patients was 1.2 +/- 0.2 (SD) mm. The greatest average movement of any seed in any patient was 1.9 mm over the entire 7-week course of radiotherapy. The smallest average movement was 0.6 mm. The greatest change in intraseed spacing in any of the patients during the full course of therapy was 6.6 mm. One seed in 1 patient was lost at the start of the third week of therapy and censored from analysis. Digitizing error in seed localization was calculated to be 0.20 +/- 0.03 (SD) mm.

CONCLUSIONS

As an aggregate, there is negligible seed migration within the prostate over the entire course of definitive radiotherapy. However, there are small, detectable movements in individual seed locations, perhaps resulting from topographic changes in the gland secondary to seed placement, anatomic changes in bladder and rectum, or treatment itself. With respect to seed migration, prostate marker seeds represent an accurate and reliable surrogate of gland location during a full course of radiotherapy.

Bulb of penis as a marker for prostatic apex in external beam radiotherapy of prostate cancer

PURPOSE

To investigate the relationship between the bulb of the penis and the peak of the urethrogram, and to compare this measurement with the ischial tuberosities (ITs) to peak distance.

METHODS AND MATERIALS

Pelvic CT scans from 50 consecutive patients with localized prostate cancer were analyzed to identify the penile bulb. Each patient was required to undergo retrograde urethrography during CT-based treatment planning with 3-mm slices. The peak of the urethrogram was defined as the last CT slice in which the contrast dye in the urethra could be visualized. Measurements were taken from the slice containing the most superior aspect of the penile bulb to the last slice of the urethrogram peak. The superior aspect of the penile bulb was defined as the CT slice nearest the peak that contained a bulbous structure at the base of the penis. This distance was defined as the bulb-peak distance. Similarly, the IT-peak distance was recorded for comparison.

RESULTS

The mean bulb-peak and IT-peak distances were calculated for 47 of 50 patients. The peak of the urethrogram was unable to be evaluated in 3 patients. The mean, median, and range bulb-peak distance was 2.4 mm (SD 1.8), 3 mm, and 0-6 mm, respectively. The mean, median, and range IT-peak distance was 20.1 mm (SD 6.6), 21 mm, and 6-33 mm, respectively. No patient had the bulb located above the apex of the urethrogram.

CONCLUSION

The bulb of the penis is a relatively consistent soft-tissue landmark compared with the ITs and is located an average of 3 mm below the peak of the urethrogram. Therefore, the bulb of the penis is another landmark for the identification of the prostatic apex and is less invasive than retrograde urethrography.

Robustness and precision of an automatic marker detection algorithm for online prostate daily targeting using a standard V-EPID

An algorithm for the daily localization of the prostate using implanted markers and a standard video-based electronic portal imaging device (V-EPID) has been tested. Prior to planning, three gold markers were implanted in the prostate of seven patients. The clinical images were acquired with a BeamViewPlus 2.1 V-EPID for each field during the normal course radiotherapy treatment and are used off-line to determine the ability of the automatic marker detection algorithm to adequately and consistently detect the markers. Clinical images were obtained with various dose levels from ranging 2.5 to 75 MU. The algorithm is based on marker attenuation characterization in the portal image and spatial distribution. A total of 1182 clinical images were taken. The results show an average efficiency of 93% for the markers detected individually and 85% for the group of markers. This algorithm accomplishes the detection and validation in 0.20-0.40 s. When the center of mass of the group of implanted markers is used, then all displacements can be corrected to within 1.0 mm in 84% of the cases and within 1.5 mm in 97% of cases. The standard video-based EPID tested provides excellent marker detection capability even with low dose levels. The V-EPID can be used successfully with radiopaque markers and the automatic detection algorithm to track and correct the daily setup deviations due to organ motions.

Clinical feasibility study for the use of implanted gold seeds in the prostate as reliable positioning markers during megavoltage irradiation

BACKGROUND AND PURPOSE

The aim of this study was to assess the feasibility of using gold seed implants in the prostate for position verification, using an a-Si flat panel imager as a detector during megavoltage irradiation of prostate carcinoma. This is a study to guarantee positioning accuracy in intensity-modulated radiotherapy.

METHODS AND MATERIALS

Ten patients with localized prostate carcinoma (T2-3) received between one and three fiducial gold markers in the prostate. All patients were treated with 3-D conformal radiotherapy with an anterior-posterior (AP) and two lateral wedge fields. The acute gastrointestinal (GI) and genitourinary (GU) toxicities were scored using common toxicity criteria scales (CTC). Using three consecutive CT scans and portal images obtained during the treatment we have studied the occurrence of any change in prostate shape (deformation), seed migration and the magnitude of translations and rotations of the prostate.

