- Email: [email protected]
PO-0922 FEASIBILITY OF DOSE ESCALATION DURING RADIOTHERAPY FOR LOCALLY ADVANCED NON SMALL CELL LUNG CANCER: A PLANNING STUDY
ESTRO 31
S363
was kept below 1.5. Mean beam-on time and amount of MUs were also analyzed. Results: PTVs were adequately covered by all plans. The V15 of small bowel was 145.9cc (SD 75.9cc), 133.4cc (SD 75.3cc) and 112.7cc (SD73.4cc) for HybridArc, RapidArc and Tomotherapy, respectively. The mean bladder dose was 24.5Gy (SD 4.1Gy), 20.6Gy (SD 2.2Gy) and 23.0Gy (SD4.7Gy) for HybridArc, RapidArc and Tomotherapy, respectively. The mean beam-on time was significantly lower (p<0.01) for HybridArc (2.8min ±0.8) compared to RapidArc (4.5min ±0.5) and Tomotherapy (11.0min ±0.7). The total amount of MUs was significantly (p<0.01) lower for RapidArc (547±44) compared to HybridArc (978±163). Conclusions: HybridArc is a feasible solution for preoperative RT with a SIB in rectal cancer patients. Compared to RapidArc and Tomotherapy, it achieved similar PTV coverage and OAR (small bowel and bladder) sparing with significant lower beam-on time. The added value of HybridArc is that same LINAC-performance and QA-schemes can be maintained. PO-0921 LOSS OF DOSE COVERAGE IN VMAT PROSTATE PLANS DUE TO VARIOUS COUCHTOPS M. van Prooijen1, D.C. Comsa2, M.K. Islam1, R.K. Heaton1 1 Princess Margaret Hospital, Radiation Physics, Toronto, Canada 2 Southlake Regional Hospital, Radiation Physics, Newmarket, Canada Purpose/Objective: Complex radiation therapy treatments are designed using computer algorithms which rely on highly specific dosimetric inputs. Optimization is continued until all dosimetric criteria are met or otherwise accepted by a clinician. Much time can be spent fine-tuning the plan to satisfy established planning criteria. However, the optimization typically takes place on data sets that do not include the treatment couchtop. Since VMAT prostate plans are usually done with a single full arc, we investigated the dosimetric impact of ignoring the couchtop for five VMAT prostate plans. Materials and Methods: Four different couchtops were considered for each of five patients: iBeam evo, Sinmed Mastercouch, Varian flatpanel and Varian unipanel with t-bar support. Each of the Varian couchtops was evaluated in two different configurations: with support bars in or out. Our starting point in each case was a previously accepted clinical case planned without consideration of a couchtop to which the various couchtops were added in succession. The original beams were maintained in each instance. The five patients were selected to span a range in target size and planning complexity. A number of parameters characterizing the dose distribution were assessed for each combination, including our standard clinical target coverage criteria of PTV maximum dose to control hotspots, and the CTV and PTV minimum doses to characterize coverage. Shifts in the prescription isodose line were measured on 5 different slices, at 3 posterior locations and 1 each at lateral and anterior locations. Assessments were also made based on isodose encompassed volumes and dosimetric phantom measurement tools available in most clinics. Results: All couches attenuate dose. For a prescribed dose of 78 Gy, the change in CTV V78 ranged from 0.2 – 40%, while the change in PTV V74.1 ranged from 0.6 – 9% (Table 1). Average prescription isodose volume contraction ranged from 1 – 8.5 mm or 4 – 36% of the equivalent radius of the CTV (prostate gland), with a slightly higher than average change posteriorly (Figure 1). Plan verification measurements performed on a multidetector QA phantom showed a significantly higher failure rate when couch effects are not included in the comparison calculation.
