- Email: [email protected]
Rapid delivery of stereotactic radiotherapy for peripheral lung tumors using volumetric intensity-modulated arcs
Radiotherapy and Oncology 93 (2009) 122–124
Contents lists available at ScienceDirect
Radiotherapy and Oncology journal homepage: www.thegreenjournal.com
Stereotactic radiotherapy
Rapid delivery of stereotactic radiotherapy for peripheral lung tumors using volumetric intensity-modulated arcs Wilko F.A.R. Verbakel *, Suresh Senan, Johan P. Cuijpers, Ben J. Slotman, Frank J. Lagerwaard Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands
a r t i c l e
i n f o
Article history: Received 10 March 2009 Received in revised form 12 May 2009 Accepted 24 May 2009 Available online 22 June 2009 Keywords: RapidArc Volumetric-modulated arc therapy SBRT
a b s t r a c t The delivery of high dose conventional stereotactic body radiotherapy (SBRT) for patients with stage I lung tumors generally takes 30–45 min per fraction. The novel volumetric intensity-modulated arc therapy (RA) for planning and delivery enabled much faster treatment for three patients with different fractionation schemes. This reduces the risk of intrafraction motion and is more patient friendly. In addition, in comparison to the conventional plans using 10 static non-coplanar fields, RA plans achieved superior dose conformity around the PTV and reduced chest wall doses. Ó 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 93 (2009) 122–124
We previously reported on outcomes of stereotactic body radiotherapy (SBRT) for stage I lung tumors, which achieves local control rates in excess of 85% and low toxicity rates using riskadapted fractionation schemes [1]. Conventional SBRT was performed using 8–12 non-coplanar static beams, and treatment delivery generally took between 20 and 45 min, including set-up prior to treatment and, if necessary, repeated set-up in-between beams. Planning and delivery with intensity-modulated radiotherapy (IMRT) could create more conformal dose distributions than 8– 12 static beams, especially for larger and irregular tumor shapes, but this would substantially increase treatment time. As prolonged treatment sessions have been associated with intrafractional shifts in patient and tumor positioning [2], faster treatment delivery is of utmost importance in SBRT. This faster treatment should, however, not reduce the conformity of the plans and the possibility for sparing of organs at risk (OAR). Recently, RapidArc (Varian Medical Systems, Palo Alto, USA) was clinically introduced at our center. RapidArc is a volumetric intensity-modulated arc technique that allows for fast planning and delivery of conformal dose distributions [3]. Recent studies have confirmed the ability of RapidArc to generate highly conformal treatment plans for tumors of the head and neck, brain, prostate and various other indications [4–8] with only one or two arcs. We describe the implementation of the RapidArc delivery technique in peripheral stage I lung cancer and compare the plans with those obtained with 10 static non-coplanar fields.
* Corresponding author. Address: Department of Radiation Oncology, VU University Medical Center, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands. E-mail address: [email protected] (W.F.A.R. Verbakel). 0167-8140/$ - see front matter Ó 2009 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2009.05.020
Materials and methods The use of RapidArc in the first patients treated with each of three risk-adapted SBRT schemes used at our center was reviewed. As described previously, the choice of fractionation scheme in a particular patient is dictated by tumor size and by the proximity to critical organs [1]. Target definition was based on contouring internal target volumes (ITVs) derived using four-dimensional CT scans (GE medical systems, Waukesha, USA), as described previously [1,9]. To account for set-up inaccuracies and potential baseline tumor shifts, a PTV margin of 3 mm was applied around the ITV. Organs at risk (OAR) such as the heart, spinal cord, large blood vessels, brachial plexus and thoracic wall were contoured when relevant, in accordance with established guidelines [10]. The SBRT dose (3 fractions of 18 Gy, 5 fractions of 11 Gy, or 8 fractions of 7.5 Gy, which slightly differed from our previous report as a result of clinical introduction of the AAA algorithm instead of the pencil beam for dose calculations) was prescribed