Abstract Book

ESTRO 37

S602

Purpose or Objective Dynamic Jaw (DJ) Mode in Helical TomoTherapy® (HT) allows the jaws to move continuously and to adapt the field width (FW) dynamically at the cranial and caudal edges of a target to reduce the cranio-caudal dose penumbra during treatment. Few studies claimed that it can improve the cranio-caudal dose distribution without prolonging the treatment time in treating different types of cancer. Also, studies suggested that DJ with a wider 5 cm FW can replace fixed jaws (FJ) with 2.5 cm FW, which can sustain the plan quality and reduce the treatment delivery time. Yet, the study on breast cancer with supraclavicular fossa (SCF) nodal involvement using DJ Mode in HT is limited. This study aims to evaluate the DJ Mode retrospectively by comparing their dosimetric quality with Normal Tissue Complication Probability (NTCP) of OARs and treatment delivery time with FJ Mode on treating left-side breast with SCF nodal involvement. The best choice of mode will be advised to maximize the patient’s benefit. Material and Methods All post-mastectomy patients, who had been irradiated for left-side breast with SCF nodal involvement and planned under HT using DJ Mode with 2.5 cm FW (DJ2.5), from November 2014 to August 2016 at Hong Kong Sanatorium & Hospital, were selected retrospectively in this study. With the same dose constraint and prescription as the treated DJ2.5 plan, 2 extra plans using DJ Mode with 5cm FW (DJ5.0) and FJ mode with 2.5 cm FW (FJ2.5) were computed for plan comparison. Homogeneity index (HI) and ICRU-recommended dose- volume specifications (e.g. D95) of PTV for chestwall (CW) and SCF, dose-volume specifications with clinical value of OARs, NTCP of heart and lung, treatment delivery time and actual modulation factor (MF) were used for comparison. Results

FJ2.5. For other OARs, including heart and lung, FJ2.5 showed a better OARs sparing than DJ5.0. Radiobiologically, NTCP of heart and lung for all plans were close to 0, while those of DJ2.5 were calculated to be the lowest. The delivery time and actual MF were found to have statistical significance. The average delivery time of DJ5.0 was significantly lower than DJ2.5 and FJ2.5 by almost 40%. The actual MF was significantly higher in FJ2.5 than DJ2.5.

Conclusion No statistical significance was found in those dosimetric and radiobiological parameters among 3 modes while the delivery time has greatly reduced by using DJ5.0. A shorter treatment time can minimize intra-fractional error and better the patient’s experience during treatment. Therefore, DJ5.0 is suggested to be the optimization mode in HT for left-breast cancer patient with SCF nodal involvement. PO-1072 Head & Neck VMAT Auto-Planning in Pinnacle. A class solution. P.G.M. Van Kollenburg 1 , L.C.W. Bouwmans 1 , J.M.A.M. Kusters 1 , E.J.L. Brunenberg 1 , M.C. Kunze-Busch 1 , T. Dijkema 1 , J.H.A.M. Kaanders 1 1 UMC St Radboud Nijmegen, Dept. of radiation oncology 874, Nijmegen, The Netherlands Purpose or Objective To create an Auto-Planning (A-P) VMAT technique for irradiation of head and neck tumors, that generates conformal and homogeneous treatment plans, with adequate coverage of the target volume and optimal sparing of the organs at risk (OARs), as good as but preferably superior to operator-driven VMAT treatment plans. Secondary aim was to reduce planning-hands-on- time. Material and Methods Ten H&N-cancer patients treated with VMAT were selected for dosimetric comparison with an A-P VMAT technique. All patients underwent a CT-scan with 3 mm slice thickness. The following organs at risk were contoured: parotid glands, submandibular glands, larynx, oral cavity, mandible, esophageal inlet, pharynx constrictor, spinal canal and brainstem. CTV’s were delineated based on EORTC 1219 H&N study protocol. A margin of 3 mm was used to generate the PTV. Treatment plans were created in the Pinnacle 3 treatment planning system V.9.10 using 2 full-arc 6 MV photon beams. A SIB technique was used to deliver 50.3 Gy to the elective and 68 Gy to the boost volume in 34 fractions, 5 times a week. For the new technique the A-P module in Pinnacle was used. Table 1 shows the set-up parameters of this new technique. Treatment was delivered on an Elekta linac with Agility collimator. PTV coverage and dose to the OARs were analyzed to compare the planning techniques. Results As can be seen in figure 1 and table 2, the A-P VMAT technique reduces the spinal canal and brainstem maximum dose and the mean dose to the other OARs.

The statistical results showed no significance in all the parameters of PTV and OARs, except for V20 of whole lung. Relatively DJ2.5 and DJ5.0 had the best and the worst dosimetric quality respectively in term of dose- volume specifications of PTV and OARs among 3 modes. Larynx and liver, sited superior and inferior to PTV, were the only OARs giving a relatively lower dose in DJ5.0 than