Conclusions: Reductions in CTV and PTV coverage observed in this study show that introducing couch information at the planning stage would produce a different plan than without couch information. These differences significantly exceed our specific plan objectives and assessment criteria. This implies that either the current planning goals for these techniques are overly precise, resulting in little patient benefit gain from time invested in fine tuning a plan, or that planning which includes a model of the treatment couch is required to achieve the precision of dose delivery specified by the clinician. PO-0922 FEASIBILITY OF DOSE ESCALATION DURING RADIOTHERAPY FOR LOCALLY ADVANCED NON SMALL CELL LUNG CANCER: A PLANNING STUDY S. Agrawal1, S. Kumar1, A.S. Maurya1, S. Kumar1 1 Sanjay Gandhi Postgraduate Institute of Medical Sciences, Department of Radiation Oncology, Lucknow UP, India Purpose/Objective: To evaluate the tumor volume reduction occurring during a course of radiotherapy (RT) by a sequential computed tomography scan at 45 Gy and its effect on possibility of adaptive treatment planning with dose escalation. Materials and Methods: A total of 20 patients underwent RT for Stage III non–small-cell lung cancer with conventional fractionation dose up to 59.4Gy/33#; 10 received concurrent chemotherapy and radiotherapy (CONC) and 10 received neoadjuvant chemotherapy (NACT) followed by external radiotherapy alone. One repeat CT scan was performed at a dose of 45 Gy for dosemetric study only. Primary gross tumor volume (GTV_P), gross nodal volume (GTV_N) and planning target volume (PTV) were evaluated for regression. A replanning (3DCRT) was done with the regressed target volume at 45 Gy in those patients who showed marked regression(>20% in GTV) to evaluate the feasibility of dose escalation keeping the limit of organs at risk tolerance below 50Gy (maximum spinal cord dose), below 21Gy (mean lung dose ), below 37%(V20 whole lung). Results: The mean GTV_P, GTV_N and PTV reduction in CONC arm were 64.8%, 15.7% and 43.7% and that of NACT arm was 41.7%, 64.58% and 28.2% respectively .The difference in GTV_P regression in concurrent arm as compared to NACT arm was significant (p=0.02)
S364
resulting in a significant PTV difference between the 2 arms (p=0.04). 10 out of 20 patients showed >20% reduction in GTV and dose escalation was possible in 5 out of these 10 patients. Mean dose escalation that could be achieved was 22.44Gy ( range 1.8Gy to 30.6Gy).
ESTRO 31
correlation between the dose variations and the amount of tumour motion.
Table:1
Conclusions: Relative large variations in DVH values are observed between respiration phases. This effect can only to a limited degree be minimised by using the mid-ventilation phase for planning. Nevertheless, since accurate delineation of the tumour is of high importance and only 0.6 Gy is separating the two extreme phases, it is beneficial to perform plan calculations in the expiration phase, as long as the actual treatment is based on the mid-ventilation phase.
Conclusions: The results of this study have demonstrated considerable reduction in the GTV_P in both arms though higher in CONC (64.8%) arm as compared to NACT (41.7%) arm. Adaptive planning led to possibility of dose escalation in about 25% of patients. PO-0923 MID-VENTILATION PHASE FOR CALCULATION OF TREATMENT PLAN FOR LUNG CANCER PATIENTS MIGHT NOT BE OPTIMAL C. Brink1, M. Nielsen2, O. Hansen1, T.B. Nielsen1 1 University of Southern Denmark, Institute of Clinical Research, Odense DK-5000, Denmark 2 Odense University Hospital, Laboratory of Radiation Physics, Odense DK-5000, Denmark Purpose/Objective: Treatment planning and irradiation of lung cancer patients is often based on the mid-ventilation phase in order to minimize planning margins. It is known that imaging artefacts are most pronounced in the mid-ventilation phase due to the rapid tumour motion in that phase. Also dose calculations depend on the respiration phase used for calculation (see figure). This study measures these differences to evaluate whether treatment planning could be performed in a different phase in order to reduce image artefacts and enhance the accuracy of tumour delineation. Materials and Methods: 20 stereotactic lung cancer treatment plans based on the mid-ventilation phase were copied to the remaining phases, keeping the monitor units constant. The isocenter was fixed relative to the tumour position for all respiration phases. Prescription was 66 Gy/3 fractions with a minimum dose to PTV of 45 Gy. PTV, GTV, lung, spinal cord, and heart contours were propagated to the remaining respiration phases to create DVH’s for the