Organ Movements and Dose Exposures in Teletherapy of Prostate Cancer using a Rectal Balloon

BACKGROUND AND PURPOSE: During radiotherapy of localized prostate cancer, organ movements for the dose exposure of organs at risk like rectum, urinary bladder and urethra play, inter alia, a significant role. One possibility of internal organ stabilizing is offered by the usage of a rectal balloon during radiotherapy. The influence on organ movements and dose allocation of the organs at risk is unknown. PATIENTS AND METHODS: Twelve patients (Table 1) were characterized based on planning-CT's regarding organ movements and organ doses using a rectal balloon, inflated with 0 ml and 60 ml air. For the determination of the organ doses, three-dimensional conformal radiation plans (3-field-pelvis box) with a cumulative dose of 59.4 Gy were created, and the dose-volume-histograms for the anterior rectal wall, the posterior rectal wall, the rectal mucosa, the whole rectum, as well as the urinary bladder were compared (Figures 1 and 2). RESULTS: The application of a 60 ml air-filled rectal balloon during each fraction of teletherapy led to significant organ movements of the anterior and posterior rectal wall and to a reduction of the transversal prostate diameter, as well as to a changed organ dose exposure of the organs at risk. A ventral shift of the anterior rectal wall (maximum 0.8 cm, mean 0.4 cm) was shown, as well as a dorsal shift of the posterior rectal wall (maximum 1.2 cm, mean 0.7 cm), associated with a transversal prostate diameter decrease (maximum 0.8 cm, mean 0.3 cm) (Table 2, Figure 3). The organ dose of the anterior rectal wall increased significantly (maximum 1.3 Gy, mean 0.5 Gy) during application of a rectal balloon, the one of the posterior rectal wall decreased significantly (maximum 18.6 Gy, mean 6.5 Gy). Related to the entire rectal mucosa and the rectum as a complete organ, a decrease of the maximum doses was shown (rectal mucosa: maximum 9.1 Gy, mean 3.0 Gy; rectum: maximum 9.4 Gy, mean 3.7 Gy). The organ dose of the urinary bladder did not show significant changes (Tables 3 and 4, Figures 4 to 7). CONCLUSION: The application of a rectal balloon in teletherapy of localized prostate cancer leads to significantly changed dose exposition of organs at risk. The decreased dose exposure of the posterior rectal wall and the rectal mucosa is opposed by the higher organ dose of the anterior rectal wall. It has to be shown weather documented organ dose exposure is associated with short and long-term consequences.     

Rectal wall sparing by dosimetric effect of rectal balloon used during intensity-modulated radiation therapy (IMRT) for prostate cancer.

The use of an air-filled rectal balloon has been shown to decrease prostate motion during prostate radiotherapy. However, the perturbation of radiation dose near the air-tissue interfaces has raised clinical concerns of underdosing the prostate gland. The aim of this study was to investigate the dosimetric effects of an air-filled rectal balloon on the rectal wall/mucosa and prostate gland. Clinical rectal toxicity and dose-volume histogram (DVH) were also assessed to evaluate for any correlation. A film phantom was constructed to simulate the 4-cm diameter air cavity created by a rectal balloon. Kodak XV2 films were utilized to measure and compare dose distribution with and without air cavity. To study the effect in a typical clinical situation, the phantom was computed tomography (CT) scanned on a Siemens DR CT scanner for intensity-modulated radiation therapy (IMRT) treatment planning. A target object was drawn on the phantom CT images to simulate the treatment of prostate cancer. Because patients were treated in prone position, the air cavity was situated superiorly to the target. The treatment used a serial tomotherapy technique with the Multivane Intensity Modulating Collimator (MIMiC) in arc treatment mode. Rectal toxicity was assessed in 116 patients treated with IMRT to a mean dose of 76 Gy over 35 fractions (2.17-Gy fraction size). They were treated in the prone position, immobilized using a Vac-Loktrade mark bag and carrier-box system. Rectal balloon inflated with 100 cc of air was used for prostate gland immobilization during daily treatment. Rectal toxicity was assessed using modifications of the Radiation Therapy Oncology Group (RTOG) and late effects Normal Tissue Task Force (LENT) scales systems. DVH of the rectum was also evaluated. From film dosimetry, there was a dose reduction at the distal air-tissue interface as much as 60% compared with the same geometry without the air cavity for 15-MV photon beam and 2x2-cm field size. The dose beyond the interface recovered quickly and the dose reductions due to air cavity were 50%, 28%, 11%, and 1% at 2, 5, 10, and 15 mm, respectively, from the distal air-tissue interface. Evaluating the dose profiles of the more clinically relevant situation revealed the dose at air-tissue interface was approximately 15% lower in comparison to that without an air cavity. The dose built up rapidly so that at 1 and 2 mm, there was only an 8% and 5% differential, respectively. The dosimetric coverage at the depth of the posterior prostate wall was essentially equal with or without the air cavity. The median follow-up was 31.3 months. Rectal toxicity profile was very favorable: 81% (94/116) patients had no rectal complaint while 10.3% (12/116), 6.9% (8/116), and 1.7% (2/116) had grade 1, 2, and 3 toxicity, respectively. There was no grade 4 rectal toxicity. DVH analysis revealed that none of the patients had more than 25% of the rectum receiving 70 Gy or greater. Rectal balloon has rendered anterior rectal wall sparing by its dosimetric effects. In addition, it has reduced rectal volume, especially posterior and lateral rectal wall receiving high-dose radiation by rectal wall distension. Both factors may have contributed to decreased rectal toxicity achieved by IMRT despite dose escalation and higher than conventional fraction size. The findings have clinical significance for future very high-dose escalation trials whereby radiation proctitis is a major limiting factor.     