at the 80% isodose line. Patient P1 had a small PTV of 12.4 cm3 in the central lower lobe, adjacent to the chest wall. P2 had a larger PTV of 30.8 cm3 in the center of the right lung, and P3 had the largest tumor with PTV volume of 60.1 cm3, located in the right upper lobe, extending from the mediastinum to the chest wall. Maximum tumor movements according to the 4DCT scans were 10 mm, 13 mm and smaller than 3 mm, respectively, for P1, P2 and P3. The RapidArc plan optimization phase used constraints specified in the ongoing randomized ROSEL trial comparing surgery and SBRT in patients with operable stage I lung cancer [10]. In addition, a 5-mm wide ring structure around the PTV was used as an additional OAR to enforce a steep dose fall-off immediately outside the target. Each RapidArc plan consisted of at least one pair
123
W.F.A.R. Verbakel et al. / Radiotherapy and Oncology 93 (2009) 122–124
of 358° clockwise and counter-clockwise coplanar arcs using 6 MV photons at a maximum dose rate of 600 monitor units (MUs)/minute. As described previously, the first optimized arc is used as a base dose plan for optimization of the second arc [4]. Since the current version of Eclipse (8.2.23) allows for a maximum of 999 (MU) to be delivered per arc, up to five arcs were needed to deliver the prescribed fraction doses, depending on the fraction dose. These multiple arcs consisted of copies of the first and second optimized arcs. Plan acceptance criteria were also based on the ROSEL protocol, with most important being that at least 95% of the PTV should receive the prescribed dose (80%) and that the maximum PTV dose should be below 110%. All final dose calculations were performed with the AAA of Eclipse version 8.2.23 on the average intensity dataset derived from the planning 4DCT. For comparison, standard SBRT plans consisting of 10 noncoplanar static beams were generated in Eclipse for each patient. These plans were reviewed by a clinician and a physicist, and where necessary, further optimized. The conformity indices (CI80, 60 or 40) were calculated for both planning techniques, with e.g. CI80 defined as the volume encompassed by the 80% isodose line divided by the volume of PTV encompassed by the same isodose level. The volume of the chest wall receiving a dose higher than 45 Gy (V45) was also compared [11]. For each RapidArc plan, patient-specific quality assurance (QA) was performed by comparing the calculated dose distributions with film dosimetry, in a coronal plane of the ‘lung’ (low density) compartment of a Quasar phantom (Modus Medical Systems, London, Canada). Double Gafchromic EBT films were used for these measurements, with an accuracy of 1.3% (1 standard deviation) [12]. Measurements were performed for both the static phantom and the lung compartment with film moving in a sinusoidal
pattern in cranial–caudal direction with amplitude equal to the tumor motion amplitude derived from the patients planning 4DCT scan. The penumbras, defined as the distance between the 20% and 80% isodose lines, were compared between measurements and calculations, as well as the dose differences above the prescribed dose (80%). Treatment delivery was performed on a Varian Trilogy linear accelerator. Set-up prior to each fraction was performed using co-registration between kV cone beam CT scan (CBCT) and the average intensity projection dataset of the 4DCT scans, based on a tumor match. The latter was verified by a physicist and/or radiation oncologist prior to treatment. If more than two arcs were needed, the CBCT-based set-up was repeated after the second arc in order to correct for possible intrafraction movement of the patient. Treatment times from start of the first CBCT until end of treatment were recorded for each treatment fraction. In addition, the delivery time was recorded as the time needed from start of the first arc until end of the last arc, excluding the time for CBCT set-up.
Results For all three patients, the conformity indices for the high dose region (CI80 and CI60) were superior for RapidArc plans than for conventional SBRT plans (Table 1). Furthermore, the volume of chest wall receiving more than 45 Gy was lower in RapidArc plans and a typical dose distribution in patient P1 is shown in Fig. 1. The results for P3 show that RapidArc is also more conformal for large tumors, although a somewhat larger volume of contralateral lung received low doses of 5 Gy or more.
