Intensity-modulated Radiotherapy For Prostate Cancer Research Articles

Use of benchmark dose–volume histograms for selection of the optimal technique between three-dimensional conformal radiation therapy and intensity-modulated radiation therapy in prostate cancer

The aim of this study was to develop and validate our own benchmark dose-volume histograms (DVHs) of bladder and rectum for both conventional three-dimensional conformal radiation therapy (3D-CRT) and intensity-modulated radiation therapy (IMRT), and to evaluate quantitatively the benefits of using IMRT vs. 3D-CRT in treating localized prostate cancer. During the implementation of IMRT for prostate cancer, our policy was to plan each patient with both 3D-CRT and IMRT. This study included 31 patients with T1b to T2c localized prostate cancer, for whom we completed double-planning using both 3D-CRT and IMRT techniques. The target volumes included prostate, either with or without proximal seminal vesicles. Bladder and rectum DVH data were summarized to obtain an average DVH for each technique and then compared using two-tailed paired t test analysis. For 3D-CRT our bladder doses were as follows: mean 28.8 Gy, v60 16.4%, v70 10.9%; rectal doses were: mean 39.3 Gy, v60 21.8%, v70 13.6%. IMRT plans resulted in similar mean dose values: bladder 26.4 Gy, rectum 34.9 Gy, but lower values of v70 for the bladder (7.8%) and rectum (9.3%). These benchmark DVHs have resulted in a critical evaluation of our 3D-CRT techniques over time. Our institution has developed benchmark DVHs for bladder and rectum based on our clinical experience with 3D-CRT and IMRT. We use these standards as well as differences in individual cases to make decisions on whether patients may benefit from IMRT treatment rather than 3D-CRT.

Daily organ tracking in intensity-modulated radiotherapy of prostate cancer using an electronic portal imaging device with a dose saving acquisition mode

Background and purposeDaily use of conventional electronic portal imaging devices (EPID) for organ tracking is limited due to the relatively high dose required for high quality image acquisition. We studied the use of a novel dose saving acquisition mode (RadMode) allowing to take images with one monitor unit per image in prostate cancer patients undergoing intensity-modulated radiotherapy (IMRT) and tracking of implanted fiducial gold markers. Patients and methodsTwenty five patients underwent implantation of three fiducial gold markers prior to the planning CT. Before each treatment of a course of 37 fractions, orthogonal localization images from the antero-posterior and from the lateral direction were acquired. Portal images of both the setup procedure and the five IMRT treatment beams were analyzed. ResultsOn average, four localization images were needed for a correct patient setup, resulting in four monitor units extra dose per fraction. The mean extra dose delivered to the patient was thereby increased by 1.2%. The procedure was precise enough to reduce the mean displacements prior to treatment to ≤0.3mm. ConclusionsThe use of a new dose saving acquisition mode enables to perform daily EPID-based prostate tracking with a cumulative extra dose of below 1Gy. This concept is efficiently used in IMRT-treated patients, where separation of setup beams from treatment beams is mandatory.

Recommendations for treatment with IMRT for prostate and head-neck cancer

Intensity-modulated radiation therapy (IMRT) is an advanced form of radiotherapy for the treatment of cancer that allows, on one hand, to administer a more homogeneous dose to the patients on the volume to irradiate (which would increase the local control of the disease), and on the other hand, to diminish the toxicity in the organs at risk. This type of treatment is based on imaging techniques, on computer dosimetry programs, and on more precise immobilization accessories. Before delivering IMRT it is necessary to establish a protocol that includes the different phases of the treatment process, that is, the obtaining of anatomical data, beam definition, calculation, dose distribution, and treatment performance and control. In this article we present the basic standards for the IMRT treatment for prostate and head-neck cancer agreed upon a consensus meeting. The follow-up of the recommendations settled down in this document will help in the establishment of a standardized clinical practice -assuring the quality- and a better evaluation of the results of the clinical intervention.

