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PO-0780 CLINICAL APPLICATION OF FLATTENING FILTER FREE BEAMS FOR EXTRACRANIAL STEREOTACTIC RADIOTHERAPY
S302
PO-0780 CLINICAL APPLICATION OF FLATTENING FILTER FREE BEAMS FOR EXTRACRANIAL STEREOTACTIC RADIOTHERAPY S. Lang1, B. Shrestha1, S. Graydon1, F. Cavelaars1, C. Linsenmeier1, G. Studer1, O. Riesterer1 1 University Hospital Zürich, Radiation Oncology, Zurich, Switzerland Purpose/Objective: When using FFF beams the dose rate can be increased up to 4 times if compared to flattened beams. Potential advantages of FFF beams when used for extracranial stereotactic radiotherapy are improved dose distribution in the surrounding lung tissue, reduced low dose exposure to the body and shortened treatment time. Concern exists among clinicians with regard to the biological effects of using unprecedented high dose rates. Here we report on our experience with clinical application of FFF beams. Materials and Methods: Patients with different tumors of the lung or upper abdomen were subjected to stereotactic volumetric arc radiotherapy at the Varian TrueBeam using 6 MV FFF or 10 MV FFF beams. In all cases the optimizer was allowed to use the maximum dose rate. Treatment times were recorded and patient, baseline and tumor shifts were assessed by pre- and post-treatment Cone Beam CT (CBCT) imaging. Tumor control and toxicity were retrospectively evaluated. Results: So far 26 patients, 18 patients with tumors in the lung and 8 patients with tumors in the upper abdomen (4 liver, 3 pancreas, 1 adrenal gland tumor), have been treated. The average dose rate per patient used by the optimizer ranged from 905 MU/min to 1389 MU/min for 6 MV FFF and from 1076-1860 for 10 MV FFF beams. The average beam-on time was 1.85 min (+/- 0.65). The mean overall treatment time including patient set-up and CBCT was 18.4 min (+/3.7 min). The average vector of the patient and baseline shifts during treatment were 1.7 mm (+/-1.2 mm) and 1.8 mm (+/- 1.0 mm) The systemic and random errors for 3 dimensional tumor shifts (vrt, lng, lat) were (0.99, 0.98, 0.54) mm and (1.08, 0.66, 0.81) mm for lung treatments and (0.29, 0.48, 0.45) mm and (0.93, 0.47, 0.47) mm for abdominal treatments. At a medium follow up-of 5.03 months (1-16) tumors were controlled in 92% of patients. Minor grade 1 toxicity occurred in 31% and 69% of patients experienced no toxicity. Conclusions: Our early clinical experience indicates that stereotactic radiotherapy using FFF beams is safe and does not result in unexpected toxicity. The substantially shortened treatment time is associated with excellent patient stability. According to Van Herk’s formula ITV to PTV margins of 6 mm are sufficient.
ESTRO 31
ART_5. Distances between each ART CTV and the remaining rCT CTVs were calculated to estimate the residual shape variation for each strategy. Required PTV margins were calculated for adapted and nonadapted treatment using a previously derived recipe for CTV shape variation: MPTV = 3.2*∑ + 0.7*σ + GM. For each strategy the average PTV volume over the whole treatment was calculated taking the fractions before and after adaptation into account. The strategy with the least overall PTV volume was selected and adapted IMRT plans were used to quantify bowel area dose reduction. Results: There was a clear and significant time trend showing rectal and CTV volume reduction (Table 1), which resulted in a GM error up to 0.5 cm at the upper anterior part of the CTV (Fig. 1). This GM error was not fully anticipated using ART in the first week, due to later occurence. Substantial systematic and random shape variation (Fig. 1) required a PTV margin up to 2.4 cm at the upper-anterior part of the CTV when no ART was used. The average overall treatment PTV volumes for the different strategies were 993, 905, 882, 875, 870, 872 cc for no_ART, ART_1, 2, 3, 4 and ART_5, respectively. With ART_4 a maximum 0.7 cm margin reduction and a significant PTV volume reduction from 993 cc to 847 cc (p<0.01) was achieved. The bowel volume receiving ≥45 and ≥50 Gy was reduced from 92 to 73 cc and 44 to 27 cc, respectively (p<0.01).
