Reproducibility of conformal radiation therapy in localized carcinoma of the prostate without rigid immobilization

Quality Control by Portal Film Analysis in Radiotherapy for Prostate Cancer: A Comparison between Two Different Institutions and Treatment Techniques

AIMS AND BACKGROUND

Accuracy and reproducibility of patient setup during radiotherapy for prostate cancer were investigated in two different Institutions (A and B), within their Quality Assurance programs. The purpose of the study was to evaluate and compare setup accuracy and reproducibility in Institutions A and B, which adopt different patient positioning and treatment techniques for prostate irradiation.

MATERIALS AND METHODS

A retrospective analysis of portal localization films taken during the treatment course was performed: 30 and 21 patients in Institutes A and B, respectively, entered the study. In Institute A, patients were treated in a prone position, utilizing an individualized immobilization cast (either an alpha cradle or a heat and vacuum-formed cellulose acetate cast) with an open table top and individual abdominal wall compressor to minimize small bowel irradiation; a 5-field conformal technique was used. In Institute B, patients were treated in a supine position without any immobilization device; a 6-field BEV-based technique (conformal only for patients treated with a radical aim) was adopted. A total of 598 portal films (420 from Institute A and 178 from Institute B) were analyzed. The mean number of films per patient was 12 (range, 4-29). Systematic and random setup errors were estimated utilizing the statistical method suggested by Bijhold et al. (1992).

RESULTS

When patients with a mean (systematic) error larger than 5, 8 and 10 mm in craniocaudal, lateral and posterior-anterior directions, respectively, were compared, no statistically significant difference between the two groups was observed. Similarly, when comparing portal films, a significant difference (P <0.01) appeared only in the craniocaudal direction (errors > 5 mm: Institute A = 24%; Institute B = 11%). In both Institutes, the SD of random and systematic error distribution ranged from 1.8 to 4.2 mm, with a small prevalence of systematic errors. Only for craniocaudal shifts in Institute A was the random error larger than the systematic error, and it was significantly worse than in Institute B (1 SD, 4.2 mm in Institute A vs 1.8 mm in Institute B).

CONCLUSIONS

Setup errors observed in Institutes A and B were similar and in accord with data reported in the literature. In Institute B, satisfactory geometrical treatment quality was achieved without patient immobilization. In Institute A, the goal of minimizing small bowel irradiation and prostate motion through the aforementioned technique, which makes patient position less comfortable, did not seem to considerably increase daily setup uncertainty.

Setup error analysis in helical tomotherapy based image-guided radiation therapy treatments

The adequacy of setup margins for various sites in patients treated with helical tomotherapy was investigated. A total of 102 patients were investigated. The breakdown of the patients were as follows: Twenty-five patients each in brain, head and neck (H and N), and pelvis, while 12 patients in lung and 15 in craniospinal irradiation (CSI). Patients were immobilized on the institutional protocol. Altogether 2686 megavoltage computed tomography images were analyzed with 672, 747, 622, 333, and 312 fractions, respectively, from brain, H and N, pelvis, lung, and CSI. Overall systematic and random errors were calculated in three translational and three rotational directions. Setup margins were evaluated using van Herk formula. The calculated margins were compared with the margins in the clinical use for various directions and sites. We found that the clinical isotropic margin of 3 mm was adequate for brain patients. However, in the longitudinal direction it was found to be out of margin by 0.7 mm. In H and N, the calculated margins were well within the isotropic margin of 5 mm which is in clinical use. In pelvis, the calculated margin was within the limits, 8.3 mm versus 10 mm only in longitudinal direction, however, in vertical and lateral directions the calculated margins were out of clinical margins 11 mm versus 10 mm, and 8.7 mm versus 7.0, mm respectively. In lung, all the calculated margins were well within the margins used clinically. In CSI, the variation was found in the middle spine in the longitudinal direction. The clinical margins used in our hospital are adequate enough for sites H and N, lung, and brain, however, for CSI and pelvis the margins were found to be out of clinical margins.