individual phases. Differences of the DVH values were measured as differences of maximum and minimum values over all respiration phases or as differences between the expiration and inspiration phases. Correlations between DVH variations and tumour motion were evaluated. Results: For each patient the maximum differences of selected DVH values were measured. The mean (and maximum) patient specific PTV difference for minimum dose (D98), median dose (D50) and maximum dose (D02) were 2.0 Gy (3.6 Gy), 1.2 Gy (2.9Gy) and 0.8 Gy (1.8 Gy), respectively. These variations could only to a limited degree be attributed to differences between calculations in the two extreme phases: expiration and inspiration. Differences between expiration and inspiration was evaluated by Wilcoxon test which showed a significant differences of 0.6 Gy for the median dose (p=0.005). The median dose for the mid-ventilation was on average between the values for the extreme phases. No statistical significant differences between the extreme phases for the minimum and maximum doses were observed. None of the DVH variations for the PTV could be correlated to the size of the tumour motion (Spearman test >0.05). Results similar to PTV were observed for GTV. Very limited changes in doses to lung, cord, and heart were observed. However, for the heart and lung it was possible to establish a
PO-0924 PLANNING COMPARISON BETWEEN STATIC AND ROTATIONAL INTENSITY MODULATED TECHNIQUES FOR HEAD-NECK CANCER F. Declich1, F. Bonsignore1, S. Broggi2, L. Perna2, G. Rinaldini2 1 A.O. della Provincia di Lecco, Servizio di Fisica Sanitaria, Lecco, Italy 2 Ospedale San Raffaele, Servizio di Fisica Sanitaria, Milano, Italy Purpose/Objective: To compare helical tomotherapy (HT) with volumetric-modulated arc techniques, Elekta VMAT and RapidArc (RA) and conventional fixed-field intensity modulated techniques (S-IMRT) treatment plans for different head-neck cancer using a simultaneous integrated boost approach. Materials and Methods: Data of 9 patients (3 nasopharynx, 3 oropharynx and 3 hypopharynx) previously treated with HT or S-IMRT were considered. For each patient 4 treatment plans were created using static and volumetric IMRT techniques.Rotational IMRT plans (double arcs) were generated using Oncentra for VMAT and Eclipse for RA. S-IMRT plans were created by considering 7 step-shoot fields optimized with Eclipse.All techniques were optimized to simultaneously deliver 66 Gy in 30 fractions to planning target volume (PTV1; GVT and enlarged nodes) and 54 Gy to PTV2 subclinical, electively treated nodes. Plans quality was assessed in terms of PTVs coverage/homogeneity and sparing of several critical structure (parotid gland, spinal cord, brain stem, mucosae, larynx, tyroid, esophagous, mandible, body). Dose maps comparisons were performed using the SunNuclear 3DVH Results: Comparable results were obtained for all four delivery techniques in terms of PTVs coverage. V51.3 and V62.7 for PTV2 and PTV1 were respectively very close to 95%, as imposed as planning objective.Significant differences were found for PTVs homogeneity (in terms of D5%-D95% ) for the 4 techniques. For PTV1, D5%-D95% was estimated equal to 4.5±0.3 Gy for HT and higher values were found both for S-IMRT ( 6.7± 0.6 Gy) and for volumetric arc techniques ( RA: 6.5± 1.1 Gy; VMAT: 7.6 ± 0.8 Gy ). A similar trend was found for PTV2: 7.4 ± 1 Gy for HT and higher values ranging between 9.3 and 10.9 for S-IMRT and arc techniques. Similar results were found for all techniques in terms of maximum dose for dose limiting structures (spinal cord, brainstem, optical structures). For the other OARs, significant lower values were found for HT both in terms of mean dose and in the range V15-V30. No significant differences were found between S-IMRT and RA. VMAT showed a lower critical structures sparing, except for larynx.Similar results were found for body mean dose for all techniques; higher values of body V5-V10 were found for HT. The comparison between spatial dose distributions showed that body superficial dose was lower in Elekta-VMAT plans. More concave low isodoses were generally created for both arc volumetric techniques compared to HT , above all in the posterior neck region. High dose spots and streaky dose pattern were found in S-IMRT plans. Conclusions: All systems used have proved capable of providing highquality plans. HT plans were superior in terms of PTVs dose homogeneity and sparing of structures very small and close to the target as mucosae, larynx and tyroid. Higher values to some critical structure for VMAT were caused by the difficulty to properly cover PTVs very close to the body surface.