RESULTS

We observed no acute major complications for prostate irradiation regarding the seed implantation. The maximum acute GU toxicity grade 2 (dysuria and frequency) was observed in seven patients during the treatment. The maximum grade 2 (diarrhoea) was scored in two patients regarding the acute GI toxicities. No significant prostate deformation could be detected in the consecutive CT scans. It appeared that the distances between the markers only slightly changed during treatment (S.D. 0.5 mm). Random prostate translations were (1 S.D.) 2.1, 3.2 and 2.2 mm in the lateral (LR), AP and cranial-caudal (CC) directions, respectively, whereas systematic translations were 3.3, 4.8 and 3.5 mm in the LR, AP and CC directions, respectively. Random prostate rotations were (1 S.D.) 3.6, 1.7 and 1.9 degrees around the LR, AP and CC axis, respectively, whereas systematic rotations were 4.7, 2.0 and 2.7 degrees around the LR, AP and CC axis, respectively.

CONCLUSIONS

We found that the fiducial gold seeds are a safe and appropriate device to verify and correct the position of prostate during megavoltage irradiation. The amount of seed migration and prostate deformation is far below our present tumour delineation accuracy.

Interfractional and intrafractional accuracy during radiotherapy of gynecologic carcinomas: a comprehensive evaluation using the ExacTrac system

PURPOSE

To evaluate positioning uncertainties with an infrared body marker-based positioning system (ExacTrac) compared with conventional laser positioning in patients treated for gynecologic carcinomas, and to investigate patient movement during therapy.

MATERIALS AND METHODS

Ten patients were positioned both with a conventional laser system and with the ExacTrac system. Positioning accuracy was evaluated using repeated electronic portal images. Average displacements and overall, systematic, and random errors were calculated and compared for the two positioning methods. Further, inter- and intrafractional patient movement including time trends in positioning displacements, respiratory amplitudes, and breathing frequencies were analyzed by online documentation of body marker movement with the ExacTrac system.

RESULTS

Average displacements ranged between -3.6 and 6.7 mm for the three coordinates. Mean systematic and random errors ranged from 1.6 to 3.7 mm and 2.2 to 3.7 mm, respectively, with no significant differences between conventional and ExacTrac positioning (p > 0.07). The main breathing direction was from dorsocaudal to anterocranial in 9 of 10 patients. The mean 3D breathing amplitude in the pelvis was 2.4 mm (0.49-6.96 mm). Significant interfractional and intrafractional time trends were observed concerning breathing amplitudes and positioning displacements.

CONCLUSIONS

The observed displacements did not vary significantly between the two evaluated positioning systems. The analysis of registered body marker positions revealed a wide variation in respiratory frequencies, breathing amplitudes, and patient displacements with interfractional and intrafractional time trends. Systems that allow the measurement of each patient's motion characteristics are a necessary requirement for all efforts at individually tailored radiation therapy.

CT-MRI image fusion for delineation of volumes in three-dimensional conformal radiation therapy in the treatment of localized prostate cancer

The objective of this study was to assess the utility of CT-MRI image fusion software and compare both prostate volume and localization with CT and MRI studies. We evaluated the differences in clinical volumes in patients undergoing three-dimensional conformal radiation therapy for localized prostate cancer. After several tests performed to ensure the quality of image fusion software, eight patients suffering from prostate adenocarcinoma were submitted to CT and MRI studies in the treatment position within an immobilization device before the start of radiotherapy. The clinical target volume (CTV) (prostate plus seminal vesicles) was delineated on CT and MRI studies and image fusion was obtained from the superimposition of anatomical fiducial markers. A comparison of dose-volume histograms relative to CTV, rectum, bladder and femoral heads was performed for both studies. Image fusion showed a mean overestimation of CTV of 34% with CT compared with MRI. Along the anterior-posterior and superior-inferior direction, CTV was a mean 5 mm larger with CT study compared with MRI. The dose-volume histograms resulting from CT and MRI comparison showed that it is possible to spare a mean 10% of rectal volume and approximately 5% of bladder and femoral heads, respectively. This study confirmed an overestimation of CTV with CT images compared with MRI. Because this finding only allows a minimal sparing of organs at risk, considering the organ motion during each radiotherapy session and the excellent outcomes of prostate cancer treatment with CT based target identification, we are still reluctant to reduce the CTV to that identified by MRI.