Adverse effect of a distended rectum in intensity-modulated radiotherapy (IMRT) treatment planning of prostate cancer

Background and purposeThe retrospective planning study for intensity-modulated radiotherapy (IMRT) of prostate cancer evaluated whether proximal rectum and supra-anal rectum/anal canal should be delineated as separated organs-at-risk (OARs) to achieve optimal dose distributions to the anorectal region. Patients and methodsFor 10 patients with localized prostate cancer IMRT plans were generated with the rectum and anal canal as separated OARs (Rec-sep) and as one single OAR (Rec-tot). Two different treatment planning systems (TPS) were utilized. Influence on dose distributions to target and OARs was analyzed. ResultsResults from both TPS showed significantly increased doses to the distal rectum/anal canal for plans Rec-tot compared with Rec-sep in case of a distended rectum in the planning CT study: doses were increased by up to mean 31% (P=0.02) and 18% (P=0.03), respectively, in both TPS. For the patient with the largest rectum, the maximum dose increase was 61%. No significant differences in doses to target, bladder, femoral head and proximal rectum were seen. ConclusionsFor patients with a distended rectum in the planning CT, delineation of separated OARs for proximal rectum and distal rectum/anal canal resulted in superior dose distributions to the anorectal region and therefore, we recommend this as standard procedure for IMRT planning of prostate cancer.

Integral radiation dose to normal structures with conformal external beam radiation

This study was designed to evaluate the integral dose (ID) received by normal tissue from intensity-modulated radiotherapy (IMRT) for prostate cancer. Twenty-five radiation treatment plans including IMRT using a conventional linac with both 6 MV (6MV-IMRT) and 20 MV (20MV-IMRT), as well as three-dimensional conformal radiotherapy (3DCRT) using 6 MV (6MV-3DCRT) and 20 MV (20MV-3DCRT) and IMRT using tomotherapy (6MV) (Tomo-IMRT), were created for 5 patients with localized prostate cancer. The ID (mean dose x tissue volume) received by normal tissue (NTID) was calculated from dose-volume histograms. The 6MV-IMRT resulted in 5.0% lower NTID than 6MV-3DCRT; 20 MV beam plans resulted in 7.7%-11.2% lower NTID than 6MV-3DCRT. Tomo-IMRT NTID was comparable to 6MV-IMRT. Compared with 6MV-3DCRT, 6MV-IMRT reduced IDs to the rectal wall and penile bulb by 6.1% and 2.7%, respectively. Tomo-IMRT further reduced these IDs by 11.9% and 16.5%, respectively. The 20 MV did not reduce IDs to those structures. The difference in NTID between 3DCRT and IMRT is small. The 20 MV plans somewhat reduced NTID compared with 6 MV plans. The advantage of tomotherapy over conventional IMRT and 3DCRT for localized prostate cancer was demonstrated in regard to dose sparing of rectal wall and penile bulb while slightly decreasing NTID as compared with 6MV-3DCRT.

Comment by E. Weiss and C. F. Hess on G. M. Villeirs et al. Interobserver Delineation Variation Using CT versus Combined CT + MRI in Intensity-Modulated Radiotherapy for Prostate Cancer

Comment by E. Weiss and C. F. Hess on G. M. Villeirs et al. Interobserver Delineation Variation Using CT versus Combined CT + MRI in Intensity-Modulated Radiotherapy for Prostate Cancer

Integral Radiation Dose to Normal Structures with Conformal External Beam Radiation

Integral Radiation Dose to Normal Structures with Conformal External Beam Radiation

Interstitial High Dose Rate (HDR) Brachytherapy + IMRT vs. HDR Monotherapy for Early Stage Prostate Cancer

Purpose/Objective: Transrectal Ultrasound (TRUS) guided interstitial implant for prostate cancer using HDR + External Beam Radiation Therapy (EBRT) technique has been reported with results comparing favorably to surgery, Low Dose Rate (LDR) brachytherapy ± EBRT, EBRT, and Intensity Modulated Radiation Therapy (IMRT). We compare our results of HDR + IMRT vs. HDR monotherapy.

Quality of Life (QOL) Outcomes After Three-Dimensional Conformal Radiation Therapy (3D-CRT) and Intensity Modulated Radiation Therapy (IMRT) for Localized Prostate Cancer

Quality of Life (QOL) Outcomes After Three-Dimensional Conformal Radiation Therapy (3D-CRT) and Intensity Modulated Radiation Therapy (IMRT) for Localized Prostate Cancer

The calculated risk of fatal secondary malignancies from intensity-modulated radiation therapy