Accuracy and reproducibility of conformal radiotherapy using data from a randomised controlled trial of conformal radiotherapy in prostate cancer (MRC RT01, ISRCTN47772397)

Aims

The MRC RT01 trial used conformal radiotherapy to the prostate, a method that reduces the volume of normal tissue treated by 40–50%. Because of the risk of geographical miss, the trial used portal imaging to examine whether treatment delivery was within the required accuracy.

Material and methods

In total, 843 patients were randomly assigned to receive 64 Gy in 32 fractions over 6.5 weeks or 74 Gy in 37 fractions over 7.5 weeks. Field displacements and corrections were recorded for all imaged fractions. Displacement trends and their association with time, disease and treatment set-up characteristics were examined using univariate and multivariate analyses. A Radiographer Trial Implementation Group (RTIG) was set up to inform the quality assurance process and to promote the development of best practice.

Results

Treatment isocentre positioning was within 5 mm in every direction on 6238 (83%) of the 7535 fractions imaged. In total, 532 (81%) of 695 included patients had at least one ≥ 3mm displacement and 415 (63%) had at least one ≥ 5mm displacement. Univariate, multivariate and stepwise models of ≥ 5mm displacements showed an increased likelihood of displacement in weeks 1 and 2 with low melting point alloy (LMPA) blocks compared with multileaf collimators, film verification compared with electronic portal imaging (EPI) and increased number of fractions imaged. Except for LMPA, this was also seen for ≥ 5mm displacements in weeks 3–6.

Conclusions

Accurate conformal treatment was delivered. The use of EPI was associated with increased reported accuracy. The RTIG was a crucial part of the quality assurance process.

High conformality radiotherapy in Europe: thirty-one centres participating in the quality assurance programme of the EORTC prostate trial 22991

Today, conformality in radiotherapy is at the centre of many investments in equipment and staffing. To estimate the current situation within the European Organisation for Research and Treatment of Cancer (EORTC) conformal radiotherapy trial for prostate cancer, a technology questionnaire was designed to assess whether participating centres can comply with the required radiotherapy procedures of EORTC trial 22991, where a high dose is prescribed to the prostate. Questions covered various items of computed tomography, data acquisition, treatment planning, delivery and verification. All centres (n=31) replied to the questionnaire. All generate beam's eye views and dose volume histograms. All, but two, centres use digitally reconstructed radiographs to display images. The vast majority of the centres perform at least weekly treatment verification and half have access to individual in vivo dosimetry. The results of the questionnaire indicate that participating centres have access to the equipment and apply the procedures that are essential for conformal prostate radiotherapy. The technology questionnaire is the first step in the extensive quality assurance programme dedicated to this high-tech radiotherapy trial.

Influence of rectal volume changes during radiotherapy for prostate cancer: A predictive model for mild-to-moderate late rectal toxicity

PURPOSE

To assess the rectal volume changes during radiotherapy for prostate cancer, to estimate an average rectal dose distribution profile during treatment, and to correlate these parameters with mild-to-moderate late rectal toxicity.

MATERIALS AND METHODS

Nine patients with localized prostate cancer underwent virtual CT simulation using a six-field conformal 18-MV photon technique. During treatment, patients underwent weekly pelvic CT scans under simulation conditions. Dosimetries were run with each CT data set using the same beam parameters as in the initial treatment plan. The influence of weekly rectal volume changes on the dose-volume histogram (DVH) profiles was studied. A polynomial function correlating the initial rectal volume with the mean percentage of change in the rectal volume during treatment was used to define a correction factor for rectal DVHs. The model was validated using data from 100 patients treated with 74 Gy according to the same technique. Areas under the curve of the initial rectal DVHs were correlated with toxicity (Radiation Therapy Oncology Group Grade 0 vs. 1-2, Student's t test), with or without the use of the above correction factor.

RESULTS

A trend for enlargement of the rectal volume during treatment was observed for most patients in the study with small rectal volumes (<75 cm(3)) at simulation, resulting in an increase in the integral rectal dose by a factor ranging from 1.3 to 2.1. Corrected, but not uncorrected, rectal DVH profiles were strongly predictive of Grade 0 vs. 1-2 late rectal morbidity.