Intrafraction prostate motion during IMRT for prostate cancer

PURPOSE

Although the interfraction motion of the prostate has been previously studied through the use of fiducial markers, CT scans, and ultrasound-based systems, intrafraction motion is not well documented. In this report, the B-mode, Acquisition, and Targeting (BAT) ultrasound system was used to measure intrafraction prostate motion during 200 intensity-modulated radiotherapy (IMRT) sessions for prostate cancer.

METHODS AND MATERIALS

Twenty men receiving treatment with IMRT for clinically localized prostate cancer were selected for the study. Pre- and posttreatment BAT ultrasound alignment images were collected immediately before and after IMRT on 10 treatment days for a total of 400 BAT alignment procedures. Any ultrasound shifts of the prostate borders in relation to the planning CT scan were recorded in 3 dimensions: right-left (RL), anteroposterior (AP), and superior-inferior (SI). Every ultrasound procedure was evaluated for image quality and alignment according to a 3-point grading scale.

RESULTS

All the BAT images were judged to be of acceptable quality and alignment. The dominant directions of intrafraction prostate motion were anteriorly and superiorly. The mean magnitude of shifts (+/-SD) was 0.01 +/- 0.4 mm, 0.2 +/- 1.3 mm, and 0.1 +/- 1.0 mm in the left, anterior, and superior directions, respectively. The maximal range of motion occurred in the AP dimension, from 6.8 mm anteriorly to 4.6 mm posteriorly. The percentage of treatments during which prostate motion was judged to be <or=5 mm was 100%, 99%, and 99.5% in the RL, AP, and SI directions, respectively. Three of the measurements were >5 mm. The extent of intrafraction motion was much smaller than that of interfraction motion. Linear regression analysis showed very little correlation between the two types of motion (r = 0.014, 0.029, and 0.191, respectively) in the RL, AP, and SI directions.

CONCLUSION

Using an ultrasound-based system, intrafraction prostate motion occurred predominantly in the anterior and superior directions, but was clinically insignificant. Intrafraction motion was much smaller than interfraction motion, and the two types of movement did not correlate.

Daily prostate targeting using implanted radiopaque markers

PURPOSE

A system has been implemented for daily localization of the prostate through radiographic localization of implanted markers. This report summarizes an initial trial to establish the accuracy of patient setup via this system.

METHODS AND MATERIALS

Before radiotherapy, three radiopaque markers are implanted in the prostate periphery. Reference positions are established from CT data. Before treatment, orthogonal radiographs are acquired. Projected marker positions are extracted semiautomatically from the radiographs and aligned to the reference positions. Computer-controlled couch adjustment is performed, followed by acquisition of a second pair of radiographs to verify prostate position. Ten patients (6 prone, 4 supine) participated in a trial of daily positioning.

RESULTS

Three hundred seventy-four fractions were treated using this system. Treatment times were on the order of 30 minutes. Initial prostate position errors (sigma) ranged from 3.1 to 5.8 mm left-right, 4.0 to 10.1 mm anterior-posterior, and 2.6 to 9.0 mm inferior-superior in prone patients. Initial position was more reproducible in supine patients, with errors of 2.8 to 5.0 mm left-right, 1.9 to 3.0 mm anterior-posterior, and 2.6 to 5.3 mm inferior-superior. After prostate localization and adjustment, the position errors were reduced to 1.3 to 3.5 mm left-right, 1.7 to 4.2 mm anterior-posterior, and 1.6 to 4.0 mm inferior-superior in prone patients, and 1.2 to 1.8 mm left-right, 0.9 to 1.8 mm anterior-posterior, and 0.8 to 1.5 mm inferior-superior in supine patients.

CONCLUSIONS

Daily targeting of the prostate has been shown to be technically feasible. The implemented system provides the ability to significantly reduce treatment margins for most patients with cancer confined to the prostate. The differences in final position accuracy between prone and supine patients suggest variations in intratreatment prostate movement related to mechanisms of patient positioning.

Fiducial-based targeting accuracy for external-beam radiotherapy

The accuracy of fiducial-based alignment of external radiotherapy beams is analyzed. The study considers three basic computational methods to determine the target position--the exact closed-form solution for three fiducials, the solution via singular value decomposition for four or more fiducials, and the iterative solution for any number of fiducials--and assesses their accuracy, robustness, and efficiency. Particular attention is paid to inaccuracies arising from the variability of fiducial positions in soft tissue. In nearly every test case it is found that all three solution methods, when properly implemented, yield the same result for the target position, but that the method of singular value decomposition must be modified to distinguish rotations from reflections. When an accurate measure of the rotation of the target site is needed, four fiducials give much better results than three, while more than five fiducials gain little further improvement.