Dose-volume and radiobiological dependence on the calculation grid size in prostate VMAT planning

This study evaluated the effects of dose-volume and radiobiological dependence on the calculation grid size in prostate volumetric-modulated arc therapy (VMAT) planning. Ten patients with prostate cancer were selected for this retrospective treatment planning study. Prostate VMAT plans were created for the patients using the 6 MV photon beam produced by a Varian TrueBEAM linac with the calculation grid size equal to 1, 2, 2.5, 3, 4, and 5?mm. Dose-volume histograms (DVHs) of targets and organs at risk were generated for different grid sizes. We calculated the radiobiological parameters of the tumor control probability (TCP) of clinical target volume (CTV) and planning target volume (PTV), and the normal tissue complication probability (NTCP) of organs at risk (rectal wall, rectum, bladder wall, bladder, left femur, and right femur). The homogeneity, conformity, and gradient indexes of CTV and PTV were calculated for different grid sizes. The TCP of PTV was found decreasing with a rate of 0.06%/mm when the calculation grid size increased from 1 to 5?mm. On the other hand, both NTCPs of rectal wall and rectum were found decreasing with rates of 0.03%/mm and 0.05%/mm, respectively, with an increase of grid size. The homogeneity index of PTV increased with a rate of 0.57/mm of the calculation grid size, whereas the conformity index of PTV decreased with a rate of 0.0075/mm. The gradient index of PTV was found increasing with a rate equal to 0.05/mm. In prostate VMAT planning, variations of dose-volume and radiobiological parameters with calculation grid size on PTV, rectal wall, and rectum were more significant than those of CTV and other organs at risk such as bladder wall, bladder, and femurs. Results in this study are important in the treatment planning quality assurance when the calculation grid size is varied to compromise a shorter dose computing time.     

Rectum dose reduction and individual treatment plan optimization for high-dose-rate prostate brachytherapy

PURPOSE: Different doses and techniques used in high-dose-rate (HDR) prostate brachytherapy make it difficult to define universal quality parameters. The aim of this study was to develop individual, objective parameters for the evaluation of an HDR brachytherapy plan for prostate radiation. METHODS: Fifty-three patients who received an HDR brachytherapy boost after external radiation were analyzed in this study. Brachytherapy was performed with a (192)Ir source after ultrasound-guided, transperineal metal needle application followed by removal of the ultrasound probe to reduce organ dose levels at the anterior rectum wall. The rectum and prostate locations as well as the dose at the anterior rectum wall were estimated under the anatomical conditions of HDR prostate brachytherapy. The doses at the organs at risk (rectum and urethra) were analyzed for several parameters, which were compared to values of former patients before the start of treatment. In cases of major deviations, modifications of the treatment plan were performed before starting the treatment. RESULTS: Deflating of the water balloon led to an increase of the space between the anterior rectal wall and the dorsal margin of the prostate (mean, 6mm; 1-10mm). The dose of the introduced "virtual rectum," represented by the ventral surface of the ultrasound probe, in the treatment plan correlated to dose measurements in the rectum. Pretreatment evaluation and comparison of the established individual quality parameters led in two cases to a treatment plan modification. CONCLUSIONS: This method allows a fast and objective individual brachytherapy treatment plan evaluation and improvement.     