Out-of-field radiation doses to normal tissues may be associated with an increased risk of secondary malignancies, particularly in long-term survivors. Step-and-shoot intensity-modulated radiation therapy (IMRT), an increasingly popular treatment modality, yields higher out-of-field doses than do conventional treatments, because of an increase in required monitor units (beam-on time). We used published risk coefficients (NRCP Report 116) and out-of-field dose equivalents to multiple organ sites to estimate a conservative maximal risk of fatal secondary malignancy associated with 6 IMRT approaches and 1 conventional external-beam approach for prostate cancer. Depending on treatment energy, the IMRT treatments required 3.5-4.9 times as many monitor units to deliver as did the conventional treatment. The conservative maximum risk of fatal second malignancy was 1.7% for conventional radiation, 2.1% for IMRT using 10-MV X-rays, and 5.1% for IMRT using 18-MV X-rays. Intermediate risks were associated with IMRT using 6-MV X-rays: 2.9% for treatment with the Varian accelerator and 3.7% for treatment with the Siemens accelerator, as well as using 15-MV X-rays: 3.4% (Varian) and 4.0% (Siemens). The risk of fatal secondary malignancy differed substantially between IMRT and conventional radiation therapy for prostate cancer, as well as between different IMRT approaches. Perhaps this risk should be considered when choosing the optimal treatment technique and delivery system for patients who will undergo prostate radiation.

Australia‐wide comparison of intensity modulated radiation therapy prostate plans*

The aim of this study was to investigate the ability of Australian centres to produce high-dose intensity modulated radiation therapy (IMRT) prostate plans, and to compare the planning parameters and resultant dose distributions. Five Australian radiation therapy departments were invited to participate. Each centre received an identical 5 mm-slice CT data set complete with contours of the prostate, seminal vesicles, rectum, bladder, femoral heads and body outline. The planning team was asked to produce the best plan possible, using published Memorial Sloan-Kettering Cancer Centre prescription and dose constraints. Three centres submitted plans for evaluation. All plans covered the planning target volume adequately; however, only one plan met all the critical organ dose constraints. Although the planning parameters, beam arrangements and planning systems were different for each centre, the resulting plans were similar. In Australia, IMRT for prostate cancer is in the early stages of implementation, with routine use limited to a few centres.

An automatic CT-guided adaptive radiation therapy technique by online modification of multileaf collimator leaf positions for prostate cancer

To propose and evaluate online adaptive radiation therapy (ART) using in-room computed tomography (CT) imaging that detects changes in the target position and shape of the prostate and seminal vesicles (SVs) and then automatically modifies the multileaf collimator (MLC) leaf pairs in a slice-by-slice fashion. For intensity-modulated radiation therapy (IMRT) using a coplanar beam arrangement, each MLC leaf pair projects onto a specific anatomic slice. The proposed strategy assumes that shape deformation is a function of only the superior-inferior (SI) position. That is, there is no shape change within a CT slice, but each slice can be displaced in the anteroposterior (AP) or right-left (RL) direction relative to adjacent slices. First, global shifts (in SI, AP, and RL directions) were calculated by three-dimensional (3D) registration of the bulk of the prostate in the treatment planning CT images with the daily CT images taken immediately before treatment. Local shifts in the AP direction were then found using slice-by-slice registration, in which the CT slices were individually registered. The translational shift within a slice could then be projected to a translational shift in the position of the corresponding MLC leaf pair for each treatment segment for each gantry angle. Global shifts in the SI direction were accounted for by moving the open portal superiorly or inferiorly by an integral number of leaf pairs. The proposed slice-by-slice registration technique was tested by using daily CT images from 46 CT image sets (23 each from 2 patients) taken before the standard delivery of IMRT for prostate cancer. A dosimetric evaluation was carried out by using an 8-field IMRT plan. The shifts and shape change of the prostate and SVs could be separated into 3D global shifts in the RL, AP, and SI directions, plus local shifts in the AP direction, which were different for each CT slice. The MLC leaf positions were successfully modified to compensate for these global shifts and local shape variations. The ART method improved geometric coverage of the prostate and SVs compared with the couch-shift method, particularly for the superior part of the prostate and all the SVs, for which the interfraction shape change was the largest. The dosimetric comparison showed that the ART method covered the target better and reduced the rectal dose more than a simple couch-translation method. ART corrected for interfraction changes in the position and shape of the prostate and SVs and gave dose distributions that were considerably closer to the planned dose distributions than could be achieved with simple alignment strategies that neglect shape change. The ART proposed in this investigation requires neither contouring of the daily CT images nor extensive calculations; therefore, it may prove to be an effective and clinically practical solution to the problem of interfraction shape changes.

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

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. 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. 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). 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.

