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tion of dose distributions in lung tumours have been used to correct DVH's planned in a static situation with pencil beam to a dynamic situation, considering breathing motions and a more correct dose calculation. Results: At 17 mo. after end of study there is one local relapse alone and 7 cases displaying distant metastasis of which 2 are combined with regional failure (1 T1 and 6 T2). Grade 3 toxicity was seen in 3 patients with dyspnoe at 3, 6 and 9 months, not correlated with pneumonitis or fibrosis. Spirometry data will be reported. Grade 3 toxicity with pain in the thorax was seen in 2 patients at 9 and 12 months not associated with rib fracture or tumor in proximity to the thoracic wall. 13/59 (22%) did not have any side effects. The most common side effect at 6 weeks was grade I skin reactions (19 patients with grade I and 4 with grade 2). At 17 mo. after end of study 11 pts. had obtained CR, 30 showed PR and 8 pats. had either local failure and/or distant metastases. Conclusion: Using 15 x 3 Gy to stage I NSCLC results in acceptable early toxicity and favourable local control rate non inferior to what has been reported with fractionated RT.

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Purpose: We developed an adaptive scheme for prostate cancer radiotherapy based on kV cone‐beam‐CT (CBCT) images that are obtained on the machine during the first six treatment days. The aim of this scheme is to improve knowledge of the average prostate position and average rectum shape and safely reduce the PTV margin. Method and Materials: CBCT‐scans, acquired our on Elekta Synergy systems, were first matched on the planning CT scan using the pelvic bones. Automatic grey‐value matching was then used to match the prostates of the CBCT‐scans to the prostate of the planning CT scan. The mean of the obtained translations and rotations was used to move the prostate of the planning CT scan to its average position. Subsequently, the rectal wall was delineated in the CBCT‐scans, and coordinates of corresponding points of the 7 rectums were averaged to obtain the average rectal wall. Based on average prostate and rectum a new IMRT treatment plan was made with a reduced PTV margin of 7 mm. Weekly CBCT‐scans were made to verify that the new PTV encompasses the prostate. Results: So far, 16 patients were successfully treated with our adaptive treatment scheme. For 85% of the CBCT‐scans a successful grey‐value match was obtained, the other scans were discarded. For 88 out of 89 verification scans the prostate was inside the PTV. The mean dose received by the rectum reduced on average by 7.6%, and the equivalent uniform dose (a=12) by 1.5%. Conclusion: This is the first routine clinical application of soft tissue image guidance for the prostate using kV CBCT. Contrary to adaptive schemes that use implanted markers, our method is non‐invasive and improves localization of both prostate and rectum. Conflict of Interest: Elekta, Inc financially supported part of this study.

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