The Impact of Varying Volumes in Rectal Balloons on Rectal Dose Sparing in Conformal Radiation Therapy of Prostate Cancer

BACKGROUND AND PURPOSE: The use of a rectal balloon leads to a protection of the posterior rectal wall in irradiation of prostate cancer. The purpose of this analysis was to quantitatively assess the optimal volume in rectal balloons concerning rectal dose sparing in different clinical target volumes (CTVs) in prostate cancer irradiation. PATIENTS AND METHODS: 14 patients with localized prostate cancer undergoing external beam radiotherapy were investigated. The prostate, the entire and the proximal seminal vesicles were delineated as CTV. Treatment plans without a rectal balloon and with a rectal balloon inflated with 40 ml (six patients) or 60 ml air (eight patients) were generated for each CTV and compared concerning rectal dose volume histograms. RESULTS: The use of a rectal balloon filled with 40 ml air led to no significant advantage in radiation exposure of the rectal wall in all CTVs. The use of a rectal balloon filled with 60 ml air resulted in a significant decrease of the exposed rectal wall volume in all CTVs with a reduced estimated risk for chronic toxicity in case of inclusion of the proximal or entire seminal vesicles into the CTV. CONCLUSION: The use of a rectal balloon filled with 60 ml air led to a significantly decreased proportion of the irradiated rectal wall for all CTVs. This volume filled in rectal balloons is therefore recommended for use. In case of irradiation of the prostate without the seminal vesicles, the use of a rectal balloon should be considered carefully concerning the patients' imaginable discomfort using a rectal balloon and a questionable advantage concerning the estimated risk for chronic toxicity.     

PEG spacer gel and adaptive planning vs single plan in external prostate radiotherapy—clinical dosimetry evaluation

Objective:

Spacer gel is used to reduce the rectal dose in prostate radiotherapy. It is injected to increase the distance between the prostate and rectum. During the course of external radiotherapy treatment, physiological changes in rectal volume exist. When using polyethylene glycol material, such as DuraSeal^® (Covidien, Mansfield, MA), gel resorption also occurs. Together, these factors alter the original dose plan distribution.

Methods:

External dose planning and calculations were simulated using images acquired from 10 patients who were treated with brachytherapy and gel. The CT series was taken relative to gel injection: pre 1 day, post 1 day, post 1 month and post 2 months. Adaptive planning was compared with a single plan.

Results:

Adaptive planning shows better results compared with the single plan used in the total treatment course; however, the effect is minor.

Conclusion:

Gel usage is clearly favourable to rectal DVH. Using adaptive planning with gel improves rectal DVH but is not necessary according to this study.

Advances in knowledge:

Spacer gel is used in prostate radiotherapy to increase distance between the prostate and the rectum, thus reducing the rectal doses. During the treatment course, gel resorption exists which affects the rectal doses. The usefulness of adaptive planning to compensate this resorption effect has not been studied before.     