S363
was kept below 1.5. Mean beam-on time and amount of MUs were also analyzed. Results: PTVs were adequately covered by all plans. The V15 of small bowel was 145.9cc (SD 75.9cc), 133.4cc (SD 75.3cc) and 112.7cc (SD73.4cc) for HybridArc, RapidArc and Tomotherapy, respectively. The mean bladder dose was 24.5Gy (SD 4.1Gy), 20.6Gy (SD 2.2Gy) and 23.0Gy (SD4.7Gy) for HybridArc, RapidArc and Tomotherapy, respectively. The mean beam-on time was significantly lower (p<0.01) for HybridArc (2.8min ±0.8) compared to RapidArc (4.5min ±0.5) and Tomotherapy (11.0min ±0.7). The total amount of MUs was significantly (p<0.01) lower for RapidArc (547±44) compared to HybridArc (978±163). Conclusions: HybridArc is a feasible solution for preoperative RT with a SIB in rectal cancer patients. Compared to RapidArc and Tomotherapy, it achieved similar PTV coverage and OAR (small bowel and bladder) sparing with significant lower beam-on time. The added value of HybridArc is that same LINAC-performance and QA-schemes can be maintained. PO-0921 LOSS OF DOSE COVERAGE IN VMAT PROSTATE PLANS DUE TO VARIOUS COUCHTOPS M. van Prooijen1, D.C. Comsa2, M.K. Islam1, R.K. Heaton1 1 Princess Margaret Hospital, Radiation Physics, Toronto, Canada 2 Southlake Regional Hospital, Radiation Physics, Newmarket, Canada Purpose/Objective: Complex radiation therapy treatments are designed using computer algorithms which rely on highly specific dosimetric inputs. Optimization is continued until all dosimetric criteria are met or otherwise accepted by a clinician. Much time can be spent fine-tuning the plan to satisfy established planning criteria. However, the optimization typically takes place on data sets that do not include the treatment couchtop. Since VMAT prostate plans are usually done with a single full arc, we investigated the dosimetric impact of ignoring the couchtop for five VMAT prostate plans. Materials and Methods: Four different couchtops were considered for each of five patients: iBeam evo, Sinmed Mastercouch, Varian flatpanel and Varian unipanel with t-bar support. Each of the Varian couchtops was evaluated in two different configurations: with support bars in or out. Our starting point in each case was a previously accepted clinical case planned without consideration of a couchtop to which the various couchtops were added in succession. The original beams were maintained in each instance. The five patients were selected to span a range in target size and planning complexity. A number of parameters characterizing the dose distribution were assessed for each combination, including our standard clinical target coverage criteria of PTV maximum dose to control hotspots, and the CTV and PTV minimum doses to characterize coverage. Shifts in the prescription isodose line were measured on 5 different slices, at 3 posterior locations and 1 each at lateral and anterior locations. Assessments were also made based on isodose encompassed volumes and dosimetric phantom measurement tools available in most clinics. Results: All couches attenuate dose. For a prescribed dose of 78 Gy, the change in CTV V78 ranged from 0.2 – 40%, while the change in PTV V74.1 ranged from 0.6 – 9% (Table 1). Average prescription isodose volume contraction ranged from 1 – 8.5 mm or 4 – 36% of the equivalent radius of the CTV (prostate gland), with a slightly higher than average change posteriorly (Figure 1). Plan verification measurements performed on a multidetector QA phantom showed a significantly higher failure rate when couch effects are not included in the comparison calculation.