Prostate brachytherapy postimplant dosimetry: a comparison of prostate quadrants

PURPOSE

To investigate postimplant dosimetry for different regions of the prostate gland in patients treated with transperineal 125Iodine brachytherapy implants for low- and intermediate-risk prostate cancer.

METHODS AND MATERIALS

Two hundred eighty-four patients treated with permanent interstitial prostate brachytherapy comprised the study population. A nonuniform, urethral-sparing algorithm was used to plan all patients. Prostate contours were outlined on postimplant CT images. Prostate volumes were then divided into four quadrants: anterior-superior quadrant (ASQ), posterior-superior quadrant (PSQ), anterior-inferior quadrant (AIQ), and posterior-inferior quadrant (PIQ). Dose-volume histograms (DVHs) were calculated for the whole prostate and each quadrant.

RESULTS

The mean postimplant V(100) +/- 95% confidence (the percent prostate volume encompassed within the isodose surface comprising the prescription dose = 144 Gy) for the ASQ was 78.5 +/- 1.9, which was significantly lower than that of the PSQ, AIQ, and PIQ in which the V(100) plus minus 95% confidence values were 94.9 +/- 0.8, 92.6 +/- 1.2, and 98.7 +/- 0.3, respectively. The mean V(100) +/- 95% confidence for the whole prostate was 90.4 +/- 0.8. Mean values for V(150) and D(90) (the minimum dose in Gy received by 90% of the target volume) for the four quadrants and the whole prostate showed similar results.

CONCLUSIONS

Underdosed areas of the planning target volume (PTV), if present, were largely confined to the ASQ, which received a significantly lower dose, on average, compared to the other three quadrants of the prostate.

Prostate immobilization using a rectal balloon

We use a rectal balloon for prostate immobilization during intensity modulated radiotherapy (IMRT) prostate treatment. To improve the accuracy of our prostate planning target volume, we have measured prostate displacements using computed tomography (CT)-CT fusion on patients that previously received gold seed implants. The study consists of ten patients that were scanned twice per week during the course of IMRT treatment. In addition to biweekly scans, breathing studies were performed on each patient to estimate organ motion during treatment. The prostate displacement in the anterior-posterior and the lateral direction is minimal, on the order of measurement uncertainty (~1 mm). The standard deviation of the superior-inferior (SI) displacements is 1.78 mm. The breathing studies show that no organ displacement was detected during normal breathing conditions with a rectal balloon.

Trans-rectal ultrasonography (TRUS) with lipiodol injection for localization of the prostatic apex before radiotherapy planning

PURPOSE

To evaluate reliability of Trans-rectal ultra-sonography (TRUS) guidance with lipiodol injection for prostate localization before radiotherapy planning.

MATERIAL AND METHODS

From October 1997 to March 2000, 31 patients with prostatic adenocarcinoma and six patients with anastomotic recurrence after radical prostatectomy had TRUS-guided injection of lipiodol. Two milliliters of lipiodol were injected into each side of the prostate and 1 ml into both seminal vesicles with a 22 Gauge CHIBA needle and US probe guide before radiotherapy planning. We had established a contrast quality index (0 for no prostate enhancement to 5 for efficient pacification without any diffusion). On simulation films, we had performed anatomic measurements for comparison with other anatomic studies.

RESULTS

For all 37 patients, TRUS-guided injection was well tolerated. Among 31 patients with the prostate in situ, three had no apex opacification and 15 had no vesicle enhancement or peri-vesicle space diffusion. However, in 19 patients there was good contrast quality with an index score of > or =3. The majority of patients had prostatic apex between 1.5 and 3.5 cm from ischial tuberosities ligne (27 from 28 evaluable for apex). Among 19 evaluable patients, 15 had seminal vesicles 2-4 cm above the top of pubis. For six patients with anastomotic recurrence after radical prostatectomy, lipiodol was precious aid to locate it. We had only one failure because of a precocious bladder absorption relating to a delay which is too long between rectal probe locating and portal films.

CONCLUSION

TRUS injection of lipiodol is a simple, inexpensive, relatively safe technique for localization of prostatic apex, but not appropriate for seminal vesicles enhancement. This is also an interesting method to locate anastomotic recurrence.