Anatomic Variations Due to Radical Prostatectomy

BACKGROUND AND PURPOSE: A quantitative estimate of the impact of prostatectomy on pelvic anatomy is unavailable, even if it would be an important prerequisite for a precise definition of clinical target volume (CTV) in post-prostatectomy radiotherapy. The purpose of this study was to investigate the impact of prostatectomy on the definition of CTV, on the position of bladder and rectum and their implications for three-dimensional conformal radiotherapy (3-D CRT). PATIENTS AND METHODS: Six patients eligible for radical retropubic prostatectomy were considered. Each patient underwent a planning CT between 1 week and 1 month before surgery (CTpre), and then CT was repeated in the same positioning 1-2 months after surgery (CTpost). For each patient the CT(pre/post) scans were matched; rectum, bladder and CTV were contoured on both CT scans for each patient by one observer. Two different CTVs were contoured: CTV1: prostate + seminal vesicles in CTpre; prostate + seminal vesicles surgical bed in CTpost; CTV2: prostate in CTpre; prostate surgical bed in CT(post). After image registration, the contours of rectum, bladder and CTV1/2 drawn on CTpost were transferred on CTpre. The corresponding planning target volumes (PTVs) were generated, and for each PTV, a conformal four field technique using 18-MV X-rays was planned. The volumes of CTV1, CTV2, PTV1, PTV2, rectum and bladder pre- and post-surgery were compared. Differences in 3-D position of these structures before and after surgery were analyzed by beam's eye view (BEV) images. Pre- and post-surgery dose-volume histograms (DVHs) of rectum and bladder were compared together with the fraction of rectum/bladder receiving at least 95% of the ICRU dose (V95), the treated volume (TV, body included in the 95% isodose) and the irradiated volume (IV, body included in the 50% isodose). RESULTS: For both CTV1 and CTV2, the volumes were significantly reduced after prostatectomy (average reduction around 30 cm3 for both; range 0-60 cm3). This reduction was mainly due to a more caudal definition of the cranial edge of CTV after prostatectomy (average difference for CTV2: 1.5 cm; range 0-2.5 cm). Concerning the bladder, a systematic posterior shift of the bladder base (average: 1.5 cm) was found and was correlated with a significant reduction of V95 for bladder (around 10 cm3; p = 0.03). V95 of the rectum, TV and IV also resulted to be significantly lower after surgery. The average reduction of V95 for the rectum was relatively small (2.5 cm3 of rectal wall). CONCLUSION: The impact of prostatectomy on CTV definition is high. A significant reduction of CTV, PTV, TV and IV may be expected after surgery with a consequent reduction of the portions of rectum/bladder irradiated with adjuvant radiotherapy.     

Dynamics of rectal balloon implant shrinkage in prostate VMAT

Purpose

To assess the effect of a shrinking rectal balloon implant (RBI) on the anorectal dose and complication risk during the course of moderately hypofractionated prostate radiotherapy.

Methods

In 15 patients with localized prostate cancer, an RBI was implanted. A weekly kilovolt cone-beam computed tomography (CBCT) scan was acquired to measure the dynamics of RBI volume and prostate–rectum separation. The absolute anorectal volume encompassed by the 2?Gy equieffective 75?Gy isodose (V_75Gy) was recalculated as well as the mean anorectal dose. The increase in estimated risk of grade 2–3 late rectal bleeding (LRB) between the start and end of treatment was predicted using nomograms. The observed acute and late toxicities were evaluated.

Results

A significant shrinkage of RBI volumes was observed, with an average volume of 70.4% of baseline at the end of the treatment. Although the prostate–rectum separation significantly decreased over time, it remained at least 1?cm. No significant increase in V_75Gy of the anorectum was observed, except in one patient whose RBI had completely deflated in the third week of treatment. No correlation between mean anorectal dose and balloon deflation was found. The increase in predicted LRB risk was not significant, except in the one patient whose RBI completely deflated. The observed toxicities confirmed these findings.

Conclusions

Despite significant decrease in RBI volume the high-dose rectal volume and the predicted LRB risk were unaffected due to a persistent spacing between the prostate and the anterior rectal wall.

     

Influence of Organ at Risk Definition on Rectal Dose-Volume Histograms in Patients with Prostate Cancer Undergoing External-Beam Radiotherapy

PURPOSE: To evaluate rectal dose-volume relations during three-dimensional conformal radiotherapy of patients with prostate cancer by means of different rectal volume contours. PATIENTS AND METHODS: 55 patients with prostate cancer underwent three-dimensional conformal external-beam radiotherapy. Rectal dose-volume histograms were calculated for four separately contoured rectal volumes in all patients resulting in four groups. In group 1 the outer rectal wall was contoured two CT slices above and below the planning target volume. The rectal contour of group 2 was drawn from the anal verge up to the sigmoid. Furthermore, the posterior half of the rectum was contoured for both volumes mentioned above (groups 1a and 2a). Statistical analysis was then performed using nonparametric Wilcoxon tests. RESULTS: The mean target dose was 72.9 Gy (standard deviation [SD] +/- 2.1 Gy). The minimum target dose was 70.2 Gy. Mean rectum dose (+/- SD) over all patients was 50.7 Gy (+/- 4.6 Gy), 45.2 Gy (+/- 5.4 Gy), 43.2 Gy (+/- 4.2 Gy), and 38.7 Gy (+/- 5.5 Gy) for group 1, 2, 1a, and 2a, respectively. The corresponding volumes receiving > or = 70 Gy for groups 1 and 2 were 14.0% (+/- 5.3%) and 11.9% (+/- 4.5%). These differences were statistically significant. Comparison of minimum and mean rectal dose also revealed a statistically significant difference toward higher doses in groups 1 and 1a (p < 0.001). Maximum rectal doses for groups 1 and 2 as well as for groups 1a and 2a revealed no statistically significant difference (p = 1.0). CONCLUSION: Data from the literature on normal-tissue complication probability (rectal bleeding) refer to different rectal contours. When applying dose restrictions to the rectum, contouring becomes a significant factor that determines the risk of rectal toxicity. The results of this study show that different ways of rectal contouring significantly influence doses to the rectum. The influence of organ at risk contouring should be considered thoroughly in conformal radiotherapy of prostate cancer patients, especially in dose escalation studies. It is recommended to calculate the doses for absolute rectal volumes and correlate these data with toxicity in order to be able to achieve comparable results among different institutions.     