POSTER: CLINICAL TRACK: TARGET AND VOLUME DEFINITION AND IMAGING PO-0781 POTENTIAL BENEFIT OF ADAPTIVE RADIOTHERAPY DURING CHEMORADIOTHERAPY OF RECTAL CANCER J. Nijkamp1, C.A.M. Marijnen2, M.B. van Herk1, B. van Triest1, J.J. Sonke1 1 The Netherlands Cancer Institute - Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands 2 Leiden University Medical Center, Clinical Oncology, Leiden, The Netherlands Purpose/Objective: Neo-adjuvant (chemo-) RT has been established as standard of care for rectal cancer. The major geometrical uncertainties during RT are caused by CTV delineation- and shapevariation. These uncertainties lead to large PTV margins, unless they can be estimated and minimized. For shape variation, adaptive RT (ART) can be used to improve the estimate of the average CTV shape during treatment using repetitive imaging data acquired during treatment, thereby minimizing systematic errors. The aim of this study was to quantify the potential margin and subsequent bowel exposure reduction with ART during RT of locally-advanced rectal cancer. Materials and Methods: Repeat CT (rCT) scans were acquired for 28 patients treated with 25x2 Gy. A total of 10 scans were acquired for each patient, 1 for treatment planning (pCT), 5 daily during the first week, followed by 4 weekly scans. The CTV, rectum and bowel area were delineated on all scans, and CTV shape variation was estimated by calculating local surface distances between the pCT CTV and the rCT CTVs. Shape variation was quantified in surface maps representing the group mean (GM), systematic (∑) and the random error (σ). Five ART strategies were tested, consisting of calculation of an average CTV over the pCT and one to five rCT CTVs, resulting in ART_1 to
Conclusions: With adaptive radiotherapy, required PTV margins can be reduced from 2.4 to 1.7 cm, resulting in significantly less dose to the bowel area. Due to time trends, it is expected that a second adaptive intervention later in the treatment could further reduce the required margins and bowel exposure.
PO-0780 CLINICAL APPLICATION OF FLATTENING FILTER FREE BEAMS FOR EXTRACRANIAL STEREOTACTIC RADIOTHERAPY S. Lang1, B. Shrestha1, S. Graydon1, F. Cavelaars1, C. Linsenmeier1, G. Studer1, O. Riesterer1 1 University Hospital Zürich, Radiation Oncology, Zurich, Switzerland Purpose/Objective: When using FFF beams the dose rate can be increased up to 4 times if compared to flattened beams. Potential advantages of FFF beams when used for extracranial stereotactic radiotherapy are improved dose distribution in the surrounding lung tissue, reduced low dose exposure to the body and shortened treatment time. Concern exists among clinicians with regard to the biological effects of using unprecedented high dose rates. Here we report on our experience with clinical application of FFF beams. Materials and Methods: Patients with different tumors of the lung or upper abdomen were subjected to stereotactic volumetric arc radiotherapy at the Varian TrueBeam using 6 MV FFF or 10 MV FFF beams. In all cases the optimizer was allowed to use the maximum dose rate. Treatment times were recorded and patient, baseline and tumor shifts were assessed by pre- and post-treatment Cone Beam CT (CBCT) imaging. Tumor control and toxicity were retrospectively evaluated. Results: So far 26 patients, 18 patients with tumors in the lung and 8 patients with tumors in the upper abdomen (4 liver, 3 pancreas, 1 adrenal gland tumor), have been treated. The average dose rate per patient used by the optimizer ranged from 905 MU/min to 1389 MU/min for 6 MV FFF and from 1076-1860 for 10 MV FFF beams. The average beam-on time was 1.85 min (+/- 0.65). The mean overall treatment time including patient set-up and CBCT was 18.4 min (+/3.7 min). The average vector of the patient and baseline shifts during treatment were 1.7 mm (+/-1.2 mm) and 1.8 mm (+/- 1.0 mm) The systemic and random errors for 3 dimensional tumor shifts (vrt, lng, lat) were (0.99, 0.98, 0.54) mm and (1.08, 0.66, 0.81) mm for lung treatments and (0.29, 0.48, 0.45) mm and (0.93, 0.47, 0.47) mm for abdominal treatments. At a medium follow up-of 5.03 months (1-16) tumors were controlled in 92% of patients. Minor grade 1 toxicity occurred in 31% and 69% of patients experienced no toxicity. Conclusions: Our early clinical experience indicates that stereotactic radiotherapy using FFF beams is safe and does not result in unexpected toxicity. The substantially shortened treatment time is associated with excellent patient stability. According to Van Herk’s formula ITV to PTV margins of 6 mm are sufficient.