CONCLUSIONS

Correcting the area under the curve of the rectal DVH at simulation by a factor that takes into account the projected volume changes during treatment correlates significantly with the probability of mild-to-moderate late rectal toxicity (Grade 1-2). This reliable predictor for mild-to-moderate late rectal morbidity may also be a practical tool for treatment planning.

[Target-volume and critical-organ delineation for conformal radiotherapy of prostate cancer: experience of French dose-escalation trials]

The delineation of target volume and organs at risk depends on the organs definition, and on the modalities for the CT-scan acquisition. Inter-observer variability in the delineation may be large, especially when patient's anatomy is unusual. During the two french multicentric studies of conformal radiotherapy for localized prostate cancer, it was made an effort to harmonize the delineation of the target volumes and organs at risk. Two cases were proposed for delineation during two workshops. In the first case, the mean prostate volume was 46.5 mL (extreme: 31.7-61.3), the mean prostate and seminal vesicles volume was 74.7 mL (extreme: 59.6-80.3), the rectal and bladder walls varied respectively in proportion from 1 to 1.45 and from 1 to 1.16; in the second case, the mean prostate volume was 53.1 mL (extreme: 40.8-73.1), the volume of prostate plus seminal vesicles was 65.1 mL (extreme: 53.2-89), the rectal wall varied proportionally from 1 to 1, 24 and the vesical wall varied from 1 to 1.67. For participating centers to the french studies of dose escalation, a quality control of contours was performed to decrease the inter-observer variability. The ways to reduce the discrepancies of volumes delineation, between different observers, are discussed. A better quality of the CT images, use of urethral opacification, and consensual definition of clinical target volumes and organs at risk may contribute to that improvement.

On-line correction of beam portals in the treatment of prostate cancer using an endorectal balloon device

BACKGROUND

Reproducible target volume assessment is required in order to optimize portal field margins in the treatment of prostate cancer. The benefits of an endorectal balloon on target volume assessment remain unclear.

MATERIAL AND METHODS

Nine patients were treated with a daily placed air filled rectal balloon. Portal films and computer-associated tomography during the treatment were used to determine the position of the structures of interest. Comparative planning with or without a balloon was performed in order to determine rectal wall exposure to radiation.

RESULTS

The range of movements during treatment predicting the position of the prostate in relation to the symphysis was 0.05-0.59 cm in the lateral direction, 0.27-2.2 cm in the antero-posterior direction, and 0.33-1.8 cm in the crano-caudal direction, as compared to the position of the prostate predicted by the balloon ranging from 0.18 to 0.76 cm in the lateral direction, 0.22-1.68 cm in the antero-posterior direction, and 0.58-2.99 cm in the crano-caudal direction. Planning target volumes (PTV) margins as defined by the position of the balloon were 10 mm in the antero-posterior direction, 6 mm in the lateral direction, and 16 mm in the crano-caudal direction. The volume of rectal wall exposed to radiation was reduced from 40 (+/- 12%) to 25% (+/- 19%) with an endorectal balloon (P < 0.05).

CONCLUSIONS

Daily online correction with portal vision for external beam set-up is improved by an endorectal balloon device, leading to improved PTV margins and reduced radiation exposure of the rectal wall.

Effectiveness of couch height-based patient set-up and an off-line correction protocol in prostate cancer radiotherapy

PURPOSE

To investigate set-up improvement caused by applying a couch height-based patient set-up method in combination with a technologist-driven off-line correction protocol in nonimmobilized radiotherapy of prostate patients.

METHODS AND MATERIALS

A three-dimensional shrinking action level correction protocol is applied in two consecutive patient cohorts with different set-up methods: the traditional "laser set-up" group (n = 43) and the "couch height set-up" group (n = 112). For all directions, left-right, ventro-dorsal, and cranio-caudal, random and systematic set-up deviations were measured.