Influence of Rectum Delineation (Rectal Volume vs. Rectal Wall) on IMRT Treatment Planning of the Prostate

PURPOSE: To evaluate the delineation of either the rectal volume (RV) or the rectal wall (RW) in intensity-modulated radiotherapy (IMRT) for prostate cancer: influence on dose distribution to the targets and organs at risk (OARs) was investigated. MATERIAL AND METHODS: For ten patients with localized prostate cancer IMRT treatment plans were generated with the RV, wall including the filling, and the RW without the lumen as OAR (plan-RV and plan-RW), respectively. Two different IMRT treatment- planning systems (TPS) were utilized. The influence on target coverage and sparing of OARs was investigated. RESULTS: No influence was seen on target coverage and sparing of the bladder and femoral heads. Doses to the RV were significantly reduced in plan-RV for all evaluated dose levels: maximum 26% and 17%, respectively, in both TPS. The dose distribution to the RW was not significantly different between plan-RV and plan-RW. CONCLUSION: The different delineation of the OAR rectum significantly affected the inverse IMRT treatment-planning process. The use of the RV as OAR resulted in improved dose distributions to the RV. Therefore, it is suggested using the RV as OAR in IMRT treatment planning of the prostate.     

Feasibility of online adaptive HDR prostate brachytherapy: A novel treatment concept

PurposeThe purpose of this study was to determine the feasibility of online adaptive transrectal ultrasound (TRUS)-based high-dose-rate prostate brachytherapy (HDRPBT) through retrospective simulation of source positioning and catheter swap errors on patient treatment plans.MethodSource positioning errors (catheter shifts in 1 mm increments in the cranial/caudal, anterior/posterior, and medial/lateral directions up to ±6 mm) and catheter swap errors (between the most and least heavily weighted) were introduced retrospectively into DICOM treatment plans of 20 patients that previously received TRUS HDRPBT. Dose volume histogram (DVH) indices were monitored as errors were introduced sequentially into individual catheters, simulating potential errors throughout treatment. Whenever DVH indices were outside institution thresholds: prostate V100% <95%, urethra D0.1cc >118% and rectum Dmax >80%, the plan was adapted using remaining catheters (i.e., simulating previous catheters as previously delivered). The final DVH indices were recorded.ResultsProstate coverage (V100% >95%) could be maintained for source position errors up to 6 mm through online plan adaptation. The source position error at which the urethra D0.1cc and rectum Dmax was able to return to clinically acceptable levels using online adaptation varied between 6 mm to 1 mm, depending on the direction of the source position error and patient anatomy. After introduction of catheter swap errors to patient plans, prostate V100% was recoverable using online adaptation to near original plan characteristics. Urethra D0.1cc and rectum Dmax showed less recoverability.ConclusionOnline adaptive HDRPBT maintains the prostate V100% to clinically acceptable values for majority of directional shifts. However, the current online adaptive method may not correct for source position errors near organs at risk.     

