Lee Gerig

Investigating treatment dose error due to beam attenuation by a carbon fiber tabletop

Carbon fiber is commonly used in radiation therapy for treatment tabletops and various immobilization and support devices, partially because it is generally perceived to be almost radiotransparent to high‐energy photons. To avoid exposure to normal tissue during modern radiation therapy, one must deliver the radiation from all gantry angles; hence, beams often transit the couch proximal to the patient. The effects of the beam attenuation by the support structure of the couch are often neglected in the planning process. In this study, we investigate the attenuation of 6‐MV and 18‐MV photon beams by a Medtec (Orange City, IA) carbon fiber couch. We have determined that neglecting the attenuation of oblique treatment fields by the carbon fiber couch can result in localized dose reduction from 4% to 16%, depending on energy, field size, and geometry. Further, we investigate the ability of a commercial treatment‐planning system (Theraplan Plus v3.8) to account for the attenuation by the treatment couch. Results show that incorporating the carbon fiber couch in the patient model reduces the dose error to less than 2%. The variation in dose reduction as a function of longitudinal couch position was also measured. In the triangular strut region of the couch, the attenuation varied ±0.5% following the periodic nature of the support structure. Based on these findings, we propose the routine incorporation of the treatment tabletop into patient treatment planning dose calculations. PACS numbers: 87.53.Dq, 87.53.Mr

Stage III carcinoma of the endometrium: A review of 41 cases

Abstract Forty-one patients with clinical-pathological stage III carcinoma of the endometrium treated between January 1958 and December 1983 at the Ottawa General Hospital were analyzed. Twenty-three patients (56%) presented with clinical stage III and 18 (44%) with pathological stage III. The 5-year survival of all stage III patients was 35%. Patients with clinical stage III had a 5-year survival of 8% compared to 70% for patients with pathological stage III ( P

Obstructive ureteropathy following radiation therapy for carcinoma of the cervix

Between January 1959 and December 1986, 10 of 328 patients (3%) treated with curative intent using primary radiation therapy for carcinoma of the cervix developed obstructive ureteropathy due to fibrosis. The mean age of the patients with obstructive ureteropathy was 45 years and the median time to obstruction was 26 months. The obstruction was unilateral in 8 cases and involved the parametrial portion of the ureter in at least 5 cases. No predisposing risk factor was found to be associated with the development of obstructive ureteropathy. After corrective surgery, renal function remained normal in 8 patients, and resolution of the hydronephrosis occurred in 4 patients.

Prognostic factors in early carcinoma of the endometrium

Two hundred forty-one patients with clinical-pathological Stage I and 58 patients with clinical-pathological Stage II carcinoma of the endometrium treated between January 1959 and December 1983 at the Ottawa General Hospital were analyzed. The adjusted survival rate at 5 years was 92% in patients with Stage I and 66% in patients with Stage II. In patients with Stage I, the most important prognostic factors were the histological grade of the tumor and the depth of myometrial invasion. In patients with Stage II, the single most important prognostic factor was the clinical extent of the disease. Grade and depth of myometrial invasion were also significant prognostic factors, particularly in patients with pathological Stage II. Combined surgery and radiation therapy was clearly superior to surgery alone in patients with Stage II but not in patients with Stage I, although, with long-term follow-up, our results may suggest improved survival in these patients as well.

Prognostic significance of endometrial extension in carcinoma of the cervix

A retrospective analysis of 343 consecutive patients with histologically proven carcinoma of the cervix, treated at the Ottawa General Hospital, was undertaken to evaluate the prognostic significance of endometrial extension. All these patient had a D + C as part of their work-up. Sixty-seven patients had a (+) D + C: 34/150 (23%) in Stage IB, 21/106 (20%) in Stage II, and 12/87 (14%) in Stage III. Survival was closely related to the D + C findings in early stages. In Stage IB, the 5-year survival of D + C (-) patients was 90% vs 50% in D + C (+) patients (P less than 0.003) and in Stage II, the 5-year survival was 77% and 55%, respectively (P = 0.089). There was, however, no difference in survival in Stage III patients (35% vs 29%). Pelvic failures were similar in both groups, stage for stage, but those with a (+) D + C had a higher incidence of distant metastasis. In Stage IB, distant metastases were found in 8.5% (10/116) of D + C (-) patients compared to 38% (13/34) in D + C (+) patients (P less than 0.001) and in Stage II, in 18.5% (16/85 and 33% (7/21) of the patients (P = 0.126), respectively. There was no difference in Stage III patients (28% vs 25%). This study suggests that endometrial extension is a significant prognostic factor in early stages and is associated with a higher risk of distant metastases. Management of these high risk patients is discussed.

Rapid palliative radiotherapy: comparing IG-IMRT with more conventional approaches

Purpose: To assess the efficiency of an integrated imaging, planning, and treatment delivery system to provide image-guided intensity-modulated radiotherapy (IG-IMRT) for patients requiring palliative radiotherapy (PRT). Methods: Between December 2006 and May 2008, 28 patients requiring urgent PRT were selected to undergo single-session megavoltage computed tomography (MV-CT) simulation, IMRT treatment planning, position verification and delivery of the first faction of radiotherapy on a helical Tomotherapy ® unit. The time required to complete each step was recorded and compared to our standard approach of using either fluoroscopic or CT-based simulation, simplified treatment planning and delivery on a megavoltage unit. Results: Twenty-eight patients were treated with our integrated IG-IMRT protocol. The median age was 72 years, with 61% men and 39% women. The indications for PRT were: painful bone and soft tissue metastasis (75%); bleeding lesions (14%); and other reasons (11%). The areas treated included the following: hip and/or pelvis (42%); spine (36%); and other areas (21%). The most commonly used dose prescription was 20 Gy in five fractions. Average times for the integrated IG-IMRT processes were as follows: image acquisition, 15 minutes; target delineation, 16 minutes; IMRT treatment planning, 9 minutes; treatment position verification, 10 minutes; and treatment delivery, 12 minutes. The average total time was 62 minutes compared to 66 minutes and 81 minutes for fluoroscopic and CT-simulation-based approaches, respectively. The IMRT dose distributions were also superior to simpler plans. Conclusions: PRT with an integrated IG-IMRT approach is efficient and convenient for patients, and has potential for future applications such as single-fraction radiotherapy.