RESULTS

The couch height set-up method improves the patient positioning compared to the laser set-up method. Without application of the correction protocol, both systematic and random errors reduced to 2.2-2.4 mm (1 SD) and 1.7-2.2 mm (1 SD), respectively. By using the correction protocol, systematic errors reduced further to 1.3-1.6 mm (1 SD). One-dimensional deviations were within 5 mm for >90% of the measured fractions. The required number of corrections per patient in the off-line correction protocol was reduced significantly during the course of treatment from 1.1 to 0.6 by the couch height set-up method. The treatment time was not prolonged by application of the correction protocol.

CONCLUSIONS

The couch height set-up method improves the set-up significantly, especially in the ventro-dorsal direction. Combination of this set-up method with an off-line correction strategy, executed by technologists, reduces the number of set-up corrections required.

Bladder opacification does not significantly influence dose distribution in conformal radiotherapy of prostate cancer

PURPOSE

To compare the results of treatment planning with or without bladder contrast during simulation of three dimensional conformal radiotherapy (3D-CRT) for prostate cancer (18 MV X-rays, six field arrangement), and to assess the potential changes in dose distribution in the target and rectal volumes.

METHODS AND MATERIALS

Based on CT-simulation using intravenous contrast media, 3-D conformal treatment planning was performed in five patients. To simulate a non-opacified bladder, the electron matrix density of the opacified bladder was virtually changed to water density. Two treatment plans were carried out, with and without bladder opacification. In each patient dose distributions were formally compared for both plans, and the increment in monitor units (MU) needed to compensate for the presence of contrast media was assessed.

RESULTS

A mean dose variation of -0.03% (range, -0.03-0.14%) and -1.13% (range, -1.85-0%) was observed for the prostate and the rectum, respectively. The average mean MU increment without bladder contrast normalized to the case with bladder contrast was 0.31% +/- 0.52.

CONCLUSIONS

Bladder opacification used during simulation does not significantly influence prostate or rectal dose distributions in prostate patients treated with 3D-CRT, 18 MV X-rays, and a six-beam arrangement.

Set-up verification using portal imaging; review of current clinical practice

In this review of current clinical practice of set-up error verification by means of portal imaging, we firstly define the various types of set-up errors using a consistent nomenclature. The different causes of set-up errors are then summarized. Next, the results of a large number of studies regarding patient set-up verification are presented for treatments of patients with head and neck, prostate, pelvis, lung and breast cancer, as well as for mantle field/total body treatments. This review focuses on the more recent studies in order to assess the criteria for good clinical practice in patient positioning. The reported set-up accuracy varies widely, depending on the treatment site, method of immobilization and institution. The standard deviation (1 SD, mm) of the systematic and random errors for currently applied treatment techniques, separately measured along the three principle axes, ranges from 1.6-4.6 and 1.1-2.5 (head and neck), 1.0-3.8 and 1.2-3.5 (prostate), 1.1-4.7 and 1.1-4.9 (pelvis), 1.8-5.1 and 2.2-5.4 (lung), and 1.0-4.7 and 1.7-14.4 (breast), respectively. Recommendations for procedures to quantify, report and reduce patient set-up errors are given based on the studies described in this review. Using these recommendations, the systematic and random set-up errors that can be achieved in routine clinical practice can be less than 2.0 mm (1 SD) for head and neck, 2.5 mm (1 SD) for prostate, 3.0 mm (1 SD) for general pelvic and 3.5 mm (1 SD) for lung cancer treatment techniques.

Potential role of intensity modulated proton beams in prostate cancer radiotherapy

PURPOSE

The present study was undertaken to assess the potential benefit of intensity modulated (IM) proton beams in optimizing the dose distribution to safely escalate the tumor dose in prostate cancer radiotherapy.