Evaluation of the ITV-margin and variables affecting bladder and mesorectal deformation during long course neoadjuvant radiotherapy for rectal cancer

Internal target volume (ITV) margins were estimated by evaluating the movement of mesorectum and bladder during neoadjuvant long-course radiation therapy (RT) for rectal cancer. In this prospective study, 23 patients with rectal cancer had planning CT (pCT) and weekly cone beam CT (CBCT) in supine position during preoperative long-course RT. Mesorectal wall motion was analyzed based on the coordinates of the most anterior, posterior, left and right points on the pCT and CBCT. Overlap volume (OV) between the pCT bladder and CBCT mesorectum was generated. Variables that might affect relative bladder volume (ratio of CBCT to pCT bladder volumes), anterior mesorectal wall position, and OV were studied. ITV margins were also calculated. In females, smaller OV and less movement of the upper anterior mesorectal wall were identified, suggesting smaller ITV margins might be required compared to males. The relative bladder volume did not change significantly over time and was correlated with OV: the larger the relative bladder volume, the less the OV. ITV margin of 0.8 to 1.1 cm in right-left direction is satisfactory. Posteriorly, only 8 to 9 mm margin is required for upper and mid rectal regions but double of this is required for inferior third. Anteriorly, 1.3 cm margin is adequate for lower and mid rectal regions and 2.4 cm is required superiorly. An anisotropic ITV expansion of clinical target volume (CTV) for rectal cancer radiotherapy contouring provides a robust method to encompass the deformation of bladder and mesorectum. The ITV margin should take into account sex and distance from the anal verge.     

Ultrasonography of seminal vesicles by the suprapubic approach. Value of their study in prostatic pathology

The transabdominal suprapubic study of the seminal vesicles was realised in 162 cases. Their relationship with the posterior bladder wall was examined. The normal distance between distal extremity of seminal vesicles-bladder wall measures 5 mm, and the distance between proximal extremity of seminal vesicles-bladder wall measures 7 mm. In case of benign hypertrophy, the seminal vesicles are pressed back and distance proximal extremity of seminal vesicles-bladder wall gets greater than 15 mm. In malignant hypertrophy of the posterior prostatic lobe, the position of the seminal vesicles does not change although the prostate has an increased volume.     

3.0T磁共振IVIM-DWI参数与直肠中分化腺癌T分期相关性的初步研究

目的：探讨3.0T MR 体素内不相干运动-扩散加权成像(IVIM-DWI)参数与直肠中分化腺癌 T 分期的关系。方法收集37例直肠中分化腺癌患者的直肠 MRI 图像(包括 IVIM-DWI 序列)及临床资料;根据肿瘤是否突破固有肌层,将 T 分期分为未突破组(T1、T2期)和突破组(T3、T4期);测量直肠癌与正常直肠壁真实扩散系数(D)、假扩散系数(D?)、灌注分数(f)和表观扩散系数(ADC)值,比较直肠癌与正常直肠壁,不同 T 分期,不同分组直肠癌各参数值之间的差异,分析 IVIM-DWI 参数和 ADC 值与直肠中分化腺癌 T 分期的关系。结果直肠癌的 D、D?、f 和 ADC 值均低于正常直肠壁,其中 D、f 和 ADC 值差异有统计学意义(P <0.05);不同 T 分期肿瘤 D 和 D?值的差异有统计学意义,LSD 法两两比较显示 T1期与 T4期,T3期与 T4期肿瘤的 D?值差异有统计学意义(P =0.017、0.003),T2期与 T3期肿瘤 D 值差异有统计学意义(P =0.005);未突破组与突破组肿瘤 D 值分别约(0.93±0.16)×10-3 mm2/s、(0.77±0.19)×10-3 mm2/s,D?值分别约(12.6±2.44)×10-3 mm2/s、(12.3±3.49)×10-3 mm2/s,f值分别约(27.1±2.94)%、(24.6±4.13)%,ADC 值分别约(0.95±0.09)×10-3 mm2/s、(0.87±0.12)×10-3 mm2/s,其中 D 值的差异有统计学意义(t=2.512,P =0.017)。结论IVIM-DWI 参数和 ADC 值能够区分直肠癌和正常直肠壁,D 值可以辅助鉴别直肠癌是否突破固有肌层。     

Intra-fractional uncertainties in image-guided intensity-modulated radiotherapy (IMRT) of prostate cancer

Purpose

To evaluate intra-fractional uncertainties during intensity-modulated radiotherapy (IMRT) of prostate cancer.

Patients and Methods

During IMRT of 21 consecutive patients, kilovolt (kV) cone-beam computed tomography (CBCT) images were acquired prior to and immediately after treatment: a total of 252 treatment fractions with 504 CBCT studies were basis of this analysis. The prostate position in anterior-posterior (AP) direction was determined using contour matching; patient set-up based on the pelvic bony anatomy was evaluated using automatic image registration. Internal variability of the prostate position was the difference between absolute prostate and patient position errors. Intra-fractional changes of prostate position, patient position, rectal distension in AP direction and bladder volume were analyzed.