To move or not to move: measurements of prostate motion by urethrography using MRI

PURPOSE

Urethrography is commonly used to aid in definition of the prostate apex during CT simulation for prostate cancer. If the position of the prostate were altered by the urethrogram itself, then systematic error could be introduced into the patient's treatment. Sagittal MRI scans were acquired immediately before and after a localization urethrogram to determine the extent of displacement.

METHODS AND MATERIALS

Thirteen patients underwent sagittal T2-weighted fast spin echo MRI scans. Patients were scanned supine in an alpha cradle cast in the treatment position. The prostate was contoured by 3 different observers to determine the apex location on the central sagittal MRI section and the center of mass relative to an immobile bony landmark. Statistical multivariate analysis was performed to establish if there was a net displacement of the prostate (systematic error), and to determine the margin required to cover the random prostate position within a 95% confidence interval.

RESULTS

There was no significant systematic motion of either the prostate nor its apex in either the anterior-posterior or superior-inferior directions. The average motion of the prostate center of mass was 0.04 +/- 0.40 cm (1 SD) and 0.01 +/- 0.33 cm in the anterior-posterior and superior-inferior direction, respectively. The corresponding figures for location of the apex were 0.05 +/- 0.30 cm and 0.01 +/- 0.33 cm, respectively. The statistical analysis revealed that a margin of 2 mm is sufficient to cover any random motion of the prostate that could occur as a result of the urethrogram 95% of the time.

CONCLUSION

Urethrography during CT simulation for prostate cancer does not cause significant prostate displacement or systematic error in planning and delivering external-beam radiation.

Simulation for localized prostate cancer: a comparison of urethrography techniques

Forty-five patients having conventional fluoroscopic, or CT scan simulation of the prostate gland from January 1999 to June 1999 were studied. Patients were consecutively assigned (not randomized) in groups of 15 to 3 different urethrography techniques: air contrast alone (group 1), hypaque contrast alone (group 2), and xylocaine jelly and hypaque contrast (group 3). Outcome measures were pain scores, visualization of the apex (indicated by urethrogram tip), and frequency of corrections necessary on the basis of verification port films. Group 3 patients had the lowest mean pain score and required fewer lateral setup corrections at the time of portal imaging on the first day of treatment. A comparison of radiographs also revealed that group 2 and 3 patients (hypaque contrast) had better delineation of the prostatic anatomy than group 1 patients (air contrast). We found that of the 3 techniques tested, urethrography utilizing xylocaine jelly and hypaque was associated with the least amount of pain, least amount of corrective shifts, and best quality in defining the prostatic anatomy.

A clinical evaluation of setup errors for a prostate immobilization system

A prostate treatment immobilization system was evaluated with respect to setup errors and efficiency for a specific treatment setup. Prostate patients were treated in the prone position with a rectal catheter using the NOMOS intensity modulated radiotherapy system. Immobilization and setup consisted of a Vac-Loktrade mark bag (MED-TEC, Orange City, IO) fitted within a registration carrier box where patients were aligned to the bag using skin marks along the lower leg. Daily setup errors were analyzed using lateral portal films, registration plates mounted to the carrier box, and the pubic symphasis as a bony reference. Two studies were conducted to evaluate setup technique. In the first study, patient setup required 3-5 minutes for patient positioning and the corresponding superior/inferior errors were found to have a standard deviation of 3.5 mm. In the second study, the technique standards were reduced to allow for faster setup times and, consequently, larger errors; setup times were 1-2 minutes and the mean and standard deviation errors were approximately 2 and 5 mm, respectively.

Detection of fiducial gold markers for automatic on-line megavoltage position verification using a marker extraction kernel (MEK)

PURPOSE

In this study automatic detection of implanted gold markers in megavoltage portal images for on-line position verification was investigated.

METHODS AND MATERIALS

A detection method for fiducial gold markers, consisting of a marker extraction kernel (MEK), was developed. The detection success rate was determined for different markers using this MEK. The localization accuracy was investigated by measuring distances between markers, which were fixed on a perspex template. In order to generate images comparable to images of patients with implanted markers, this template was placed on the skin of patients before the start of the treatment. Portal images were taken of lateral prostate fields at 18 MV within 1-2 monitor units (MU).

RESULTS

The detection success rates for markers of 5 mm length and 1.2 and 1.4 mm diameter were 0.95 and 0.99 respectively when placed at the beam entry and 0.39 and 0.86 when placed at the beam exit. The localization accuracy appears to be better than 0.6 mm for all markers.

