Michael C. Baird

A Prolonged Time Interval Between Trauma and Prophylactic Radiation Therapy Significantly Increases the Risk of Heterotopic Ossification

PURPOSE To ascertain whether the time from injury to prophylactic radiation therapy (RT) influences the rate of heterotopic ossification (HO) after operative treatment of displaced acetabular fractures. METHODS AND MATERIALS This is a single-institution, retrospective analysis of patients referred for RT for the prevention of HO. Between January 2000 and January 2009, 585 patients with displaced acetabular fractures were treated surgically followed by RT for HO prevention. We analyzed the effect of time from injury on prevention of HO by RT. In all patients, 700 cGy was prescribed in a single fraction and delivered within 72 hours postsurgery. The patients were stratified into five groups according to time interval (in days) from the date of their accident to the date of RT: Groups A ≤3, B ≤7, C ≤14, D ≤21, and E >21 days. RESULTS Of the 585 patients with displaced acetabular fractures treated with RT, (18%) 106 patients developed HO within the irradiated field. The risk of HO after RT increased from 10% for RT delivered ≤3 days to 92% for treatment delivered >21 days after the initial injury. Wilcoxon test showed a significant correlation between the risk of HO and the length of time from injury to RT (p < 0.0001). Chi-square test and multiple logistic regression analysis showed no significant association between all other factors and the risk of HO (race, gender, cause and type of fracture, surgical approach, or the use of indomethacin). CONCLUSIONS Our data suggest that there is higher incidence and risk of HO if prophylactic RT is significantly delayed after a displaced acetabular fracture. Thus, RT should be administered as early as clinically possible after the trauma. Patients undergoing RT >3 weeks from their displaced acetabular fracture should be informed of the higher risk (>90%) of developing HO despite prophylaxis.

The Impact of Body Mass Index on Heterotopic Ossification

PURPOSE To analyze the impact of different body mass index (BMI) as a surrogate marker for heterotopic ossification (HO) in patients who underwent surgical repair (SR) for displaced acetabular fractures (DAF) followed by radiation therapy (RT). METHODS AND MATERIALS This is a single-institution retrospective study of 395 patients. All patients underwent SR for DAF followed by RT ± indomethacin. All patients received postoperative RT, 7 Gy, within 72 h. The patients were separated into four groups based on their BMI: <18.5, 18.5-24.9, 25-29.9, and >30. The end point of this study was to evaluate the efficacy of RT ± indomethacin in preventing HO in patients with different BMI. RESULTS Analysis of BMI showed an increasing incidence of HO with increasing BMI: <18.5, (0%) 0/6 patients; 18.5-24.9 (6%), 6 of 105 patients developed HO; 25-29.9 (19%), 22 of 117; >30 (31%), 51 of 167. Chi-square and multivariate logistic regression analysis showed that the correlation between odds of HO and BMI is significant, p < 0.0001. As the BMI increased, the risk of HO and Brooker Classes 3, 4 HO increased. The risk of developing HO is 1.0× (10%) more likely among those with higher BMI compared with those with lower BMI. For a one-unit increase in BMI the log odds of HO increases by 1.0, 95% CI (1.06-1.14). Chi-square test shows no significant difference among all other factors and HO (e.g., indomethacin, race, gender). CONCLUSIONS Despite similar surgical treatment and prophylactic measures (RT ± indomethacin), the risk of HO appears to significantly increase in patients with higher BMI after DAF. Higher single-fraction doses or multiple fractions and/or combination therapy with nonsteroidal inflammatory drugs may be of greater benefit to these patients.

3D Image based Customized versus Standard Treatment Planning for Cervical Cancer High Dose Rate Brachytherapy with Tandem and Ovoids

Purpose and Objective(s): To investigate the advantages of volumetric treatment planning in HDR brachytherapy for cervical carcinoma compared to standardized loading based on 2-D planning techniques. Materials and Methods: Our institution uses volume-based 3-D planning for each tandem and ovoid (T&O) insertion for HDR brachytherapy in the treatment of advanced cervical carcinoma. Here, we attempt to define the benefits of this approach. We re-planned 48 CT-based treatment plans on 12 patients (treated in our facility between February, 2009 and February, 2010) using a commonly used 2-D standard HDR loading of the T&O. All patients had received 4 fractions of 6.5 Gy or 5 fractions of 5.5 Gy to point H or A. The following organs at risk (OARs) were contoured: rectum, bladder, sigmoid, and small bowel. Our customized planning approach required the adjustment of source dwell times and positions to keep doses to the OARs below 80% of the prescription dose. The standardized HDR planning, however, bases the loading time on the length of the tandem. The dwell time for each tandem source position is the same. The dwell time multipliers for the ovoids were 0.33, 0.665 and 1.0, proportionate to the 2 cm, 4 cm, and 6 cm tandem length, respectively. The dose to the highest 2 cc (D2cc) of the OARs were also determined and analyzed. Results: There was a marked change in the value and location of the D2cc for all OARs from one HDR session to the next in both the standard and customized plans. When the data for the 48 plans were analyzed together, there were no significant differences between the customized plans and the standardized plans. However, when data for the individual plans were analyzed, 35% of the 2-D based plans did not meet our treatment planning objectives. Conclusion: Using customized plans for HDR T&O brachytherapy did not always reduce the doses to the rectum, bladder, sigmoid, and small bowels compared to the standardized plans. The dose to the small bowel could be up to 15% higher than the dose to point H or A in the standard plans indicating that customized plans may be superior to the standardized ones for the treatment of patients where this dose is critical.