Results

With a median treatment time of 16 min, intra-fractional drifts of the prostate were > 5 mm in 12% of all fractions and a margin of 6 mm was calculated for compensation of this uncertainty. Mobility of the prostate was independent from the bony anatomy with poor correlation between absolute prostate motion and motion of the bony anatomy (R² = 0.24). A systematic increase of bladder filling by 41 ccm on average was observed; however, these changes did not influence the prostate position. Small variations of the prostate position occurred independently from intra-fractional changes of the rectal distension; a weak correlation between large internal prostate motion and changes of the rectal volume was observed (R² = 0.55).

Conclusion

Clinically significant intra-fractional changes of the prostate position were observed and margins of 6 mm were calculated for this intra-fractional uncertainty. Repeated or continuous verification of the prostate position may allow further margin reduction.     

Einfluss von bildgestützter translatorischer Isozentrumskorrektur auf die Dosisverteilung bei 3-D-Konformationsbestrahlung der Prostata

BACKGROUND AND PURPOSE: Interfractional prostate motion during radiotherapy due to variation in rectal distension can have negative consequences. The authors investigated the dosimetric consequences of a linear translational position correction based on image guidance when a three-dimensional conformal treatment technique was used. MATERIAL AND METHODS: Planning CTs of seven patients with empty and distended rectum were analyzed. A reference plan for the planning target volume (PTV) and the boost were calculated on the CT dataset with the empty rectum with a standard four-field technique. The treatment plan was transferred to the CT with the distended rectum for an uncorrected setup (referenced to bony anatomy) and a corrected setup after position correction of the isocenter. The dosimetric consequences were analyzed. RESULTS: Organ motion decreased the coverage of the prostate by the 95% isodose during simulated single treatment fractions by up to -21.0 percentage points (%-p; boost plan) and by up to -14.9%-p for the seminal vesicles (PTV plan). The mean rectum dose increased by up to 18.3%-p (PTV plan). Linear translational correction (mean 6.4 +/- 3.4 mm, maximum 10.8 mm) increased the coverage of the prostate by the 95% isodose by up to 12.7%-p (boost plan), while the mean rectum dose was reduced by up to -8.9%-p (PTV plan). For the complete treatment a reduction of complication probability of the rectum of approximately 5%-p was calculated. CONCLUSION: The use of an image guidance system with linear translational correction can improve radiation treatment accuracy for prostate cancer, if geometric changes are within certain limits.     

Untersuchungen zur Positionierungsgenauigkeit bei Prostatakonformationsbestrahlungen mittels Portal-Imaging

BACKGROUND: Conformal radiotherapy techniques as used in prostate treatment allow to spare normal tissue by conforming the radiation fields to the shape of the planning target volume (PTV). To be able to fully utilize the advantages of these techniques correct patient positioning is an important prerequisite. This study employing an electronic portal imaging device (EPID) investigated the positioning uncertainties that occur in the pelvic region for different patient positioning devices. PATIENTS AND METHODS: 15 patients with prostate cancer were irradiated with or without rectal balloon/pelvic mask at a linear accelerator with multileaf collimator (MLC). For each patient multiple portal images were taken from different directions and compared to the digitally reconstructed radiographs (DRRs) of the treatment planning system and to simulation films (Table 1, Figure 1). RESULTS: In spite of different positioning devices, all patients showed comparable total positioning uncertainties of 4.0 mm (lateral), 4.5 mm (cranio-caudal) and 1.7 mm (dorso-ventral). The lateral positioning error was reduced for the pelvic mask patients while the cranio-caudal error increased (Table 2, Figure 2). A systematic and a random component sum up to the total positioning error, and a good estimate of the magnitudes of the two is possible from six to eight portal images (Figure 3). CONCLUSIONS: With a small number of portal images it is possible to find out the systematic and random positioning error of a patient. Knowledge of the random error can be used to resize the treatment margin which is clinically relevant since this error differs greatly for different patients (Figure 4). Image analysis with EPID is convenient, yet has some problems. For example, one only gets indirect information on the movement of the ventral rectum wall (Figure 5). The successful operation of positioning devices, although, needs further improvement--especially if one focuses on IMRT.