Magnetic resonance assessment of prostate localization variability in intensity-modulated radiotherapy for prostate cancer

To measure prostate motion with magnetic resonance imaging (MRI) during a course of intensity-modulated radiotherapy. Seven patients with prostate carcinoma were scanned supine on a 1.5-Tesla MRI system with weekly pretreatment and on-treatment HASTE T2-weighted images in 3 orthogonal planes. The bladder and rectal volumes and position of the prostatic midpoint (PMP) and margins relative to the bony pelvis were measured. All pretreatment positions were at the mean position as computed from the on-treatment scans in each patient. The PMP variability (given as 1 SD) in the anterior-posterior (AP), superior-inferior (SI), and right-left (RL) directions was 2.6, 2.4, and 1.0 mm, respectively. The largest variabilities occurred at the posterior (3.2 mm), superior (2.6 mm), and inferior (2.6 mm) margins. A strong correlation was found between large rectal volume (>95th percentile) and anterior PMP displacement. A weak correlation was found between bladder volume and superior PMP displacement. All pretreatment positions were representative of the subsequent on-treatment positions. A clinical target volume (CTV) expansion of 5.3 mm in any direction was sufficient to ascertain a 95% coverage of the CTV within the planning target volume (PTV), provided that a rectal suppository is administered to avoid rectal overdistension and that the patient has a comfortably filled bladder (<300 mL).

Intensity-modulated radiotherapy as primary treatment for prostate cancer: Acute toxicity in 114 patients

Introduction Dose escalation improves local control in prostate cancer. At Ghent University Hospital, intensity-modulated radiotherapy (IMRT) is used to increase the dose to the prostate and/or seminal vesicles. We report on acute toxicity in 114 patients who received IMRT for prostate cancer. Methods and materials Intensity-modulated radiotherapy was initiated after approval of our ethics committee. A class solution was used to plan all cases. Three beams (gantry 0°, 116°, and 244°) and anatomy-based segmentation were used to create an intensity-modulated dose distribution. Maximal rectal dose was set at 2 Gy per fraction. Detailed dose–volume histograms for all relevant structures were present. For all patients, we determined the pretreatment morbidity by a detailed preradiotherapy, in-house developed symptom scale. All patients were treated with 18 MV photons of an Elekta linear accelerator. Patients were seen on a weekly basis during treatment, and 1 month (M1) and 3 months (M3) thereafter. The registration of acute toxicity was standardized by a fixed questionnaire. The Radiation Therapy Oncology Group (RTOG) toxicity scale served as a basis, but additional symptoms, such as rectal blood loss, urgency, and incontinence, were scored as well. Results All 114 IMRT plans were delivered successfully without any interruption or technical problem. Daily treatment time was always less than 8 min and less than 6 min in 90% of the cases. Grade 1 and Grade 2 gastrointestinal (GI) toxicities were observed in 44% and 29% of the patients, respectively, during the whole period. If only the RTOG scale was used, Grade 1 and Grade 2 GI toxicities were noted in 39% and 27% of the patients, respectively, leaving 34% free of acute RTOG-scaled toxicity. Grade 3 genitourinary (GU) toxicity was seen in 8 patients (7%), all but 1 during treatment. Grade 2 and Grade 1 GU toxicities were seen in 36% and 47% of the patients, respectively, leaving only 10% free of acute GU toxicity. Discussion Anatomy-based IMRT to treat prostate cancer is incorporated into our daily routine without any problem. Acute toxicity is very low. Most of the recorded symptoms decrease over time, except for GI urgency and incontinence. The incorporation of additional symptoms makes the scoring more detailed.

Impact of high-dose radiation on Erectile Function in patients treated with intensity-modulated radiation therapy for prostate cancer