ESTRO 31
ART_5. Distances between each ART CTV and the remaining rCT CTVs were calculated to estimate the residual shape variation for each strategy. Required PTV margins were calculated for adapted and nonadapted treatment using a previously derived recipe for CTV shape variation: MPTV = 3.2*∑ + 0.7*σ + GM. For each strategy the average PTV volume over the whole treatment was calculated taking the fractions before and after adaptation into account. The strategy with the least overall PTV volume was selected and adapted IMRT plans were used to quantify bowel area dose reduction. Results: There was a clear and significant time trend showing rectal and CTV volume reduction (Table 1), which resulted in a GM error up to 0.5 cm at the upper anterior part of the CTV (Fig. 1). This GM error was not fully anticipated using ART in the first week, due to later occurence. Substantial systematic and random shape variation (Fig. 1) required a PTV margin up to 2.4 cm at the upper-anterior part of the CTV when no ART was used. The average overall treatment PTV volumes for the different strategies were 993, 905, 882, 875, 870, 872 cc for no_ART, ART_1, 2, 3, 4 and ART_5, respectively. With ART_4 a maximum 0.7 cm margin reduction and a significant PTV volume reduction from 993 cc to 847 cc (p<0.01) was achieved. The bowel volume receiving ≥45 and ≥50 Gy was reduced from 92 to 73 cc and 44 to 27 cc, respectively (p<0.01).
POSTER: CLINICAL TRACK: TARGET AND VOLUME DEFINITION AND IMAGING PO-0781 POTENTIAL BENEFIT OF ADAPTIVE RADIOTHERAPY DURING CHEMORADIOTHERAPY OF RECTAL CANCER J. Nijkamp1, C.A.M. Marijnen2, M.B. van Herk1, B. van Triest1, J.J. Sonke1 1 The Netherlands Cancer Institute - Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands 2 Leiden University Medical Center, Clinical Oncology, Leiden, The Netherlands Purpose/Objective: Neo-adjuvant (chemo-) RT has been established as standard of care for rectal cancer. The major geometrical uncertainties during RT are caused by CTV delineation- and shapevariation. These uncertainties lead to large PTV margins, unless they can be estimated and minimized. For shape variation, adaptive RT (ART) can be used to improve the estimate of the average CTV shape during treatment using repetitive imaging data acquired during treatment, thereby minimizing systematic errors. The aim of this study was to quantify the potential margin and subsequent bowel exposure reduction with ART during RT of locally-advanced rectal cancer. Materials and Methods: Repeat CT (rCT) scans were acquired for 28 patients treated with 25x2 Gy. A total of 10 scans were acquired for each patient, 1 for treatment planning (pCT), 5 daily during the first week, followed by 4 weekly scans. The CTV, rectum and bowel area were delineated on all scans, and CTV shape variation was estimated by calculating local surface distances between the pCT CTV and the rCT CTVs. Shape variation was quantified in surface maps representing the group mean (GM), systematic (∑) and the random error (σ). Five ART strategies were tested, consisting of calculation of an average CTV over the pCT and one to five rCT CTVs, resulting in ART_1 to
Conclusions: With adaptive radiotherapy, required PTV margins can be reduced from 2.4 to 1.7 cm, resulting in significantly less dose to the bowel area. Due to time trends, it is expected that a second adaptive intervention later in the treatment could further reduce the required margins and bowel exposure.