Table 1 Comparison between RA and 10-beam SBRT for the three patients: number of monitor units, conformity index at different dose levels, the volume of chest wall treated to more than 45 Gy, and the delivery times of RapidArc treatment, both including and excluding the time required for CBCT set-up. RapidArc
Fractionation (# Gy) No. of arcs Delivery time (min) Treatment time incl. CBCT (min) Monitor units CI80 CI60 CI40 V45 chest wall (cm3)
Conventional SBRT
P1
P2
P3
3 18 5 11 31 4829 1.09 2.18 5.00 1.28
5 11 3 6.4 18.4 2217 1.09 2.13 4.71 0.40
8 7.5 2 4.5 10.5 1853 1.08 2.27 5.71 1.33
Average
1.09 2.19 5.14 1.00
P1
P2
P3
3 18
5 11
8 7.5
12
11.2
12.9
3029 1.28 2.48 4.92 3.09
1981 1.14 2.20 4.55 0.7
1126 1.24 2.54 5.53 2.64
Average
1.22 2.41 5.00 3.42
Fig. 1. Comparison of dose distributions for patient P1 with both RapidArc (left, squares) and 10-beam SBRT (right, triangles). The DVH shows the PTV (red), adjacent thoracic wall (purple), ipsilateral lung (red) and contralateral lung (green).
124
Rapid delivery of SBRT using RapidArc
Film dosimetry in the static Quasar phantom corresponded well with calculated dose distributions. Measured penumbras in both cranial–caudal as in lateral direction corresponded within 1 mm with the calculated penumbras, and the maximum dose deviations between measurements and calculations were 4%. Film measurements in the moving phantom also corresponded well with calculation. The maximum dose difference due to the motion was 7%, but still the dose remained well above the prescription dose. The penumbra broadening in the cranial–caudal direction was according to what could be expected from the phantom movement, i.e. weighted summation of calculations according to phase shift corresponded with the measurements. The mean number of monitor units for RapidArc plans was 31% higher than those needed for dynamic conformal arc plans, a finding which is indicative of the amount of leaf motion in the RapidArc plans. Treatment delivery times, both including and excluding the setup procedure, are shown in Table 1. Typical delivery times for our conventional 10-beam SBRT, excluding set-up, range from 10 min for the 8-fraction low dose scheme to 14 min for the 3-fraction scheme. At least 30% of the fractions require a second set-up during their treatment fraction because of significant intrafraction movement of the patient during their radiation delivery. Discussion In this report, we demonstrate the planning, dosimetric verification, and delivery of highly conformal RapidArc plans for SBRT of peripheral lung tumors. The three patients described in this report are representative of the spectrum of tumor sizes and locations for which risk-adapted fractionation is mandated in our protocol. The reduction achieved in delivery time is a major advantage of RA delivery. For the scheme with the lowest fraction dose, it was reduced by a factor 2 compared to delivery time of our conventional 10 static field non-coplanar plans. The delivery time for the 3 18 Gy scheme is still relatively large with RA, but this will be further reduced as the latest clinical release of RapidArc (8.6.10) has no constraints with respect to the maximum number of MUs for one arc. After upgrading RapidArc to the latest release, we plan to treat these patients on a Novalis Tx unit with a dose rate of 1000 MU/min, which will permit patients to be treated with 2 arcs in less than 6 min beam-on time for a fraction dose of 18 Gy. Shorter treatment time will reduce the likelihood of intrafraction baseline shifts in tumor position [2]. Based on our experience, we expect that a single CBCT set-up will be sufficient if the entire radiation delivery can be completed in 6 min. At present, each CBCTbased set-up procedure, including verification of the automatic co-registration, takes 4–5 min. We are now studying the incidence and extent of baseline shifts in tumor position as a function of the delivery time. Several centers use only 4–6 coplanar beams for SBRT, which allows for a quicker delivery than our 10 non-coplanar beams. However, the use of 10 non-coplanar beams improves the conformity of the high dose region, particularly at the site of the chest wall. This can account for the low reported incidence of rib fractures of only 2% in the first 206 treated patients [1]. This contrasts with a far higher incidence of rib fractures [11] or severe skin reactions [13] in centers using 4–6 coplanar beams. For each patient, RapidArc achieved a better target dose conformity in comparison to our conventional SBRT non-coplanar 10-field approach. Specifically, this leads to improved sparing of the chest wall (Fig. 1), though, at the cost of a minimally larger CI40. Although the minor increase in dose to contralateral lung was not considered significant in these three patients, additional analysis in patients with large tumors located in the lower lobe will be performed to study this issue further. It should be noted,
