T. Haycocks

Positioning errors and prostate motion during conformal prostate radiotherapy using on-line isocentre set-up verification and implanted prostate markers.

PURPOSE To evaluate treatment errors from set-up and inter-fraction prostatic motion with port films and implanted prostate fiducial markers during conformal radiotherapy for localized prostate cancer. METHODS Errors from isocentre positioning and inter-fraction prostate motion were investigated in 13 men treated with escalated dose conformal radiotherapy for localized prostate cancer. To limit the effect of inter-fraction prostate motion, patients were planned and treated with an empty rectum and a comfortably full bladder, and were instructed regarding dietary management, fluid intake and laxative use. Field placement was determined and corrected with daily on-line portal imaging. A lateral portal film was taken three times weekly over the course of therapy. From these films, random and systematic placement errors were measured by matching corresponding bony landmarks to the simulator film. Superior-inferior and anterior-posterior prostate motion was measured from the displacement of three gold pins implanted into the prostate before planning. A planning target volume (PTV) was derived to account for the measured prostate motion and field placement errors. RESULTS From 272 port films the random and systematic isocentre positioning error was 2.2 mm (range 0.2-7.3 mm) and 1.4 mm (range 0.2-3.3 mm), respectively. Prostate motion was largest at the base compared to the apex. Base: anterior, standard deviation (SD) 2.9 mm; superior, SD 2.1 mm. Apex: anterior, SD 2.1 mm; superior, SD 2.1 mm. The margin of PTV required to give a 99% probability of the gland remaining within the 95% isodose line during the course of therapy is superior 5.8 mm, and inferior 5.6 mm. In the anterior and posterior direction, this margin is 7.2 mm at the base, 6.5 mm at the mid-gland and 6.0 mm at the apex. CONCLUSIONS Systematic set-up errors were small using real-time isocentre placement corrections. Patient instruction to help control variation in bladder and rectal distension during therapy may explain the observed small SD for prostate motion in this group of patients. Inter-fraction prostate motion remained the largest source of treatment error, and observed motion was greatest at the gland base. In the absence of real-time pre-treatment imaging of prostate position, sequential portal films of implanted prostatic markers should improve quality assurance by confirming organ position within the treatment field over the course of therapy.

Magnetic resonance imaging in the radiation treatment planning of localized prostate cancer using intra-prostatic fiducial markers for computed tomography co-registration.

PURPOSE To assess the feasibility, and potential implications, of using intra-prostatic fiducial markers, rather than bony landmarks, for the co-registration of computed tomography (CT) and magnetic resonance (MR) images in the radiation treatment planning of localized prostate cancer. METHODS All men treated with conformal therapy for localized prostate cancer underwent routine pre-treatment insertion of prostatic fiducial markers to assist with gross target volume (GTV) delineation and to identify prostate positioning during therapy. Six of these men were selected for investigation. Phantom MRI measurements were obtained to quantify image distortion, to determine the most suitable gold alloy marker composition, and to identify the spin-echo sequences that optimized both marker identification and the contrast between the prostate and the surrounding tissues. The GTV for each patient was contoured independently by three radiation oncologists on axial planning CT slices, and on axial MRI slices fused to the CT slices by matching the implanted fiducial markers. From each set of contours the scan common volume (SCV), and the scan encompassing volume (SEV), were obtained. The ratio SEV/SCV for a given scan is a measure of inter-observer variation in contouring. For each of the 18 patient-observer combinations the observer common volume (OCV) and the observer encompassing volume (OEV) was obtained. The ratio OEV/OCV for a given patient-observer combination is a measure of the inter-modality variation in contouring. The distance from the treatment planning isocenter to the prostate contours was measured and the discrepancy between the CT- and the MR-defined contour recorded. The discrepancies between the CT- and MR-defined contours of the posterior prostate were recorded in the sagittal plane at 1-cm intervals above and below the isocenter. RESULTS Phantom measurements demonstrated trivial image distortion within the required field of view, and an 18K Au/Cu alloy to be the marker composition most suitable for CT-MRI image fusion purposes. Inter-observer variation in prostate contouring was significantly less for MR compared to CT. The mean SEV/SCV ratio was 1.58 (confidence interval (CI): 1.47-1.69) for CT scans and 1.37 (CI: 1.33-1.41) for MR scans (paired t-test; P=0.036). The overall magnitude of contoured GTV was similar for MR and CT; however, there were spatial discrepancies in contouring between the two modalities. The greatest systematic discrepancy was at the posterior apical prostate border, which was defined 3.6 mm (SD 3.5 mm) more posterior on MR- than CT-defined contouring. CONCLUSIONS Prostate contouring on MR is associated with less inter-observer variation than on CT. In addition, we have demonstrated the feasibility of using intra-prostatic fiducial markers, rather than bony landmarks, for the co-registration of CT and MR images in the radiation treatment planning of localized prostate cancer. This technique, together with on-line correction of treatment set-up according to the fiducial marker position on electronic portal imaging, may enable a reduction in the planning target volume (PTV) margin needed to account for inter-observer error in target delineation, and for prostate motion.

