the planning CT for each patient; one with clinically used clinical target volume (CTV) to planning target volume (PTV) margins for image guided radiotherapy (non-ART) and one with expected clinical oART CTV-PTV margins. The oART margins were reduced 5 mm isotropically and 5 mm anteriorly for the positive lymph nodes and elective target, respectively, compared to non-ART; corresponding to a 50 % reduction from non-ART. Treatment sessions were simulated on six weekly CBCT images for one of the patients. Bladder, rectum and bowel cavity structures were automatically delineated and manually adjusted to match the anatomy on the CBCT. For non-ART, the reference plan was re-calculated on the anatomy of the day with rigidly propagated targets. For oART, the plan was re-optimized using targets that were either deformed or rigidly propagated, according to the expected clinical workflow. The volume of bowel cavity receiving 30 and 45 Gy (V 30Gy and V 45Gy , respectively) was evaluated and compared between the two workflows. Results: For the reference treatment plans, the CTV-PTV margin reduction in oART resulted in a median reduction (interquartile range) for bowel cavity V 30Gy and V 45Gy of 6.4 (3.8 − 8.5) % and 11.4 (9.7 − 14.0) %, respectively. This corresponded to 39.4 (17.9 − 80.2) cc and 48.5 (35.6 − 61.3) cc, respectively. Re-optimizing the plan to the anatomy of the six CBCT images lead to a median reduction [minimum; maximum] for bowel cavity V 30Gy and V 45Gy of 9.2 [7.5; 12.3] % and 13.1 [10.8; 15.3] %, respectively, corresponding to 58.0 [46.5; 86.0] cc and 56.9 [46.4; 70.3] cc. Conclusion: Simulated oART of anal cancer resulted in reduced dose to the bowel cavity, indicating potential reduction in gastrointestinal toxicity.Early results demonstrated that the superiority of oART compared to non-ART was consistent during the course of treatment. Further oART sessions of other patients are planned to be simulated before the ASTRO conference. A single-arm phase II trial to investigate the clinical impact has been developed and awaits approval.

No abstract

6058 Background: Intraoperative Radiation Therapy (IORT) allows precise delivery of radiation therapy (RT) to a limited target area at high risk of cancer recurrence while minimizing RT to nearby organs at risk. IORT may be particularly beneficial for patients with locally recurrent head and neck cancer (HNC) in a previously irradiated field and for locally advanced HNC cases, in which obtaining negative surgical margins may be difficult. This study aims to present a single institution experience with IORT for HNC patients. Methods: This study included HNC patients treated consecutively with IORT at our institution between 2014 and 2018. Charts were reviewed for patients’ and tumors’ characteristics, IORT technical details, IORT-induced adverse events, and treatment outcomes. Results: The study included 23 eligible patients. Median patient age was 66 (range 33-91). Tumor sites included parotid gland (43%), lymph nodes (43%), oral tongue (9%), and ear (4%). 52% of patients received IORT upfront with or without postoperative adjuvant external beam radiation therapy (EBRT), while 48% received salvage IORT after local tumor recurrence. The median prescribed IORT dose was 7.5 Gy (range 5-14 Gy) in a single fraction prescribed to 5 mm depth with Flat applicators (median diameter of 5 cm). 92% of patients did not experience wound healing complications. One patient (4%) developed postoperative acute thromboembolic stroke, a second patient (4%) experienced protracted wound healing. At a median follow-up of 36 months (range 2-81), 42% of patients presented with no evidence of disease (NED), overall survival was 54%, 13% of patients were alive with disease, and 46% died with disease. Local-regional recurrence rate was 39% (median time to local recurrence was 18 months, range 2-60), rate of distant metastasis was 43% (median time to distant metastasis was 23 months, range 5-60), and 30% of patients had both local-regional recurrence and distant metastases. The percent of local-regional recurrence and distant metastases among patients receiving salvage IORT was 64% and 73% respectively, compared to 23%, and 15% respectively in those receiving upfront IORT with or without adjuvant EBRT. Conclusions: In this single institution chart review study, IORT to locally advanced and recurrent HN cancer patients was a safe treatment modality, with tumor control comparable to historical EBRT data. Larger prospective studies are needed to further assess the utility of IORT in the management of locally advanced and recurrent HN cancer.