though, that for these three patients no objectives were used to reduce the lung dose, and that adding such objective can achieve reduced contralateral lung doses. The use of multiple partial arcs could also lead to a lower contralateral lung dose. IMRT could also provide much better dose conformity compared to the conventional 10-field approach, but IMRT is generally not used for most lung SBRT because it increases the delivery time. It should be pointed out that the use of RapidArc for SBRT planning of lung tumors is presently not straightforward as the optimization algorithm is based on convolution of spatially invariant point spread kernels, whereas the final dose calculation by the AAA better takes into account the tissue inhomogeneities. This difference between optimization and dose calculation can lead to a larger dose inhomogeneity in the PTV, though this often falls within our plan acceptance criteria [10]. It is reassuring that dosimetric verification with films in a moving phantom confirmed that the combination of RA (moving leaves) with a moving tumor up to a maximum amplitude of 13 mm did not result in dose inhomogeneity in the PTV due to interplay effects. This study shows that RapidArc allows delivering of hypofractionated doses much faster than conventional SBRT, with the additional advantage that the plans are more conformal compared to those of conventional SBRT. The use of the Novalix Tx with smaller leaf width of the MLC and a higher MU output will further improve these results.
Conflicts of interest statement The VUmc has a research collaboration with Varian Medical Systems.
References [1] Lagerwaard FJ, Haasbeek CJ, Smit EF, et al. Outcomes of risk-adapted fractionated stereotactic radiotherapy for stage I non-small-cell lung cancer. Int J Radiat Oncol Biol Phys 2008;70:685–92. [2] Purdie TG, Bissonnette JP, Franks K, et al. Cone-beam computed tomography for on-line image guidance of lung stereotactic radiotherapy: localization, verification, and intrafraction tumor position. Int J Radiat Oncol Biol Phys 2007;68:243–52. [3] Otto K. Volumetric modulated arc therapy: IMRT in a single gantry arc. Med Phys 2008;35:310–7. [4] Verbakel WFAR, Cuijpers JP, Hoffmans D, et al. Volumetric intensity modulated arc therapy versus conventional IMRT in head and neck cancer: a comparative planning and dosimetric study. Int J Radiat Oncol Biol Phys 2009;74:252–9. [5] Lagerwaard FJ, Meijer OWN, van der Hoorn EAP, et al. Volumetric modulated arc radiotherapy for vestibular schwannomas. Int J Radiat Oncol Biol Phys 2009;74:610–6. [6] Kjaer-Kristoffersen F, Ohlhues L, Medin J, et al. RapidArc volumetric modulated therapy planning for prostate cancer patients. Acta Oncol 2009;48:227–32. [7] Verbakel WFAR, Senan S, Lagerwaard F, et al. Dosimetric validation of RapidArc treatment plans for 5 treatment sites. Med Phys 2008;35:2967. [8] Clivio A, Fogliata A, Franzetti-Pellanda A, et al. Volumetric-modulated arc radiotherapy for carcinomas of the anal canal: A treatment planning comparison with fixed field IMRT. Radiother Oncol 2009;92:118–24. [9] Underberg RW, Lagerwaard FJ, Cuijpers JP, et al. Four-dimensional CT scans for treatment planning in stereotactic radiotherapy for stage I lung cancer. Int J Radiat Oncol Biol Phys 2004;60:1283–90. [10] Hurkmans CW, Cuijpers JP, Lagerwaard FJ, et al. Recommendations for implementing stereotactic radiotherapy in peripheral stage IA non-small cell lung cancer: report from the Quality Assurance Working Party of the randomised phase III ROSEL study. Radiat Oncol 2009;4:1. [11] Pettersson N, Nyman J, Johansson K-A. Radiation-induced rib fractures after hypofractionated stereotactic body radiation therapy of non-small cell lung cancer: a dose- and volume-response analysis. Radiother Oncol 2009;91: 360–8. [12] van Battum LJ, Hoffmans D, Piersma H, et al. Accurate dosimetry with GafChromic EBT film of a 6 MV photon beam in water: what level is achievable? Med Phys 2008;35:704–16. [13] Hoppe BS, Laser B, Kowalski AV, et al. Acute skin toxicity following stereotactic body radiation therapy for stage I non-small-cell lung cancer: who’s at risk? Int J Radiat Oncol Biol Phys 2008;72:1283–6.