CONCLUSION

Automatic marker detection with an acceptable accuracy at the start of a radiotherapy fraction is feasible. Further minimization of marker diameters may be achieved with the help of an a-Si flat panel imager and may increase the clinical acceptance of this technique.

Prostate position late in the course of external beam therapy: patterns and predictors

PURPOSE

To examine prostate and seminal vesicles position late in the course of radiation therapy and to determine the effect and predictive value of the bladder and rectum on prostate and seminal vesicles positioning.

METHODS AND MATERIALS

Twenty-four patients with localized prostate cancer underwent a computerized tomography scan (CT1) before the start of radiation therapy. After 4-5 weeks of radiation therapy, a second CT scan (CT2) was obtained. All patients were scanned in the supine treatment position with instructions to maintain a full bladder. The prostate, seminal vesicles, bladder, and rectum were contoured. CT2 was aligned via fixed bony anatomy to CT1. The geometrical center and volume of each structure were obtained and directly compared.

RESULTS

The prostate shifted along a diagonal axis extending from an anterior-superior position to a posterior-inferior position. The dominant shift was to a more posterior-inferior position. On average, bladder and rectal volumes decreased to 51% (+/-29%) and 82% (+/-45%) of their pretreatment values, respectively. Multiple regression analysis (MRA) revealed that bladder movement and volume change and upper rectum movement were independently associated with prostate motion (p = 0.016, p = 0. 003, and p = 0.052 respectively).

CONCLUSION

Patients are often instructed to maintain a full bladder during a course of external beam radiation therapy, in the hopes of decreasing bladder and small bowel toxicity. However, our study shows that large bladder volumes late in therapy are strongly associated with posterior prostate displacement. This prostate displacement may result in marginal miss.

Imaging and radiotherapy of the prostate

Prostate cancer is the most common malignancy diagnosed in men. Over the past 10 to 20 years, advances in screening and diagnostic and management paradigms have led to improved treatment outcomes. This article offers an overview of the evolution of the role and nature of diagnostic imaging techniques in the management of prostate cancer.

[Evolution of the use of the portal imaging device: prospective study over three years]

PURPOSE

To describe the evolution of the use of the electronic portal imaging device (EPID) over three periods.

MATERIAL AND METHODS

From 1990, as part of the quality assurance research programs, the radiotherapy department of the G.-F. Leclerc Centre of Dijon used EPID systems in a prospective fashion. During the first of the three periods (PER 1:1990-1993), the study consisted of analysis criteria determination, software efficiency improvement and a selection of patients who could benefit from the method. Eight hundred and forty-five images of 40 patients were analysed qualitatively and quantitatively. Two verifications per week were planned, and the action level for correction was 10 mm. Head and neck images were also displayed in 'cinema' presentation for internal movements analysis. From 1994 to 1995 (PER 2), off-line procedure (OLP) based upon early correction of the systematic error and the rules calculated from our previous experience were tested for checking the brain, head and neck (LOC 1: 396 images) and many of the pelvic irradiations (LOC 2: 260 images). A double-exposure procedure and/or movie loop presentation was reserved for other patients. During the last period (PER 3: 1996-1997), the OLP procedure was routinely performed in 54 patients (images: 321 LOC 1, 680 LOC 2).

RESULTS

LOC 1: deviations of < 3 mm increased from 75.5% during PER 1 to 81% during PER 2 to 83% during PER3. Conversely, deviations of 3-5 mm dropped from 19.5 to 13%, while deviations of more than 5 mm remained stable, around 5%. The actual standard error of the mean deviation observed was 2 mm. LOC 2: deviations of < 5 mm were observed in 81% of the cases during PER 1 and in 91% during PER 3 (89.5% in PER 2). These good results led to a decrease in deviation of 5 to 7 mm (11 to 6%) and also to a significant drop in deviations of more than 7 mm, 8 to 3% respectively. The actual precision obtained was 2.5 mm +/- 3 mm SD.

CONCLUSIONS

The OLP based upon the early correction of the systematic error led to a significant increase of setup accuracy of patients irradiated for the brain, head and neck, and especially for pelvic lesions.

Effects of urethrography on prostate position: considerations for radiotherapy treatment planning of prostate carcinoma

PURPOSE

Retrograde urethrography is commonly used to define the prostate apex at simulation. This study evaluated the hypothesis that urethrography causes prostate displacement, resulting in an error in treatment planning.