SU-F-J-52: A Novel Approach to X-Ray Tube Quality Assurance for CBCT Systems in Order to Better Assess the Patient Imaging Dose in a Large, Multi-Unit Treatment Facility

PURPOSE To standardize the tube calibration for Elekta XVI cone beam CT (CBCT) systems in order to provide a meaningful estimate of the daily imaging dose and reduce the variation between units in a large centre with multiple treatment units. METHODS Initial measurements of the output from the CBCT systems were made using a Farmer chamber and standard CTDI phantom. The correlation between the measured CTDI and the tube current was confirmed using an Unfors Xi detector which was then used to perform a tube current calibration on each unit. RESULTS Initial measurements showed measured tube current variations of up to 25% between units for scans with the same image settings. In order to reasonably estimate the imaging dose, a systematic approach to x-ray generator calibration was adopted to ensure that the imaging dose was consistent across all units at the centre and was adopted as part of the routine quality assurance program. Subsequent measurements show that the variation in measured dose across nine units is on the order of 5%. CONCLUSION Increasingly, patients receiving radiation therapy have extended life expectancies and therefore the cumulative dose from daily imaging should not be ignored. In theory, an estimate of imaging dose can be made from the imaging parameters. However, measurements have shown that there are large differences in the x-ray generator calibration as installed at the clinic. Current protocols recommend routine checks of dose to ensure constancy. The present study suggests that in addition to constancy checks on a single machine, a tube current calibration should be performed on every unit to ensure agreement across multiple machines. This is crucial at a large centre with multiple units in order to provide physicians with a meaningful estimate of the daily imaging dose.

SU‐FF‐T‐200: Independent MU Calculation for Pre‐Treatment Verification of TomoTherapy StatRT

Purpose: To develop an independent calculator of MU as a check prior to delivery of a Helical TomoTherapy (HT) StatRT Treatment.Methods: StatRT provides rapid palliation whereby the patient gets an MVCT, and then PTVs and PRVs are defined, an inverse Tx plan is created and delivered, all in under 40 minutes without the patient leaving the Tx couch. This provides no opportunity for conventional IMRT verification (e.g. patient DQA), and no method exists for independent calculation in the short time between the plan calculation and Tx. Full dose calculation would require forward computation using the planned sinogram applied to the patient CT, something very difficult to achieve in the time clinically allotted between completion of planning and Tx delivery. Our approach is to verify the “sanity” of the total MU to be delivered prior to beam on. The approach assumes a single transverse image represents the entire contour, PTV and PRVs. It also assumes unit density and that modulation in the representative slice is constant and equal to the overall average modulation factor. An Excel application with graphic interface allows the user to quickly input an approximate patient contour (multiple XY pairs or the major‐minor axes of an ellipse), PTV position, blocking structures, modulation factor, bed speed, Tx distance, field length. From these the total MU is estimated and compared to the HT TPS value. Results: 10 patients were treated with single fraction StatRT. Delivered dose was verified post Tx with film and ion chamber. The average absolute difference between the HT TPS MU and our calculation was 5.3%, SD = 3.6% and a maximum difference of 9.8%. Time required for data input and calculation was less than 1 minute. Conclusion: an independent sanity check of HT MU is possible for simple treatments with small modulation factors.

Po‐Poster ‐ 05: An evaluation of treatment dose error due to beam attenuation from a carbon fiber table top

The emergence of carbonfiber materials for use in radiation therapy was largely due to its high mechanical strength, low specific density, and its perceived radio‐translucence. These characteristics made it an ideal material for the patient support assembly utilized during treatments. Modern radiation therapy commonly employs beams delivered at oblique angles. With the introduction of carbon fiber table tops the attenuation of the couch is often ignored during treatment planning and there is little effort to avoid intersection of the beam with the table during patient setup. The perception that carbon fiber is relatively radio‐translucent has permitted it to be used while neglecting to consider the effects it may have on the dose to the patient. In this study we have measured the attenuation of the couch under various conditions for 6 and 18 MV photons. We have found dose reductions in phantom of greater than 7%. We further investigate the ability of a commercial treatment planning system (Theraplan Plus) to properly model this effect during the planning stage. Our results show that incorporating the carbon fiber couch in the patient model reduces the dose error to less than 2%. These results reveal that it is worthwhile addressing this real clinical problem in such a manner that it can be routinely considered for all patient treatments. Thus, practical suggestions are proposed for the incorporation of the treatment tabletop into patient treatment planningdose calculations.

Stage III Carcinoma of the Endometrium: A Review of 41 Cases

Forty-one patients with clinical-pathological stage III carcinoma of the endometrium treated between January 1958 and December 1983 at the Ottawa General Hospital were analyzed. Twenty-three patients (56%) presented with clinical stage III and 18 (44%) with pathological stage III. The 5-year survival of all stage III patients was 35%. Patients with clinical stage III had a 5-year survival of 8% compared to 70% for patients with pathological stage III (P less than 0.01). Twenty-nine patients (71%) failed. The site of first recurrence was limited to the abdominal cavity in all but 6 patients (79%). The role of whole abdominal radiation is discussed.