Contents lists available at ScienceDirect
Radiotherapy and Oncology journal homepage: www.thegreenjournal.com
Stereotactic radiotherapy
Rapid delivery of stereotactic radiotherapy for peripheral lung tumors using volumetric intensity-modulated arcs Wilko F.A.R. Verbakel *, Suresh Senan, Johan P. Cuijpers, Ben J. Slotman, Frank J. Lagerwaard Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands
a r t i c l e
i n f o
Article history: Received 10 March 2009 Received in revised form 12 May 2009 Accepted 24 May 2009 Available online 22 June 2009 Keywords: RapidArc Volumetric-modulated arc therapy SBRT
a b s t r a c t The delivery of high dose conventional stereotactic body radiotherapy (SBRT) for patients with stage I lung tumors generally takes 30–45 min per fraction. The novel volumetric intensity-modulated arc therapy (RA) for planning and delivery enabled much faster treatment for three patients with different fractionation schemes. This reduces the risk of intrafraction motion and is more patient friendly. In addition, in comparison to the conventional plans using 10 static non-coplanar fields, RA plans achieved superior dose conformity around the PTV and reduced chest wall doses. Ó 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 93 (2009) 122–124
We previously reported on outcomes of stereotactic body radiotherapy (SBRT) for stage I lung tumors, which achieves local control rates in excess of 85% and low toxicity rates using riskadapted fractionation schemes [1]. Conventional SBRT was performed using 8–12 non-coplanar static beams, and treatment delivery generally took between 20 and 45 min, including set-up prior to treatment and, if necessary, repeated set-up in-between beams. Planning and delivery with intensity-modulated radiotherapy (IMRT) could create more conformal dose distributions than 8– 12 static beams, especially for larger and irregular tumor shapes, but this would substantially increase treatment time. As prolonged treatment sessions have been associated with intrafractional shifts in patient and tumor positioning [2], faster treatment delivery is of utmost importance in SBRT. This faster treatment should, however, not reduce the conformity of the plans and the possibility for sparing of organs at risk (OAR). Recently, RapidArc (Varian Medical Systems, Palo Alto, USA) was clinically introduced at our center. RapidArc is a volumetric intensity-modulated arc technique that allows for fast planning and delivery of conformal dose distributions [3]. Recent studies have confirmed the ability of RapidArc to generate highly conformal treatment plans for tumors of the head and neck, brain, prostate and various other indications [4–8] with only one or two arcs. We describe the implementation of the RapidArc delivery technique in peripheral stage I lung cancer and compare the plans with those obtained with 10 static non-coplanar fields.