SU‐GG‐T‐44: Evaluation of Xoft Electronic Brachytherapy System for Intraoperative Treatments

Purpose: The main objective was to evaluate and commission the Xoft Electronic Brachytherapy System for intraoperative treatments.Method and Materials: Using the manufacturer supplied phantom, we evaluated and commissioned the Xoft Electronic Brachytherapy system. We tested well‐chamber constancy and intercomparison, beam output stability with time, start/end effects, and performed radiation surveys. Other checks recommended by the AAPM TG152 were evaluated. Results The Ir‐192 calibrated well chamber is 3.7 times more sensitive when irradiated with the Xoft source than the Xoft calibrated well chamber. Expectation was that both chambers would give approximately the same reading because the Xoft integrated well chamber is cross calibrated in I‐125 source. It takes about 28s for the doserate from the Xoft Unit to ramp up to the treatmentdoserate. The unit delivers 9s worth of treatment during the ramp up phase. For treatment times less than 100s, this would introduce a dosimetry error of about 10% which will be repeated if there is a treatment interruption. Xoft Unit output may vary by up to 5% between 0.25 and 30 minutes. This variation is source dependent. So the minimum time used to collect charges for the AAPM‐TG61 calibration should be 1 minute, not 0.25 minute. Radiation surveys during treatment indicate that surface and intraoperative treatments give rise to exposure rates of 200mR/hr, 30cm from treatment area. Conclusions: Well chamber calibration inconsistency for Xoft Unit needs further investigation. Ramp up time accounts for 9s equivalent treatment time. Correction needed for treatment time less than 150 seconds. Charges for TG61 calibration should be collected for at least 1 minute, not 0.25 minute. Intraoperative and surface treatment produces exposure rate of 200mR/hr at distances of 30 cm. Exercise extreme caution by standing behind lead shield or wearing a lead apron.

SU‐E‐T‐588: 3D Image‐Based Customized Radiation Treatment Planning for Syed Interstitial HDR Brachytherapy for Patients with Parametrial Spread of Cervix Carcinoma

Purpose: To investigate the clinical parameters involved in Syed interstitial HDR brachytherapy for patients with cervical carcinoma. Method and Materials:Our institution uses 3‐D image based treatment planning for all Syed Interstitial implants for advanced cervical carcinoma patients with known parametrical spread. Between 8/2009 and 10/2010, eight such patients were treated at our facility with Syed Interstitial HDR brachytherapy. The high risk CTV (HRCTV), intermediate risk CTV (IRCTV), and organs at risk (OAR: rectum, bladder, sigmoid, and small bowel) were contoured on the planning CT scan. All patients received 7 fractions of 4.2 Gy each in 4 days given bid prescribed to the HRCTV. Planning objectives included keeping doses to the OARs below 80% of prescription dose, to have D90 for the HRCTV greater than 90% of the prescription dose, and for V150 and V200 to be less than 50% and 20%, respectively. OARs were evaluated using doses to 2cc (D2cc). CT scan was performed each day. Results: Between 18 and 21 needles were implanted but only 14 needles were used. Because of needle movements, an average of 2.4 plans were required. The average dose to 2cc (D2cc) of bladder was 63±5% of the prescription dose (29.4 Gy). This value for the rectum was 64%±3.4% . The sigmoid and the small bowel D2cc values were generally all less than 40% of the prescription dose. The D90 for the HRCTV was on average 31.7 Gy, whereas the D90 for the IRCTV was on average 20 Gy. V150 and V200 were on average, 51% and 28%, respectively. Conclusion: It is recommended that CT/MRI images be used for Syed interstitial HDR planning to adequately define the HRCTV, IRCTV, and OAR. The implanted needles always move during patient transfers to and from the HDR suite; thus, CT scans should be repeated daily.