METHODS AND MATERIALS

Forty-five patients with carcinoma of the prostate were evaluated. Gold seeds were placed in the apex, posterior wall, and base of the gland. In the first 20 patients, the position of the seed-defined apex was compared at simulation (with urethrogram) and on day 1 of treatment (without urethrogram). In the second cohort of 25 patients, the effects of urethrography on prostate position were evaluated directly at simulation by comparing the position of apex pre- and post-urethrography. An analysis was performed to estimate the possible impact of urethrogram-induced prostate motion on target coverage.

RESULTS

The mean superior displacement in the first and second cohort was 5.2 mm and 6.8 mm, respectively (combined mean shift 6.1 mm). With a 10-mm field margin below the tip of the urethrogram cone, 56% of patients in this study would have inadequate planning target volume (PTV) coverage.

CONCLUSION

Retrograde urethrography causes a significant superior shift of the prostate. Strict reliance on urethrography in determining the inferior field margin could result in inadequate treatment.

The contribution of magnetic resonance imaging to the three-dimensional treatment planning of localized prostate cancer

PURPOSE

To investigate whether the use of transaxial and coronal MR imaging improves the ability to localize the apex of the prostate and the anterior part of the rectum compared to the use of transaxial CT alone, and whether the incorporation of MR could improve the coverage of the prostate by the radiotherapy field and change the volume of rectum irradiated.

METHODS AND MATERIALS

Ten consecutive patients with localized prostate carcinoma underwent a CT and an axial and coronal MR scan in treatment position. The CT and MR images were mathematically aligned, and three observers were asked to contour independently the prostate and the rectum on CT and on MR. The interobserver variability of the prostatic apex location and of the delineation of the anterior rectal wall were assessed for each image modality. A dosimetry study was performed to evaluate the dose to the rectum when MR was used in addition to CT to localize the pelvic organs.

RESULTS

The interobserver variation of the prostatic apex location was largest on CT ranging from 0.54 to 1.07 cm, and smallest on coronal MR ranging from 0.17 to 0.25 cm. The interobserver variation of the delineation of the anterior rectum on MR was small and constant along the whole length of the prostate (0.09+/-0.02 cm), while for CT it was comparable to that for the MR delineation at the base of the prostate, but it increased gradually towards the apex, where the variation reached 0.39 cm. The volume of MR rectum receiving more than 80% of the prescribed dose was on average reduced by 23.8+/-11.2% from the CT to the MR treatment plan.

CONCLUSION

It can be concluded that the additional use of axial and coronal MR scans, in designing the treatment plan for localized prostate carcinoma, improves substantially the localization accuracy of the prostatic apex and the anterior aspect of the rectum, resulting in a better coverage of the prostate and a potential to reduce the volume of the rectum irradiated to a high dose.

[Conformal radiotherapy of prostatic cancer: a general review]

Recent progress in radiotherapeutic management of localized prostate cancer is reviewed. Clinical aspects--including dose-effect beyond 70 Gy, relative role of conformal radiation therapy techniques and of early hormonal treatment--are discussed as well as technical components--including patient immobilization, organ motion, prostate contouring, beam arrangement, 3-D treatment planning and portal imaging. The local control and biological relapse-free survival rates appear to be improved by high dose conformal radiotherapy from 20 to 30% for patients with intermediate and high risk of relapse. A benefit of overall survival is expected but not yet demonstrated. Late reactions, especially the rectal toxicity, remain moderate despite the dose escalation. However, conformal radiotherapy demands a high precision at all steps of the procedure.

[Definition of prostatic contours using tomodensitometric slices: study of differences among radiotherapists and between examinations]

Accuracy of conformal treatment planning for prostatic radiotherapy is based on the contours of target volumes (prostate +/- seminal vesicles) and normal tissues (rectum and bladder), drawn on CT (computed tomography) images by radiation oncologists. The interpretation of a given CT image can be different from one radiation oncologist to another, and may change in time with the state of filling of the bladder and of the rectum during the treatment. In order to quantify these variations, 12 patients treated with conformal radiotherapy for prostate carcinoma (pelvis 40 Gy/20 sessions + prostate 30 Gy/15 sessions) had two series of CT at one month intervals. Contouring of prostate, rectum and bladder were performed independently on each CT by two radiation oncologists. The first CT scan (planning CT) and the first series of contours (planning contours) were used for treatment planning. The contours of the second scan were compared to the planning contours after image fusion based on manual superimposition of bony anatomy of the two sets of CT images. Coherence ratio were defined to measure discrepancies in prostate volumes between radiation oncologists (RCE) and between scans (RCT). The mean RCE was 38 +/- 7% (1 standard deviation). Those discrepancies were primarily located at the prostate apex and at the interface between bladder and prostate and between rectum and prostate. The mean RCT was 42 +/- 8% (1 sigma). Those discrepancies were due to the prostate motion related to the state of filling of the rectum and bladder. For bladder and rectal walls, less important differences were observed between the two radiation oncologists for the same CT (4.5% for rectal volume receiving 65 Gy or more, 3% for bladder volume receiving 65 Gy or more). However, important differences in bladder and rectal volumes receiving 65 Gy or more (16% and 7% respectively) were noted for the same patient from a CT to another due to the variation in bladder or rectal filling. New techniques for planning CT acquisition are needed to decrease the discrepancies due to contouring. The treatment must, as far as possible, be delivered with an empty bladder and rectum in order to ensure a good reproduction of the initially planned treatment.