* Corresponding author. Address: Department of Radiation Oncology, VU University Medical Center, De Boelelaan 1117, 1081 HV Amsterdam, The Netherlands. E-mail address: [email protected] (W.F.A.R. Verbakel). 0167-8140/$ - see front matter Ó 2009 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2009.05.020
Materials and methods The use of RapidArc in the first patients treated with each of three risk-adapted SBRT schemes used at our center was reviewed. As described previously, the choice of fractionation scheme in a particular patient is dictated by tumor size and by the proximity to critical organs [1]. Target definition was based on contouring internal target volumes (ITVs) derived using four-dimensional CT scans (GE medical systems, Waukesha, USA), as described previously [1,9]. To account for set-up inaccuracies and potential baseline tumor shifts, a PTV margin of 3 mm was applied around the ITV. Organs at risk (OAR) such as the heart, spinal cord, large blood vessels, brachial plexus and thoracic wall were contoured when relevant, in accordance with established guidelines [10]. The SBRT dose (3 fractions of 18 Gy, 5 fractions of 11 Gy, or 8 fractions of 7.5 Gy, which slightly differed from our previous report as a result of clinical introduction of the AAA algorithm instead of the pencil beam for dose calculations) was prescribed at the 80% isodose line. Patient P1 had a small PTV of 12.4 cm3 in the central lower lobe, adjacent to the chest wall. P2 had a larger PTV of 30.8 cm3 in the center of the right lung, and P3 had the largest tumor with PTV volume of 60.1 cm3, located in the right upper lobe, extending from the mediastinum to the chest wall. Maximum tumor movements according to the 4DCT scans were 10 mm, 13 mm and smaller than 3 mm, respectively, for P1, P2 and P3. The RapidArc plan optimization phase used constraints specified in the ongoing randomized ROSEL trial comparing surgery and SBRT in patients with operable stage I lung cancer [10]. In addition, a 5-mm wide ring structure around the PTV was used as an additional OAR to enforce a steep dose fall-off immediately outside the target. Each RapidArc plan consisted of at least one pair
123
W.F.A.R. Verbakel et al. / Radiotherapy and Oncology 93 (2009) 122–124
of 358° clockwise and counter-clockwise coplanar arcs using 6 MV photons at a maximum dose rate of 600 monitor units (MUs)/minute. As described previously, the first optimized arc is used as a base dose plan for optimization of the second arc [4]. Since the current version of Eclipse (8.2.23) allows for a maximum of 999 (MU) to be delivered per arc, up to five arcs were needed to deliver the prescribed fraction doses, depending on the fraction dose. These multiple arcs consisted of copies of the first and second optimized arcs. Plan acceptance criteria were also based on the ROSEL protocol, with most important being that at least 95% of the PTV should receive the prescribed dose (80%) and that the maximum PTV dose should be below 110%. All final dose calculations were performed with the AAA of Eclipse version 8.2.23 on the average intensity dataset derived from the planning 4DCT. For comparison, standard SBRT plans consisting of 10 noncoplanar static beams were generated in Eclipse for each patient. These plans were reviewed by a clinician and a physicist, and where necessary, further optimized. The conformity indices (CI80, 60 or 40) were calculated for both planning techniques, with e.g. CI80 defined as the volume encompassed by the 80% isodose line divided by the volume of PTV encompassed by the same isodose level. The volume of the chest wall receiving a dose higher than 45 Gy (V45) was also compared [11]. For each RapidArc plan, patient-specific quality assurance (QA) was performed by comparing the calculated dose distributions with film dosimetry, in a coronal plane of the ‘lung’ (low density) compartment of a Quasar phantom (Modus Medical Systems, London, Canada). Double Gafchromic EBT films were used for these measurements, with an accuracy of 1.3% (1 standard deviation) [12]. Measurements were performed for both the static phantom and the lung compartment with film moving in a sinusoidal
pattern in cranial–caudal direction with amplitude equal to the tumor motion amplitude derived from the patients planning 4DCT scan. The penumbras, defined as the distance between the 20% and 80% isodose lines, were compared between measurements and calculations, as well as the dose differences above the prescribed dose (80%). Treatment delivery was performed on a Varian Trilogy linear accelerator. Set-up prior to each fraction was performed using co-registration between kV cone beam CT scan (CBCT) and the average intensity projection dataset of the 4DCT scans, based on a tumor match. The latter was verified by a physicist and/or radiation oncologist prior to treatment. If more than two arcs were needed, the CBCT-based set-up was repeated after the second arc in order to correct for possible intrafraction movement of the patient. Treatment times from start of the first CBCT until end of treatment were recorded for each treatment fraction. In addition, the delivery time was recorded as the time needed from start of the first arc until end of the last arc, excluding the time for CBCT set-up.