METHODS AND MATERIALS

Four treatment plans were compared in a prostate cancer patient aiming to deliver 81 Gy to the target: 1) conformal 18 MV X-rays, 6-fields; 2) 214 MeV protons, 2-fields; 3) IM 15 MV X-rays, 5-fields; and 4) 177-200 Mev IM protons, 5-fields as in Plan 3. In addition, IM methods were used to further escalate the tumor dose to 99 Gy. Dose-volume histograms (DVH) were used to physically compare the treatment plans. DVH data were also used to obtain normal tissue complication probabilities (NTCP) for the rectum, bladder, femoral heads, and tumor control probabilities.

RESULTS

Although the planning target volume dose distribution was satisfactory with the four treatment plans, the homogeneity was slightly reduced in both X-ray plans (IM and standard) and the low-to-medium doses delivered to all organs at risk, and other normal tissues were significantly reduced by both proton plans. For a prescribed dose of 81 Gy, only the IM X-ray and IM proton plans both succeeded in predicting an acceptably low NTCP for the rectum (<5%, Grade 3). The integral nontarget dose was significantly reduced with IM proton beams (i.e., 3.1, 1.3, and 1.7 times less than Plans 1, 2, and 3, respectively). When escalating the dose to 99 Gy, no additional improvement between IM protons and IM X-ray beams was observed.

CONCLUSION

Both IM X-ray and proton beams were able to optimize the dose distribution and comply with the goal of delivering the highest dose to the target while reducing the risk of severe morbidity to acceptable levels. The main advantage compared to IM X-rays was that IM protons succeeded in significantly reducing the low-to-medium dose to the nontarget tissues and achieved a small improvement in planning target volume (PTV) dose heterogeneity.

Patient positioning in prostate radiotherapy: is prone better than supine?

PURPOSE

To assess potential dose reductions to the rectum and to the bladder with three-dimensional conformal radiotherapy (3D-CRT) to the prostate in the prone as compared with the supine position; and to retrospectively evaluate treatment position reproducibility without immobilization devices.

METHODS AND MATERIALS

Eighteen patients with localized prostate cancer underwent pelvic CT scans and 3D treatment planning in prone and supine positions. Dose-volume histograms (DVHs) were constructed for the clinical target volume, the rectum and the bladder for every patient in both treatment positions. "Comparative DVHs" (cDVHs) were defined for the rectum and for the bladder: cDVH was obtained by subtracting the organ volume receiving a given dose increment in the prone position from the corresponding value in the supine position. These values were then integrated over the entire dose range. The prescribed dose to the planning target volume (PTV) was 74 Gy using a 6-field technique. To evaluate reproducibility, portal films were subsequently reviewed in 12 patients treated prone and 10 contemporary patients treated supine (controls). No immobilization devices were used. Deviations in the anterio-posterior (X) and cranio-caudal (Y) axes were measured. Mean treatment position variation, total setup variation, systematic setup variation, and random setup variation were obtained.

RESULTS

Prone position was associated with a higher dose to the rectum or to the bladder in 6 (33%) and 7 (39%) patients, respectively. A simultaneously higher dose to rectum and bladder was noted in 2 (11%) patients in prone and in 7 (39%) patients in supine. Rectal and bladder volumes were frequently larger in prone than in supine: mean prone/supine volume ratios were 1.21 (SD, 0.68) and 1.03 (SD, 1.32), respectively. In these cases cDVH analysis more often favored the prone position. Mean treatment position variation and total setup variation were similar for both prone and supine plans. A higher systematic setup variation was observed in prone positioning: 2.7 mm vs. 1.9 mm (X axis) and 4.1 mm vs. 2.2 mm (Y axis). The random variation was similar for both prone and supine: 4. 0 mm vs. 3.6 mm (X axis) and 3.7 mm vs. 3.6 mm (Y axis).

CONCLUSIONS

Prone position 3D-CRT is frequently, but not always, associated with an apparent dose reduction to the rectum and/or to the bladder for prostate cancer patients. As suggested by the increased mean prone/supine rectal volume ratio, the advantage of prone positioning for the rectum may be artifactual, at least partly reflecting a position-dependent rectal air volume, which may significantly vary from treatment to treatment. In the absence of immobilization devices, daily setup reproducibility appears less accurate for the prone position, primarily due to systematic setup variations.

