Abstract book - ESTRO meets Asia

S32 ESTRO meets Asia 2018

central insert includes a film holder for supplementary measurements. The custom phantom includes removable lungs that are replaced with solid water inserts to investigate the effect of inhomogeneity on IMRT and VMAT deliveries. Figure 1 shows the custom phantom. The CC13 chambers are connected to the TomoTherapy ® TomoElectrometer, an 8 channel reference class electrometer for simultaneous measurement on all chambers for each audit case. The IMRT and VMAT planning cases were designed for addition to the conformal Level III audit. Clinical plans were prepared based on the AAPM Publication TG119 [1] and adapted for use in the ACDS audit program.

treatment in stage II and III rectal cancer.There are two approaches to pelvic RT for resectable rectal cancer: short-course radiation and long course chemoradiotherapy(CRT).Polish and Australian randomized studies compared short-course radiation and immediate surgery with long-course CRT and delayed surgery.In these studies similar long-term survival and local control have been reported for both these approaches but pathological complete response(pCR) is not better with short course RT. Moreover studies have shown better tumor downstaging with delayed surgery. So the idea is to combine the benefits of delayed surgery for improved tumor downstaging with short course RT and adding two cycles of chemotherapy between short course RT and surgery to improve pCR rates. In this context the use of short-course radiotherapy may have some advantages and needs to be tested in clinical trials. Aim: To compare the tumour response clinically, radiologically and histopathologically To compare the toxicities between the two arms Material and Methods This prospective randomized study was a two arm study in which short course radiotherapy followed by two cycles of chemotherapy was compared with conventional neoadjuvant chemoradiotherapy in rectal cancer.Patients assigned to study group(short course RT) were given 25 Gy(5 Gy/fraction) in 5 days.Following a gap of 1 week after RT, patients were given two cycles of Capecitabine and Oxaliplatin (CAPOX)based chemotherapy.Patients assigned to control group(conventional CRT) were given radiation of 50.4 Gy in 28 fractions along with tablet Capecitabine on RT days.Patients were assessed for surgery after 4-6 weeks of completion of chemoradiation.Overall treatment time to surgery was similar in both the arms i.e. 10-12 weeks. Results Of the 28 entered patients, 27 were eligible for analysis; 14 in study arm and 13 in control arm.The pCR rate was 6.7% in study arm while it was 0 in control arm(p=0.343). 33.3% patients in study arm and 53.8% patients in control arm had partial response(p=0.274). 53.3% patients in study arm and 46.2% patients in control arm had stable disease(p=0.705).None of the patients in both the arms had progressive disease.Acute toxicities were lower in study arm.The absence of hematological toxicity in 60% patients in study arm was statistically significant (p=.001).20% patients in study arm and 92.3% patients in control arm had grade 2-3 toxicity (p=0.005).The absence of skin toxicity in 73.3% patients in study arm was statistically significant(p=.001).Grade 3 toxicity was seen in 15.4% patients in control arm and no patient in study arm(p=0.116). Conclusion pCR rates in the two arms are comparable.But the major advantage for the 5*5 Gy regimen with chemotherapy in neo-adjuvant setting is the improved toxicity profile compared with conventional CRT with significant reduction in acute toxicities in short course RT arm. OC-083 Good prognostic value of increased lymph node yield in rectal cancer with neoadjuvant chemoradiation Y. Wang 1 1 Fudan University Shanghai Cancer Center, Department of Radiation Oncology, Shanghai, China Purpose or Objective It is debated whether a lymph node yield (LNY) of at least 12 is necessary for locally advanced rectal cancer (LARC) patients with neoadjuvant chemoradiotherapy (neo-CRT). Some studies showed that an LNY of less than 12 in those patients was related to good tumor response. This study

Figure 1 Images of the custom CIRS phantom for the new ACDS Level III audit, showing the removable lungs and removable central insert. Results The measurement of conformal beams together with IMRT/VMAT allows the ACDS to assess many components of the TPS beam model. Audit cases are designed to build in complexity case by case, looking first at reference dosimetry, then modelling of PDD, asymmetric fields, lung and wedges. After establishment of the correct modelling of base components, the ACDS can then diagnose more subtle issues with IMRT/VMAT delivery due to MLC and couch modelling. Figure 2 shows the results from 246 audit plans as a function of the exclusion volume dose (organ at risk dose). As less dose is delivered to the OAR, the plans become more complex, and there is an increased spread in the difference between planned and measured doses. This indicates that some beam models are starting to fail to model the complex plans accurately.

Figure 2 Absolute dose difference of planned and measured IMRT/VMAT doses for 246 plans as a function of the exclusion volume dose (organ at risk dose). Conclusion The ACDS custom end to end audit including both conformal and IMRT/VMAT capability has good diagnostic ability to assess many components of a TPS beam model.

Proffered paper: CL3: Lower GI and radiomics

OC-082 Long-course NACRT vs short course RT(5*5GY) plus consolidation CT with delayed surgery in CA rectum N. Thakur 1 1 Post Graduate Institute of Medical Education and Resear ch, Radiation Oncology, Chandigarh, India Purpose or Objective Background: Chemoradiotherapy followed by surgery followed by adjuvant chemotherapy is the mainstay of