T. Rabin

Continuous Positive Airway Pressure for Motion Management in Stereotactic Body Radiation Therapy to the Lung: A Controlled Pilot Study.

OBJECTIVE To determine the effect of continuous positive airway pressure (CPAP) on tumor motion, lung volume, and dose to critical organs in patients receiving stereotactic body radiation therapy (SBRT) for lung tumors. METHODS AND MATERIALS After institutional review board approval in December 2013, patients with primary or secondary lung tumors referred for SBRT underwent 4-dimensional computed tomographic simulation twice: with free breathing and with CPAP. Tumor excursion was calculated by subtracting the vector of the greatest dimension of the gross tumor volume (GTV) from the internal target volume (ITV). Volumetric and dosimetric determinations were compared with the Wilcoxon signed-rank test. CPAP was used during treatment if judged beneficial. RESULTS CPAP was tolerated well in 10 of the 11 patients enrolled. Ten patients with 18 lesions were evaluated. The use of CPAP decreased tumor excursion by 0.5 ± 0.8 cm, 0.4 ± 0.7 cm, and 0.6 ± 0.8 cm in the superior-inferior, right-left, and anterior-posterior planes, respectively (P ≤ .02). Relative to free breathing, the mean ITV reduction was 27% (95% confidence interval [CI] 16%-39%, P<.001). CPAP significantly augmented lung volume, with a mean absolute increase of 915 ± 432 cm(3) and a relative increase of 32% (95% CI 21%-42%, P=.003), contributing to a 22% relative reduction (95% CI 13%-32%, P=.001) in mean lung dose. The use of CPAP was also associated with a relative reduction in mean heart dose by 29% (95% CI 23%-36%, P=.001). CONCLUSION In this pilot study, CPAP significantly reduced lung tumor motion compared with free breathing. The smaller ITV, the planning target volume (PTV), and the increase in total lung volume associated with CPAP contributed to a reduction in lung and heart dose. CPAP was well tolerated, reproducible, and simple to implement in the treatment room and should be evaluated further as a novel strategy for motion management in radiation therapy.

SU‐FF‐T‐452: Whole Pelvic IMRT Treatment for Post Operative Gynecological Malignancies, Sheba Experience

Background and Purpose: To evaluate the use of IMRTtreatment for post operative gynecological malignancies for better organ at risk sparing to reduce treatment toxicity that will allow dose escalation. Creating guidelines for that treatment. Recognize treatment limits and problems in ITPS and QA for very large IMRT volumes and fields. Patients and Methods: five patients were selected with endometrial cancertreated postoperatively. Bladder, rectal wall, small and large bowel were delineated as organs at risk. A seven field IMRT plan prescription dose 50.4 Gy and compared with conformal 4 fields plan (DVH). QA performed with film dosimetry and ion chambers.Results: significant improvements were observed for irradiated volume of rectal wall and bladder. With IMRT the average irradiated volume of small and large bowel was reduced dramatically and the impact of IMRT was large for postoperative patients, in one patient treatment volume was very large (24833cc of normal tissue) the ITPS could calculate only if the calculation grid was 10mm spaced, that lead to a disagreement in QA and patient was treated 3D conformal. Conclusions: IMRT significantly reduced the absolute volume of rectal wall, bladder and bowel irradiated at the prescribed dose level in post operative gynecologic patients. Due to large treatment volumes we have to be aware of disagreements between calculations and dosimetry.

SU‐FF‐T‐185: Dosimetric Comparison of Tandem and Ovoids Vs. Tandem and Ring for Intracavitary Gynaecologic Applications

Purpose: To evaluate dosimetric differences in Tandem and Ovoid (TO) and Tandem and Ring (TR) gynaecologic brachytherapy applicators. Method and Materials: 10 patients with cervical cancer (stages II–IV) were treated with three brachytherapy applications: either one TO and two TR, or one TR and two TO applications. All patients underwent CT scans at 2.5 mm slice thickness. Contrast was inserted into the bladder prior to scan. Patients were prescribed 8 Gy to ICRU points A, with additional optimization goals of maintaining the traditional pear‐shaped dose distribution and limiting bladder and rectum doses to below 6 Gy. ICRU bladder and rectum point doses, as well as mean and maximum doses were calculated. Maximum dose was defined as the highest dose received by 2cc of the organ. Total treatment time and volumes treated to 95%, 85% and 50% of the prescription dose were also compared. Data were analyzed using the Mann‐Whitney rank‐sum test. Results: There were no significant differences between TO and TR applicators in doses to prescription points or to critical organs. However, there was a significant difference (p<0.05) between the applicators in the treated volumes and total treatment time. The TO treated larger volumes over a longer time. The treated volumes were also found to be significantly different between applicators within each patient (p<0.05, Chi‐square). Conclusion: Our results demonstrate that the two applicators, whilst delivering prescription doses to points A and keeping critical organdoses within tolerance, treat significantly different volumes. It is unclear if this difference is clinically advantageous or not. TO applicators may be treating healthy tissue unnecessarily, or TR applicators may be underdosing tumortissue. Further investigation with appropriate imaging modalities is required for accurate delineation of target volumes. Clearly, the TR and TO are not identical, and should not be used interchangeably without further study.