A randomized study of the use of a customized immobilization system in the treatment of prostate cancer with conformal radiotherapy

PURPOSE

To evaluate the impact of a customized immobilisation system on field placement accuracy, simulation and treatment delivery time, radiographer convenience and patient acceptability.

PATIENTS AND METHODS

Thirty men receiving radical radiotherapy for prostate cancer were randomised using a cross over trial design to have radiotherapy planning and treatment given either in a conventional treatment position (CTP) or using an immobilisation system (IMS). The randomisation was to have either the CTP or IMS for the initial 3 weeks of radiotherapy after which patients were replanned and changed to the alternative treatment set-up. Treatment accuracy was measured using an electronic portal imaging device. Radiographers and patients completed weekly questionnaires.

RESULTS

Median simulation time was 22.5 min (range 20-30 min) in the CTP and 25 min (range 15-40 min) for the IMS (P < 0.001). Median treatment time was 9 min for CTP (range 8-10 min), and 10 min (range 8.5-13.5 min) for IMS (P < 0.001). Median isocentre displacement for anterior fields was 1.7 mm from the simulated isocentre for the CTP compared to 2.0 mm for IMS (P = 0.07). For left lateral fields values were 1.8 and 1.8 mm (P = 0.98), and for right lateral fields 2.1 and 1.7 mm (P = 0.06), respectively. No clinically significant reduction in either systematic or random field placement errors was demonstrated. Radiographers reported that patients found the IMS more comfortable than CTP (P < 0.001), but when using the IMS, they noticed greater difficulty in patient positioning (P < 0.001), and alignment to skin tattoos (P < 0.001).

CONCLUSIONS

Although IMS may have been more comfortable, treatment accuracy was not improved compared to the CTP in our department. In addition, treatment took longer and patient set-up was more difficult.

The performance of a fluoroscopic electronic portal imaging device modified for portability

Advances in external beam therapy technology have made routine, efficient conformal therapy a reality. With it comes the increasing need for online treatment verification, which is only achievable at present through the use of electronic portal imaging devices (EPIDs). For a large radiotherapy centre, the provision of one EPID per treatment machine proves extremely expensive. This paper details modifications to the design of a commercial fluoroscopic EPID (the SRI-100) to produce a portable system, capable of providing quick, high quality imaging on more than one treatment machine. We describe the necessary hardware and software changes made to the system, as well as the variety of mechanical and quality control checks performed for testing the stability and quality of the imaging. The modified system has been found to be both electronically and mechanically robust, with associated image quality, scaling, distortion and movement similar to other EPIDs in the department. Although the modification was designed specifically to allow for the acquisition of images from multiple treatment machines, it may also enable the operation of the EPID for other uses such as total body irradiation (TBI) treatment verification and a further range of quality control procedures on the linear accelerator itself.

Radiotherapy of pelvic malignancies: impact of two types of rigid immobilisation devices on localisation errors

BACKGROUND AND PURPOSE

To determine the distribution of set-up errors for patients treated with and without two rigid partial immobilisation devices for pelvic malignancies.

MATERIALS AND METHODS

30 patients receiving pelvic irradiation with a four field technique underwent a total of 524 portal films. The patients are divided into 3 cohorts of 10 patients. The first cohort is treated on a standard treatment couch without immobilisation device (NI); the second and third cohorts are treated with a custom-made immobilisation device used in an attempt to improve set-up accuracy: an Alpha-Cradle mattress (AM) or an Orfit cast (OC). Set-up deviations are analysed in the X, Y, Z directions of a fixed coordinate system, corresponding to the lateral, cranio-caudal and antero-posterior direction, respectively.