High-dose external beam radiation has improved outcomes significantly for prostate cancer. There is a concern that these higher doses may compromise quality of life (QOL) and be a factor in the development of post-treatment erectile dysfunction (ED). There are reports that intensity-modulated radiation therapy (IMRT) can reduce the dose received by erectile tissues. In this study, we evaluated the impact of IMRT for localized adenocarcinoma of the prostate (CaP) on sexual function. The effect of sildenafil on ED after IMRT was also studied. The records of 194 patients who underwent IMRT for CaP from 3/98 to 12/03 were retrospectively reviewed. Patients were treated to 70 to 78 Gy at 2 to 2.5 Gy/fx. Eighty-six percent were treated to 70 Gy at 2.5 Gy/fx. Potency was assessed with the self-administered International Index of Erectile Function (IIEF5) questionnaire before and after IMRT. This questionnaire consists of 5 questions regarding sexual function with a maximum score of 25 points. ED was defined as a decrease of 5 points or more between the pretreatment and post-IMRT IIEF5 scores. Penile bulb dosimetry (total dose, mean dose, D50, V10, V20, V30, V40, V50, V60, V70, V80, V90, and V100) was available for 87 (54%) patients. To determine if penile bulb dose contributed to ED, penile bulb dosimetry was compared to a decrease in IIEF5 score of 5 points or more using logistic regression. One hundred sixty-five (85%) patients were sexually active before the start of treatment. Four (2%) of these patients were not sexually active after treatment. Follow-up IIEF5 scores of the remaining 161 patients were used to assess sexual function. Median time to IIEF5 assessment was 24.3 months (range: 0 to 63.6). For the 66 (41%) patients who reported using sildenafil at the time of the post-treatment IIEF5 questionnaire, an IIEF5 score with and without the use of sildenafil was recorded. A paired t-test was done to see if there was an improvement in sexual function with the aid of sildenafil. Sixty-six (41%) out of 161 patients had a decrease in IIEF5 of 5 points or more. The rate of ED for time intervals less than 12 months, 12 to less than 24 months, 24 to less than 36 months, and 36 months or more were 12%, 52%, 44%, and 49% respectively. For the patients who took sildenafil for ED, the mean IIEF5 score without sildenafil was 9.7 vs. 16.4 with the use of sildenafil (p < 0.0001). The use of sildenafil increased sexual function by 5 points or more in 73% of the patients. The improvement in ED with the use of sildenafil for time intervals less than 12 months, 12 to less than 24 months, 24 to less than 36 months, and 36 months or more were 70%, 82%, 72%, and 63% respectively. The median penile bulb volume was 7.0 cc, median mean penile bulb dose was 40.4 Gy, and median D50 was 41.7 Gy. The median V10 was 93.4%, V20 was 76.1%, V30 was 66.3%, V40 was 60.4%, V50 was 55.7%, V60 was 49.2%, V70 was 43.7%, V80 was 37.5%, V90 was 29.2%, and V100 was 17.8%. None of the dosimetric variables examined (total dose, penile bulb mean dose, D50, V10, V20, V30, V40, V50, V60, V70, V80, V90, and V100) predicted for ED after IMRT. IMRT is being used increasingly to escalate the dose to the target and reduce the dose to the surrounding normal tissues in prostate cancer. To date, this is the first analysis of sexual QOL in patients treated with IMRT for prostate cancer. The preservation of sexual function is not compromised when higher doses are delivered with IMRT, validating the ability of IMRT to limit erectile tissue dose. Sildenafil significantly improved sexual function for those with ED, even after long time intervals from IMRT

Field-based verified IMRT for prostate cancer using a newly developed dual kilovoltage (KV) on-board imager-evaluation of inter/intra-fraction motion and early clinical outcomes

A new on-board image-guided system has been design and implemented for the treatment of prostate cancer with Intensity Modulated Radiotherapy (IMRT). The aim of this study was to investigate the magnitude of organ motion uncertainty at setup and at intra-fraction. In addition, the effect of an intra-field reposition method and clinical outcomes were assessed.

Field-based verified IMRT for prostate cancer using a newly developed dual kilovoltage (KV) on-board imager-evaluation of inter/intra-fraction motion and early clinical outcomes

Field-based verified IMRT for prostate cancer using a newly developed dual kilovoltage (KV) on-board imager-evaluation of inter/intra-fraction motion and early clinical outcomes

Evaluation of a contour-alignment technique for CT-guided prostate radiotherapy: an intra- and interobserver study