Volume-based radiotherapy targeting in soft tissue sarcoma.

RT targeting of STS presents considerable challenges in realizing optimum outcome in tissue and function preservation while maintaining high local control for most anatomic sites. While a highly effective adjuvant, RT delivered improperly may cause substantial disability by excessive volume or dose delivery. The advent of very precise treatment planning and delivery systems, including 3D CRT and IMRT, means it is now possible to choose to treat ideal volumes rather than ones that are merely feasible. At the same time precise knowledge of appropriate targets continues to evolve for the different clinical scenarios and will likely be greatly influenced in the future by enhanced imaging capability. Clinical trials are needed that include relevant end-points to measure improvements in the therapeutic ratio resulting from more precise RT targeting and without loss of local control. In addition, advancement of 3D CRT and IMRT over the next decade will rely on the consistent reporting and sharing of results concerning outcome of normal tissue from volumetric treatment planning.

Comparison of Correction Protocols for Image-Guided Radiation Therapy

In radiation therapy, patient positioning uncertainty and organ motion require that a volume larger than the actual tumour (named the planning target volume [PTV]) be irradiated to ensure that the tumour receives the prescribed dose. Image-guided patient positioning can correct targeting errors, thus reducing the uncertainty in the position of the tumour, and therefore, the size of the PTV. Positioning uncertainties are reduced with frequent imaging, but require increasing the overall time of a treatment session. We compare conventional ‘off-line’ error correction protocols to our ‘on-line’ correction protocol for prostate cancer patients. Simulations indicate that our on-line protocol leads to much smaller residual uncertainty than off-line protocols, leading to much smaller PTV margins. Our on-line protocol allows the irradiation of substantially smaller volumes than conventional off-line protocols, leading to reduced normal tissue complications. Introducing an “intervention threshold” has retained some of the efficiency of the off-line strategy.

Intensity-Modulated Radiation Therapy for Lymph Node Metastases in Bladder Cancer

Bladder cancer is a common disease, and causes significant morbidity and mortality. It is estimated that there will be 56,500 new cases diagnosed in the United States in 2002, and that 12,600 men and women will die of the disease (Jemal et al. 2002). The majority of tumors are transitional cell carcinomas that arise from the transitional epithelium of the bladder mucosa. Superficial transitional cell carcinomas account for about 75% of all cases, and have a low potential to metastasize. The remaining 25% of tumors invade the muscularis propria of the bladder wall, and frequently spread to the lymph nodes and distant sites, making muscle-invasive bladder cancer a life-threatening disease. Lymph node involvement at diagnosis is associated with a high risk of occult metastatic disease and has an ominous prognosis.

Is a Diagnosis of Inflammatory Bowel Disease

Abstract Introduction Inflammatory bowel disease (IBD) is generally considered a contraindication to irradiation in the pelvis. This case study hypothesises that a diagnosis of IBD no longer precludes the delivery of high doses to treat prostate cancer when using conformal radiotherapy techniques. Methods The case histories of 2 IBD patients who had recently received dose-escalated radiotherapy for prostate cancer were reviewed. A DVH ranking comparison was made for the rectal wall of these patients using 4 prostate treatment plans to examine each techniques contribution to rectal dose. Results The actual treatment plan delivered the dose prescribed (75.6Gy) without any toxicity to date. This plan reduced the maximum dose whilst minimizing the rectum contained within the 40Gy and 60Gy volumes. Discussion Avoidance of radiation therapy, secondary to toxicity concerns, may jeopardize the clinical outcome for IBD patients. The results from this case study support the hypothesis that precision techniques may be employed to safely treat IBD patients to a tumoricidal dose but an investigation with larger patient numbers is warranted to reach any fi rm conclusions regarding the safe management of these patients.