RESULTS

Considering the percentage of discrepancies < or = 5 mm between the simulation films and the portal films as a measure of set-up accuracy, immobilisation devices seem to increase accuracy: 88.5% (X) 79% (Y) and 100% (Z) with AM; 84% (X-Y), 97.5% (Z) with OC and only 76.5% (X), 40% (Y) and 65.5% (Z) for NI. The distribution of the systematic set-up errors for the three patient cohorts, defined as the mean patient displacement for the treatment course, had a mean and a standard deviation of (0.7 +/- 2.7) mm in the X-axis, (-5.5 +/- 2.6) mm in the Y-axis and (-0.9 +/- 2.2) mm in the Z-axis when no immobilisation is added; (0.8 +/- 1.7) mm, (-2 +/- 2.7) mm and (0.3 +/- 0.4) mm for the Alpha-Cradle group; (0.3 +/- 1.4) mm, (0.5 +/- 1.1) mm and (0.5 +/- 0.6) mm for the Orfit cast group. The distribution of random errors about the mean approximated a normal distribution and the standard deviations are 4.4 mm (X), 4.2 mm (Y) and 4.8 mm (Z) for NI; 3.3, 3.5 and 2.5 mm for the AM; 3.4, 3.3 and 2.7 mm for the OC.

CONCLUSIONS

The two rigid immobilisation devices improve the reproducibility of a given pelvic field but there is a small benefit comparative to the cost and the cumbersome place of the devices.

Accuracy of patient positioning during radiotherapy for bladder and brain tumours

We report the results of a prospective study to quantify inaccuracies in patient set-up during routine radiotherapy for tumours of the brain and bladder, which took place as part of our departmental development. Knowledge of these inaccuracies is required to put into practice the ICRU 50 recommendations regarding clinical target volume and planning target volume. We measured inaccuracies in two dimensions by comparing portal beam films with the simulator check film. Our method used manual measurements, proved to be a very laborious technique, and demonstrated the need for portal imaging. Ninety-five brain and 97 bladder portal films from 30 brain and 30 bladder patients were examined. Displacements greater than 0.5 cm were seen in 13% of brain treatments in the supero-inferior direction and 1% in the anteroposterior direction. With bladder treatments, displacements greater than 0.75 cm were seen in 12% in the supero-inferior direction and 5% in the lateral direction. These results are consistent with other previous studies. We identified a very small systematic error in the department, which was not [corrected] considered to be clinically significant. These results are discussed with reference to other similar studies and the ICRU 50 recommendations.

Set-up error in supine-positioned patients immobilized with two different modalities during conformal radiotherapy of prostate cancer

BACKGROUND

Conformal radiotherapy requires reduced margins around the clinical target volume (CTV) with respect to traditional radiotherapy techniques. Therefore, high set-up accuracy and reproducibility are mandatory.

PURPOSE

To investigate the effectiveness of two different immobilization techniques during conformal radiotherapy of prostate cancer with small fields.

MATERIALS AND METHODS

52 patients with prostate cancer were treated by conformal three- or four-field techniques with radical or adjuvant intent between November 1996 and March 1998. In total, 539 portal images were collected on a weekly basis for at least the first 4 weeks of the treatment on lateral and anterior 18 MV X-ray fields. The average number of sessions monitored per patient was 5.7 (range 4-10). All patients were immobilized with an alpha-cradle system; 25 of them were immobilized at the pelvis level (group A) and the remaining 27 patients were immobilized in the legs (group B). The shifts with respect to the simulation condition were assessed by measuring the distances between the same bony landmarks and the field edges. The global distributions of cranio-caudal (CC), posterior-anterior (PA) and left-right (LR) shifts were considered; for each patient random and systematic error components were assessed by following the procedure suggested by Bijhold et al. (Bijhold J, Lebesque JV, Hart AAM, Vijlbrief RE. Maximising set-up accuracy using portal images as applied to a conformal boost technique for prostatic cancer. Radiother. Oncol. 1992;24:261-271). For each patient the average isocentre (3D) shift was assessed as the quadratic sum of the average shifts in the three directions.