e12620 Background: Neoadjuvant chemotherapy (NAC) is commonly utilized in women with locally advanced breast cancer, usually followed by surgery and radiation therapy (RT). Many studies aimed to address the risk factors contributing to a higher incidence of lymphedema in patients with breast cancer. Our group previously reported the extent of surgery increases the risk of lymphedema. Adjuvant chemotherapy with taxane-based regimens are associated with an increased risk of lymphedema likely due an increase in interstitial extracellular fluid volume therefore resulting in fluid retention. This study aims to directly compare and characterize the risk of lymphedema in patients receiving paclitaxel versus docetaxel-based NAC. Methods: This is a retrospective study approved by our institutional review board. The study included women with breast cancer treated consecutively at our institution with taxane-based NAC followed by surgery and RT from 2006 to 2018. Patients and tumor characteristics including age, race, body mass index (BMI), clinical stage, hormone receptor, HER2 status, type of surgery, RT techniques, and type of NAC (Paclitaxel versus Docetaxel), and its association to risk of lymphedema were analyzed using univariable and multivariable binary logic regression tests. Lymphedema was assessed before RT and at follow up visits, and was identified by >2.0-cm increase in arm circumference, or >10% increase in limb volume, or new self-reported lymphedema symptoms. Results: A total of 263 patients treated with either paclitaxel or docetaxel-based NAC were identified and analyzed. At a median follow up of 28.4 months (range 3.5-158.7 months). 26.2% (69/263) of patients developed lymphedema. On a multivariable analysis, patients who underwent axillary lymph node dissection (ALND) had a significantly higher rate of lymphedema (42.6%) compared to those who had only a sentinel lymph node biopsy (SLNB, 10.5%, p<0.05). Regardless of the type of surgery, there was no significant difference in rates of lymphedema between patients who received paclitaxel versus docetaxel-based NAC (28.7% vs 21.3%). However, among high-risk patients who underwent mastectomy with ALND, NAC with Paclitaxel was associated with a significantly higher rate of lymphedema compared to docetaxel (56.8% vs 22.7%, RR 2.50, p<0.05). Conclusions: This represents one of the largest studies examining the impact of taxane-based NAC on the risk of lymphedema in women with breast cancer. In this study, paclitaxel-based NAC was associated with a significantly higher risk of lymphedema in women who underwent mastectomy and ALND compared to docetaxel based chemotherapy. A larger, balanced, prospective study is warranted to verify this previously unidentified lymphedema risk from paclitaxel and guide individualized NAC decision.

array and an ion chamber for absolute dose measurement at isocenter. MU changes were evaluated in four groups: 0-25%, 26-50%, 51-75% and above 75% difference between adapted and original plans. Overall, MU changes ranged from 0%-102% between original and adapted fractions. Results: The pass rates of the adapted plans were statistically significantly different from those of the original plans by up to 5% (2-tailed t-test). In 6 cases (19%) the relative dose passed and the absolute dose did not. In 4 cases (12%), the QA did not pass even using a 3mm/3% criteria. In 22 of 27 plans (82%) the ion chamber measurements were within 3% of the predicted dose. The other 5 plans had a larger than 3% difference between planned and measured dose at isocenter. In 5 plans the isocenter was too lateral to measure with an ion chamber. Conclusion: Our results show the vendor-supplied QA tool is in good correspondence with post treatment QA measurement. We did not find any correlation between the magnitude of MU changes between the original and adapted plans and the QA pass rates, which is reassuring given the large differences in some of the plans. Treatment site was not correlated to either the magnitude of MU changes or the QA passing rates. The vendorsupplied QA tool cannot be used when treating a patient with the original plan, and thus could not be assessed on these plans. As the optimization algorithm is identical in the treatment planning system and in the adaptive planning process in the treatment delivery computer, the underlying cause for the differences observed in passing rates is unclear. Overall, our results prove the reliability of the vendor-supplied MU secondary check as adequate real-time QA for confidently treating adapted plans.