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QUANTIFICATION OF ORGAN MOTION DURING CHEMO-RADIATION OF RECTAL CANCER USING CONE BEAM CT (MEASURE STUDY)
S 182
IGRT: VOLUME DEFINITION , TREATMENT MARGINS AND GEOMETRIC UNCERTANTIES
Conclusions: The "optimized" 3D CT scan protocol was found to perform best as it accurately reflects the full excursion of the moving object with equivalent CTDIvol values to the other protocols. For ITV-based treatment plan strategy, 3D CT scan parameters should be adapted as proposed here to obtain ITV close to those obtained with 4D CT. Care should be taken, to modify the protocol according to patient breathing signal. 490 poster (Physics Track) PATIENT FIXATION WITH A BITE FRAME: A DOSIMETRIC VALIDATION OF A 2 MM PTV MARGIN USING MEASURED SETUP INACCURACIES Y. van Herten1 , J. Van de Kamer1 , R. Pool1 , J. Wiersma1 , L. Blank1 , A. Bel1 1 ACADEMIC M EDICAL C ENTER, Radiation Oncology, Amsterdam, Netherlands
Purpose: To validate the 2 mm PTV margin by investigating dosimetric effects on CTV coverage of measured setup inaccuracies. Materials: For 6 patients with optic nerve meningeoma a treatment plan with a 2 mm PTV margin was made (3 to 8 beams, multiple couch rotations, 30 fractions of 180cGy). All patients were positioned in prone position, fixed to the couch with a bite frame and supported by a transparent mask. The first four patients were positioned to the isocentre using the laser system only. The last two were positioned on pre-determined couch coordinates and fine tuned using the laser in longitudinal and vertical direction only. For each beam we used the actual couch positions during treatment to calculate the cumulative dose distribution of all delivered fractions. These calculations were performed with APlan (research module derived from Plato RTS (Nucletron, NL)). We calculated the DVHs of CTV, PTV, eye, lens and hypophysis, for both planned and actually delivered dose distributions and used this as a basis for validation of the 2 mm PTV margin. Results: For patients positioned on the lasers only the spread in couch coordinates is large (up to +/- 1.5cm), in particular for the lateral position which is difficult to assess: the laser indicating the lateral position is blocked by the head and is therefore hardly visible on both bite frame and mask while the patient is in prone position. Hence, positioning on "fixed" couch coordinates is more accurate, as independently confirmed with conebeam-CT. Obviously, for a small spread in couch coordinates the target coverage was maintained and no clinically relevant dose increase to the organs at risk was observed. However, for situations with a larger variation dose coverage differences became more serious as shown by the following example. We simulated a ’systematic’ lateral shift of 2 cm in 14% of the fractions, resulting in a target coverage of only 50% (see figure). For such a large shift the dose to the organs at risk increased, although in this case it seemed to be limited.