RESULTS

Group B showed a better accuracy and reproducibility than group A for PA shifts (2.6 versus 4.4 mm, 1 SD), LR shifts (2.4 versus 3.6 mm, 1 SD) and CC shifts (2.7 versus 3.3 mm, 1 SD). Furthermore, group B showed a rate of large PA shifts (>5 mm) equal to 4.4% with respect to the 21.6% of group A (P<0.0001). This value was also better than the corresponding value found in a previously investigated group of 21 non-immobilized patients (Italia C, Fiorino C, Ciocca M, et al. Quality control by portal film analysis of the conformal radiotherapy of prostate cancer: comparison between two different institutions and treatment techniques (abstract). Radiother. Oncol. 1997;43(Suppl. 2):S16, 16.8%, P = 0.001). For both groups there was no clear prevalence of one component (systematic or random) with respect to the other. The average isocentre shifts (averaged on all patients) were 3.0 mm (+/-1.4 mm, 1 SD) for group B and 5.0 mm (+/-2.8 mm, 1 SD) for group A against a value of 4.4 mm (+/-2.4 mm, 1 SD) for the previously investigated non-immobilized patient group.

CONCLUSIONS

Immobilization of the legs with an alpha-cradle system seems to improve both the accuracy and reproducibility of the positioning of patients treated for prostate cancer with respect to alpha-cradle pelvic-abdomen immobilization. Based on these data, we decided to use the legs immobilization system and to reduce the margin around the CTV (from 10 to 8 mm) in the PA direction.

[Conformational radiotherapy in cancers of the prostate: contribution of pelvic immobilization and new fiducial markers]

PURPOSE

To reduce errors in the positioning of patients treated with external conformal radiotherapy for prostate cancer, we evaluated both the use of an immobilization device and new fiducial markers.

MATERIALS AND METHODS

The immobilization device consisted of an individual mold made of polyurethan foam. Two sets of skin markers located on the anterior tibial surfaces were used to identify the pelvic isocenter. The patient's position was evaluated by orthogonal port films which were then compared with the original simulation film.

RESULTS

Results are presented with respect to orthogonal axes. Comparison with classic procedures without immobilization showed that use of the mold and new fiducial markers led to a decrease in set-up errors which were less than 5 mm.

CONCLUSION

With the use of an immobilization device and optimized techniques for patients' positioning, conformal radiotherapy of prostate cancer is more accurate.

Improvement in total positioning error for lateral prostatic fields using a soft immobilization device

PURPOSE

To prospectively measure the total positioning error present in lateral pelvic fields of patients undergoing prostatic irradiation, and to evaluate the effect of a rigid table insert and soft immobilization on the magnitude of the measured error.

MATERIALS AND METHODS

Sixty-one consecutive patients receiving radical prostatic irradiation with a four field technique underwent a total of 234 lateral portal films during the first, third, fifth and seventh week of treatment. The position of the isocentre was compared to the isocentre on the corresponding simulator films and the magnitude and direction of deviations recorded. The patients were divided in to three cohorts of 15 patients, 15 patients and 31 patients. The first cohort was treated on a standard treatment couch, the second cohort treated with the table top stiffened using a 1 cm polycarbonate insert, and the third cohort treated with a soft immobilization device supporting the lower legs, and the polycarbonate insert.

RESULTS

There was no difference in the mean deviation of the vector of the isocentre displacement in the y and z directions identified at any of the four times when measurements were taken during therapy between the cohorts treated with or without the polycarbonate insert, but without immobilization. The overall mean deviation for these first two cohorts of patients was 3.9 mm. The positioning of patients treated with immobilization was compared to those treated without, and the immobilized patients had a significantly improved overall mean deviation of 2.6 mm (P = 0.002). This was a result of improvement in both the random and systematic components of the total error. In addition, the proportion of errors greater than 5 mm was reduced from 17% of set-ups to 8% of set-ups. The time during the course of treatment when the measurement was taken had no effect on positioning error for any of the treatment groups.

CONCLUSION

Stiffening the treatment couch with a 1 cm thick polycarbonate insert had no effect on reducing total positioning error, but immobilization with an inexpensive and non-customized foam rubber leg support reduced total positioning error in a statistically significant way.