Definition of the prostate in CT and MRI: a multi-observer study

PURPOSE

To determine, in three-dimensions, the difference between prostate delineation in magnetic resonance (MR) and computer tomography (CT) images for radiotherapy treatment planning.

PATIENTS AND METHODS

Three radiation oncologists, considered experts in the field, outlined the prostate without seminal vesicles both on CT, and axial, coronal, and sagittal MR images for 18 patients. To compare the resulting delineated prostates, the CT and MR scans were matched in three-dimensions using chamfer matching on bony structures. The volumes were measured and the interscan and interobserver variation was determined. The spatial difference between delineation in CT and MR (interscan variation) as well as the interobserver variation were quantified and mapped three-dimensionally (3D) using polar coordinates. A urethrogram was performed and the location of the tip of the dye column was compared with the apex delineated in CT and MR images.

RESULTS

Interscan variation: CT volumes were larger than the axial MR volumes in 52 of 54 delineations. The average ratio between the CT and MR volumes was 1.4 (standard error of mean, SE: 0.04) which was significantly different from 1 (p < 0.005). Only small differences were observed between the volumes outlined in the various MR scans, although the coronal MR volumes were smallest. The CT derived prostate was 8 mm (standard deviation, SD: 6 mm) larger at the base of the seminal vesicles and 6 mm (SD 4 mm) larger at the apex of the prostate than the axial MRI. Similar figures were obtained for the CT and the other MRI scans. Interobserver variation: The average ratio between the volume derived by one observer for a particular scan and patient and the average volume was 0.95, 0.97, and 1.08 (SE 0.01) for the three observers, respectively. The 3D pattern of the overall observer variation (1 SD) for CT and axial MRI was similar and equal to 3.5 to 2.8 mm at the base of the seminal vesicles and 3 mm at the apex.

CONCLUSION

CT-derived prostate volumes are larger than MR derived volumes, especially toward the seminal vesicles and the apex of the prostate. This interscan variation was found to be larger than the interobserver variation. Using MRI for delineation of the prostate reduces the amount of irradiated rectal wall, and could reduce rectal and urological complications.

High-precision conformal radiotherapy (HPCRT) of prostate cancer--a new technique for exact positioning of the prostate at the time of treatment

PURPOSE

Biopsies taken 2 years after radiotherapy of localized prostate cancer indicate residual tumor cells in 20-60% of cases, and the prognosis for these patients is unfavorable. More precise methods of localization of the prostate are desirable to increase the dose to the prostate tumor and minimize the volume of adjacent sensitive tissues that are currently included in the planning target volume. We have sought a method to more accurately locate the prostate at the time of treatment, allowing a reduction of the volume of rectum and bladder included in the high dose region during dose escalation.

METHODS AND RESULTS

We have developed a new technique using a special urethral catheter (patent pending), containing markers that can be visualized by the radiotherapy machine for accurate positioning of the prostate. The catheter is used throughout the treatment planning procedure and the isocenter is placed on one of the markers. On the treatment couch the markers are visualized on port-films and with portal imaging immediately before dose delivery. A beam-center-marker on the accelerator makes it possible to adjust the isocenter position to within 1 mm, giving very high precision, independent of external fixation. The technique involves a simple patient setup. The method has been tested in five patients with conventional dose level (70 Gy) and in 24 patients in the first Scandinavian dose escalation study with external beam radiotherapy. No increase in acute side-effects was observed.

CONCLUSION

With the new high precision conformal radiotherapy (HPCRT) technique we have developed a technique that allows us to increase the dose to the prostate without excessive side effects. The method reduces the uncertainties in prostate localization, is easy to handle, and feasible in routine treatment.