Example of a simulation with a 2 cm spread in lateral couch coordinates for 14% of the fractions. CTV coverage decreased from 99.3% to 50%. Conclusions: Fixation with a bite frame combined with positioning based on couch coordinates is easy and facilitates a 2 mm setup margin while maintaining excellent target coverage. 491 poster (Physics Track) QUANTIFICATION OF ORGAN MOTION DURING CHEMORADIATION OF RECTAL CANCER USING CONE BEAM CT (MEASURE STUDY) I. Chong1 , M. Hawkins1 , B. O’Neill1 , K. Thomas2 , H. McNair3 , V. N. Hansen3 , A. Aitken3 , D. Tait1 1 R OYAL M ARSDEN NHS F OUNDATION T RUST, Clinical Oncology, Sutton, United Kingdom 2 R OYAL M ARSDEN NHS F OUNDATION T RUST, Statistics, Sutton, United Kingdom 3 R OYAL M ARSDEN NHS F OUNDATION T RUST, Sutton, United Kingdom
Purpose: Preoperative chemo-radiotherapy for locally advanced rectal cancer reduces the risk of local recurrence and increases rates of complete surgical resection. There is limited data regarding motion of and changes in rectal and bladder volume during radiotherapy for rectal cancer. The purpose of this study was to quantify these variables using cone beam CT (CBCT), to eventually improve the certainty of target volume coverage and to decrease the dose delivered to organs at risk. Materials: 14 patients with adenocarcinoma of the rectum undergoing preoperative chemo-radiotherapy were consented for this study. They received 45Gy in 25 fractions for phase one followed by 9 Gy in 5 fractions for phase 2 over 6 weeks. Pelvic CBCTs were acquired on days one to three of treatment, and weekly thereafter. Each CBCT was fused to the planning CT in Pinnacle planning system using local correlation software (bony anatomy). The rectum and bladder were outlined on all CBCT images. Inter-fraction movement of the rectum from the CBCT with respect to the planning CT was recorded by documenting the anterior/posterior and lateral points on both scans measured from the sacrum and mid left femoral head respectively at different rectal locations above the anal verge (grouped as ’upper’, ’mid’ and ’low’). The standard deviation (SD) of distances over all scans was recorded for each patient. Analysis of variance was used to examine the relationship between per-patient SD in each direction and rectal location (with reference to ’low’). Maximum rectal diameter at three different rectal locations was also measured. Results: 107 CBCTs were analysed. Rectal wall movement differed significantly by rectal location. Rectal diameter changed more in ’mid’ and ’upper’ than in the ’low’ rectum.
The mean planning CT rectal and bladder volumes were 140mls (range 63 335) and 319mls (range 93-663) respectively. The mean CBCT rectal and bladder volumes were 150mls (range 54-337) and 319.37mls (range 93-663) respectively. The mean per-patient SD of rectal and bladder volumes over all scans was 27mls and 108mls respectively. Conclusions: Rectal movement and diameter changes more in ’mid’ and ’upper’ compared to ’low’ rectum during rectal radiotherapy. Anterior and lateral rectal wall movements appear larger than posterior wall movements in the mid and upper rectum. Bladder volume was variable throughout treatment. Asymmetric margins may be warranted to adequately cover phase 2.
IGRT: VOLUME DEFINITION , TREATMENT MARGINS AND GEOMETRIC UNCERTANTIES
Conclusions: The "optimized" 3D CT scan protocol was found to perform best as it accurately reflects the full excursion of the moving object with equivalent CTDIvol values to the other protocols. For ITV-based treatment plan strategy, 3D CT scan parameters should be adapted as proposed here to obtain ITV close to those obtained with 4D CT. Care should be taken, to modify the protocol according to patient breathing signal. 490 poster (Physics Track) PATIENT FIXATION WITH A BITE FRAME: A DOSIMETRIC VALIDATION OF A 2 MM PTV MARGIN USING MEASURED SETUP INACCURACIES Y. van Herten1 , J. Van de Kamer1 , R. Pool1 , J. Wiersma1 , L. Blank1 , A. Bel1 1 ACADEMIC M EDICAL C ENTER, Radiation Oncology, Amsterdam, Netherlands
Purpose: To validate the 2 mm PTV margin by investigating dosimetric effects on CTV coverage of measured setup inaccuracies. Materials: For 6 patients with optic nerve meningeoma a treatment plan with a 2 mm PTV margin was made (3 to 8 beams, multiple couch rotations, 30 fractions of 180cGy). All patients were positioned in prone position, fixed to the couch with a bite frame and supported by a transparent mask. The first four patients were positioned to the isocentre using the laser system only. The last two were positioned on pre-determined couch coordinates and fine tuned using the laser in longitudinal and vertical direction only. For each beam we used the actual couch positions during treatment to calculate the cumulative dose distribution of all delivered fractions. These calculations were performed with APlan (research module derived from Plato RTS (Nucletron, NL)). We calculated the DVHs of CTV, PTV, eye, lens and hypophysis, for both planned and actually delivered dose distributions and used this as a basis for validation of the 2 mm PTV margin. Results: For patients positioned on the lasers only the spread in couch coordinates is large (up to +/- 1.5cm), in particular for the lateral position which is difficult to assess: the laser indicating the lateral position is blocked by the head and is therefore hardly visible on both bite frame and mask while the patient is in prone position. Hence, positioning on "fixed" couch coordinates is more accurate, as independently confirmed with conebeam-CT. Obviously, for a small spread in couch coordinates the target coverage was maintained and no clinically relevant dose increase to the organs at risk was observed. However, for situations with a larger variation dose coverage differences became more serious as shown by the following example. We simulated a ’systematic’ lateral shift of 2 cm in 14% of the fractions, resulting in a target coverage of only 50% (see figure). For such a large shift the dose to the organs at risk increased, although in this case it seemed to be limited.