Purpose The recent introduction of integrated CT/linear accelerator systems may mean that daily CT localization can become a reality in the clinic, possibly allowing further dose escalation to the prostate while limiting unwanted doses to the rectum and bladder. However, the implementation of CT localization is currently impeded by the lack of precise and robust techniques to align the treatment plan with the daily CT images. The purpose of this study was to evaluate a manual alignment technique, in which the gross target volume contours are overlaid on the daily CT images and then shifted to match the structures visible in the images. Methods and materials A total of 28 CT image sets were taken before the standard delivery of intensity-modulated radiotherapy for prostate cancer for 2 patients. Seven observers (four radiation oncologists and three medical physicists) manually shifted the gross target volume contours from the treatment plan to best match the daily CT images. One observer repeated the process 1 week later to evaluate intraobserver variations. The experiment was then repeated, but the CT images from the original treatment plan were used as a reference to reduce interobserver uncertainty when aligning the contours. The shifts in prostate position found by different observers, both with and without reference data, were evaluated using a factorial analysis of variance to determine the standard errors of measurement for the intra- and interobserver uncertainty (SEM intra and SEM inter, respectively). The differences in the SEM for the two groups of observers (radiation oncologists and medical physicists), the two alignment techniques (with and without reference information), and the two patients were evaluated using the t test at 90% confidence levels. Results With no reference information, the SEM inter using one patient data set (Patient 1) was 0.8 mm, 2.0 mm, and 2.2 mm in the right–left (RL), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. The use of the treatment plan as a reference reduced the SEM inter to 0.7 mm, 1.0 mm, and 1.6 mm in the RL, AP, and SI directions, respectively. In Patient 2, localization of the prostate was more difficult; the best SEM inter achieved with this patient was 0.8 mm, 1.9 mm, and 2.0 mm in the RL, AP, and SI directions, respectively. The SEM intra values with Patient 1 were also slightly better than with Patient 2. When reference data were used, the SEM intra value was 0.5 mm, 0.7 mm, and 0.5 mm for Patient 1 and 0.6 mm, 1.0 mm, and 0.7 mm for Patient 2 in the RL, AP, and SI directions, respectively. Despite the larger than expected interobserver variation reported here, the SEM inter was smaller than the typical day-to-day variation in prostate position. The contour alignment technique may still be useful to aid daily prostate localization or in a correction scheme to minimize the effect of target positional error. Conclusion The interobserver uncertainties associated with aligning the gross target volume contours with daily CT images were sufficiently small that this method may be used for daily CT localization of the prostate. The use of a reference image is important to improve the consistency among different users in this technique.

Stereotactic IMRT for prostate cancer: Setup accuracy of a new stereotactic body localization system

The purpose of this work is to prospectively assess the setup accuracy that can be achieved with a stereotactic body localizer (SBL) in immobilizing patients for stereotactic intensity-modulated radiotherapy (IMRT) for prostate cancer. By quantifying this important factor and target mobility in the SBL, we expect to provide a guideline for selecting planning target volume margins for stereotactic treatment planning. We analyzed data from 40 computed tomography (CT) studies (with slice thickness of 3 mm) involving 10 patients with prostate cancer. Each patient had four sets of CT scans during the course of radiotherapy. For the purpose of this study, all four sets of CT scans were obtained with the patients immobilized in a customized body pillow formed by vacuum suction. Unlike other immobilization devices, this system consists not only of a customized body pillow, but also of a fixation sheet used to suppress patient respiratory motion, a stereotactic body frame to provide stereotaxy, and a carbon fiber base board to which both the body cushion and the frame are affixed. We identified four bony landmarks and measured their coordinates in the stereotactic body frame on each set of CT scans. The displacements of the bony landmarks from their corresponding positions on the simulation scan (first CT scan) were analyzed in three dimensions in terms of overall, systematic, and random categories. The initial planned isocenter was also marked on the patients' skin with fiducials for each CT study. The distance from each bony landmark to the fiducial-based isocenter was measured and compared among the four sets of CT scans. The deviations in distances were also compared to those measured from the landmarks to the stereotactic frame center, in order to determine the effectiveness of the rigid body frame in positioning patients with prostate cancer. Target inter-fraction motion in this system was also studied for five patients by measuring the deviations in distances from the target geometric center to the bony landmarks. Our results showed that the overall setup accuracy had standard deviations (SDs) of 2.58 mm, 2.41 mm, and 3.51 mm in lateral (LAT), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. The random component had SDs of 1.72 mm, 2.06 mm, and 2.79 mm, and the systematic component showed SDs of 0.92 mm, –0.27 mm, and 0.90 mm in these three directions. In terms of three-dimensional vector, the mean displacement over 116 measurements was 3.0 mm with an SD of 1.29 mm. Compared to the rigid reference, the skin-mark-based reference was less reliable for patient repositioning in terms of reproducing known bony landmark positions. The mean target mobility relative to the bony landmarks was , and in the AP, LAT, and SI directions, respectively. In conclusion, the body immobilization system has the ability to immobilize prostate cancer patients with satisfactory setup accuracy for fractionated extracranial stereotactic radiotherapy. A rigid frame system serves as a reliable alignment reference in terms of repositioning patients into the planning position, while skin-based reference showed larger deviations in repositioning patients. PACS number: 87.53Ly