Conclusions: Reductions in CTV and PTV coverage observed in this study show that introducing couch information at the planning stage would produce a different plan than without couch information. These differences significantly exceed our specific plan objectives and assessment criteria. This implies that either the current planning goals for these techniques are overly precise, resulting in little patient benefit gain from time invested in fine tuning a plan, or that planning which includes a model of the treatment couch is required to achieve the precision of dose delivery specified by the clinician. PO-0922 FEASIBILITY OF DOSE ESCALATION DURING RADIOTHERAPY FOR LOCALLY ADVANCED NON SMALL CELL LUNG CANCER: A PLANNING STUDY S. Agrawal1, S. Kumar1, A.S. Maurya1, S. Kumar1 1 Sanjay Gandhi Postgraduate Institute of Medical Sciences, Department of Radiation Oncology, Lucknow UP, India Purpose/Objective: To evaluate the tumor volume reduction occurring during a course of radiotherapy (RT) by a sequential computed tomography scan at 45 Gy and its effect on possibility of adaptive treatment planning with dose escalation. Materials and Methods: A total of 20 patients underwent RT for Stage III non–small-cell lung cancer with conventional fractionation dose up to 59.4Gy/33#; 10 received concurrent chemotherapy and radiotherapy (CONC) and 10 received neoadjuvant chemotherapy (NACT) followed by external radiotherapy alone. One repeat CT scan was performed at a dose of 45 Gy for dosemetric study only. Primary gross tumor volume (GTV_P), gross nodal volume (GTV_N) and planning target volume (PTV) were evaluated for regression. A replanning (3DCRT) was done with the regressed target volume at 45 Gy in those patients who showed marked regression(>20% in GTV) to evaluate the feasibility of dose escalation keeping the limit of organs at risk tolerance below 50Gy (maximum spinal cord dose), below 21Gy (mean lung dose ), below 37%(V20 whole lung). Results: The mean GTV_P, GTV_N and PTV reduction in CONC arm were 64.8%, 15.7% and 43.7% and that of NACT arm was 41.7%, 64.58% and 28.2% respectively .The difference in GTV_P regression in concurrent arm as compared to NACT arm was significant (p=0.02)
S364
resulting in a significant PTV difference between the 2 arms (p=0.04). 10 out of 20 patients showed >20% reduction in GTV and dose escalation was possible in 5 out of these 10 patients. Mean dose escalation that could be achieved was 22.44Gy ( range 1.8Gy to 30.6Gy).
ESTRO 31
correlation between the dose variations and the amount of tumour motion.
Table:1
Conclusions: Relative large variations in DVH values are observed between respiration phases. This effect can only to a limited degree be minimised by using the mid-ventilation phase for planning. Nevertheless, since accurate delineation of the tumour is of high importance and only 0.6 Gy is separating the two extreme phases, it is beneficial to perform plan calculations in the expiration phase, as long as the actual treatment is based on the mid-ventilation phase.
Conclusions: The results of this study have demonstrated considerable reduction in the GTV_P in both arms though higher in CONC (64.8%) arm as compared to NACT (41.7%) arm. Adaptive planning led to possibility of dose escalation in about 25% of patients. PO-0923 MID-VENTILATION PHASE FOR CALCULATION OF TREATMENT PLAN FOR LUNG CANCER PATIENTS MIGHT NOT BE OPTIMAL C. Brink1, M. Nielsen2, O. Hansen1, T.B. Nielsen1 1 University of Southern Denmark, Institute of Clinical Research, Odense DK-5000, Denmark 2 Odense University Hospital, Laboratory of Radiation Physics, Odense DK-5000, Denmark Purpose/Objective: Treatment planning and irradiation of lung cancer patients is often based on the mid-ventilation phase in order to minimize planning margins. It is known that imaging artefacts are most pronounced in the mid-ventilation phase due to the rapid tumour motion in that phase. Also dose calculations depend on the respiration phase used for calculation (see figure). This study measures these differences to evaluate whether treatment planning could be performed in a different phase in order to reduce image artefacts and enhance the accuracy of tumour delineation. Materials and Methods: 20 stereotactic lung cancer treatment plans based on the mid-ventilation phase were copied to the remaining phases, keeping the monitor units constant. The isocenter was fixed relative to the tumour position for all respiration phases. Prescription was 66 Gy/3 fractions with a minimum dose to PTV of 45 Gy. PTV, GTV, lung, spinal cord, and heart contours were propagated to the remaining respiration phases to create DVH’s for the