Example of a simulation with a 2 cm spread in lateral couch coordinates for 14% of the fractions. CTV coverage decreased from 99.3% to 50%. Conclusions: Fixation with a bite frame combined with positioning based on couch coordinates is easy and facilitates a 2 mm setup margin while maintaining excellent target coverage. 491 poster (Physics Track) QUANTIFICATION OF ORGAN MOTION DURING CHEMORADIATION OF RECTAL CANCER USING CONE BEAM CT (MEASURE STUDY) I. Chong1 , M. Hawkins1 , B. O’Neill1 , K. Thomas2 , H. McNair3 , V. N. Hansen3 , A. Aitken3 , D. Tait1 1 R OYAL M ARSDEN NHS F OUNDATION T RUST, Clinical Oncology, Sutton, United Kingdom 2 R OYAL M ARSDEN NHS F OUNDATION T RUST, Statistics, Sutton, United Kingdom 3 R OYAL M ARSDEN NHS F OUNDATION T RUST, Sutton, United Kingdom
Purpose: Preoperative chemo-radiotherapy for locally advanced rectal cancer reduces the risk of local recurrence and increases rates of complete surgical resection. There is limited data regarding motion of and changes in rectal and bladder volume during radiotherapy for rectal cancer. The purpose of this study was to quantify these variables using cone beam CT (CBCT), to eventually improve the certainty of target volume coverage and to decrease the dose delivered to organs at risk. Materials: 14 patients with adenocarcinoma of the rectum undergoing preoperative chemo-radiotherapy were consented for this study. They received 45Gy in 25 fractions for phase one followed by 9 Gy in 5 fractions for phase 2 over 6 weeks. Pelvic CBCTs were acquired on days one to three of treatment, and weekly thereafter. Each CBCT was fused to the planning CT in Pinnacle planning system using local correlation software (bony anatomy). The rectum and bladder were outlined on all CBCT images. Inter-fraction movement of the rectum from the CBCT with respect to the planning CT was recorded by documenting the anterior/posterior and lateral points on both scans measured from the sacrum and mid left femoral head respectively at different rectal locations above the anal verge (grouped as ’upper’, ’mid’ and ’low’). The standard deviation (SD) of distances over all scans was recorded for each patient. Analysis of variance was used to examine the relationship between per-patient SD in each direction and rectal location (with reference to ’low’). Maximum rectal diameter at three different rectal locations was also measured. Results: 107 CBCTs were analysed. Rectal wall movement differed significantly by rectal location. Rectal diameter changed more in ’mid’ and ’upper’ than in the ’low’ rectum.
The mean planning CT rectal and bladder volumes were 140mls (range 63 335) and 319mls (range 93-663) respectively. The mean CBCT rectal and bladder volumes were 150mls (range 54-337) and 319.37mls (range 93-663) respectively. The mean per-patient SD of rectal and bladder volumes over all scans was 27mls and 108mls respectively. Conclusions: Rectal movement and diameter changes more in ’mid’ and ’upper’ compared to ’low’ rectum during rectal radiotherapy. Anterior and lateral rectal wall movements appear larger than posterior wall movements in the mid and upper rectum. Bladder volume was variable throughout treatment. Asymmetric margins may be warranted to adequately cover phase 2.