individual phases. Differences of the DVH values were measured as differences of maximum and minimum values over all respiration phases or as differences between the expiration and inspiration phases. Correlations between DVH variations and tumour motion were evaluated. Results: For each patient the maximum differences of selected DVH values were measured. The mean (and maximum) patient specific PTV difference for minimum dose (D98), median dose (D50) and maximum dose (D02) were 2.0 Gy (3.6 Gy), 1.2 Gy (2.9Gy) and 0.8 Gy (1.8 Gy), respectively. These variations could only to a limited degree be attributed to differences between calculations in the two extreme phases: expiration and inspiration. Differences between expiration and inspiration was evaluated by Wilcoxon test which showed a significant differences of 0.6 Gy for the median dose (p=0.005). The median dose for the mid-ventilation was on average between the values for the extreme phases. No statistical significant differences between the extreme phases for the minimum and maximum doses were observed. None of the DVH variations for the PTV could be correlated to the size of the tumour motion (Spearman test >0.05). Results similar to PTV were observed for GTV. Very limited changes in doses to lung, cord, and heart were observed. However, for the heart and lung it was possible to establish a
PO-0924 PLANNING COMPARISON BETWEEN STATIC AND ROTATIONAL INTENSITY MODULATED TECHNIQUES FOR HEAD-NECK CANCER F. Declich1, F. Bonsignore1, S. Broggi2, L. Perna2, G. Rinaldini2 1 A.O. della Provincia di Lecco, Servizio di Fisica Sanitaria, Lecco, Italy 2 Ospedale San Raffaele, Servizio di Fisica Sanitaria, Milano, Italy Purpose/Objective: To compare helical tomotherapy (HT) with volumetric-modulated arc techniques, Elekta VMAT and RapidArc (RA) and conventional fixed-field intensity modulated techniques (S-IMRT) treatment plans for different head-neck cancer using a simultaneous integrated boost approach. Materials and Methods: Data of 9 patients (3 nasopharynx, 3 oropharynx and 3 hypopharynx) previously treated with HT or S-IMRT were considered. For each patient 4 treatment plans were created using static and volumetric IMRT techniques.Rotational IMRT plans (double arcs) were generated using Oncentra for VMAT and Eclipse for RA. S-IMRT plans were created by considering 7 step-shoot fields optimized with Eclipse.All techniques were optimized to simultaneously deliver 66 Gy in 30 fractions to planning target volume (PTV1; GVT and enlarged nodes) and 54 Gy to PTV2 subclinical, electively treated nodes. Plans quality was assessed in terms of PTVs coverage/homogeneity and sparing of several critical structure (parotid gland, spinal cord, brain stem, mucosae, larynx, tyroid, esophagous, mandible, body). Dose maps comparisons were performed using the SunNuclear 3DVH Results: Comparable results were obtained for all four delivery techniques in terms of PTVs coverage. V51.3 and V62.7 for PTV2 and PTV1 were respectively very close to 95%, as imposed as planning objective.Significant differences were found for PTVs homogeneity (in terms of D5%-D95% ) for the 4 techniques. For PTV1, D5%-D95% was estimated equal to 4.5±0.3 Gy for HT and higher values were found both for S-IMRT ( 6.7± 0.6 Gy) and for volumetric arc techniques ( RA: 6.5± 1.1 Gy; VMAT: 7.6 ± 0.8 Gy ). A similar trend was found for PTV2: 7.4 ± 1 Gy for HT and higher values ranging between 9.3 and 10.9 for S-IMRT and arc techniques. Similar results were found for all techniques in terms of maximum dose for dose limiting structures (spinal cord, brainstem, optical structures). For the other OARs, significant lower values were found for HT both in terms of mean dose and in the range V15-V30. No significant differences were found between S-IMRT and RA. VMAT showed a lower critical structures sparing, except for larynx.Similar results were found for body mean dose for all techniques; higher values of body V5-V10 were found for HT. The comparison between spatial dose distributions showed that body superficial dose was lower in Elekta-VMAT plans. More concave low isodoses were generally created for both arc volumetric techniques compared to HT , above all in the posterior neck region. High dose spots and streaky dose pattern were found in S-IMRT plans. Conclusions: All systems used have proved capable of providing highquality plans. HT plans were superior in terms of PTVs dose homogeneity and sparing of structures very small and close to the target as mucosae, larynx and tyroid. Higher values to some critical structure for VMAT were caused by the difficulty to properly cover PTVs very close to the body surface.