Results For all three patients, the conformity indices for the high dose region (CI80 and CI60) were superior for RapidArc plans than for conventional SBRT plans (Table 1). Furthermore, the volume of chest wall receiving more than 45 Gy was lower in RapidArc plans and a typical dose distribution in patient P1 is shown in Fig. 1. The results for P3 show that RapidArc is also more conformal for large tumors, although a somewhat larger volume of contralateral lung received low doses of 5 Gy or more.
Table 1 Comparison between RA and 10-beam SBRT for the three patients: number of monitor units, conformity index at different dose levels, the volume of chest wall treated to more than 45 Gy, and the delivery times of RapidArc treatment, both including and excluding the time required for CBCT set-up. RapidArc
Fractionation (# Gy) No. of arcs Delivery time (min) Treatment time incl. CBCT (min) Monitor units CI80 CI60 CI40 V45 chest wall (cm3)
Conventional SBRT
P1
P2
P3
3 18 5 11 31 4829 1.09 2.18 5.00 1.28
5 11 3 6.4 18.4 2217 1.09 2.13 4.71 0.40
8 7.5 2 4.5 10.5 1853 1.08 2.27 5.71 1.33
Average
1.09 2.19 5.14 1.00
P1
P2
P3
3 18
5 11
8 7.5
12
11.2
12.9
3029 1.28 2.48 4.92 3.09
1981 1.14 2.20 4.55 0.7
1126 1.24 2.54 5.53 2.64
Average
1.22 2.41 5.00 3.42
Fig. 1. Comparison of dose distributions for patient P1 with both RapidArc (left, squares) and 10-beam SBRT (right, triangles). The DVH shows the PTV (red), adjacent thoracic wall (purple), ipsilateral lung (red) and contralateral lung (green).
124
Rapid delivery of SBRT using RapidArc
Film dosimetry in the static Quasar phantom corresponded well with calculated dose distributions. Measured penumbras in both cranial–caudal as in lateral direction corresponded within 1 mm with the calculated penumbras, and the maximum dose deviations between measurements and calculations were 4%. Film measurements in the moving phantom also corresponded well with calculation. The maximum dose difference due to the motion was 7%, but still the dose remained well above the prescription dose. The penumbra broadening in the cranial–caudal direction was according to what could be expected from the phantom movement, i.e. weighted summation of calculations according to phase shift corresponded with the measurements. The mean number of monitor units for RapidArc plans was 31% higher than those needed for dynamic conformal arc plans, a finding which is indicative of the amount of leaf motion in the RapidArc plans. Treatment delivery times, both including and excluding the setup procedure, are shown in Table 1. Typical delivery times for our conventional 10-beam SBRT, excluding set-up, range from 10 min for the 8-fraction low dose scheme to 14 min for the 3-fraction scheme. At least 30% of the fractions require a second set-up during their treatment fraction because of significant intrafraction movement of the patient during their radiation delivery. Discussion In this report, we demonstrate the planning, dosimetric verification, and delivery of highly conformal RapidArc plans for SBRT of peripheral lung tumors. The three patients described in this report are representative of the spectrum of tumor sizes and locations for which risk-adapted fractionation is mandated in our protocol. The reduction achieved in delivery time is a major advantage of RA delivery. For the scheme with the lowest fraction dose, it was reduced by a factor 2 compared to delivery time of our conventional 10 static field non-coplanar plans. The delivery time for the 3 18 Gy scheme is still relatively large with RA, but this will be further reduced as the latest clinical release of RapidArc (8.6.10) has no constraints with respect to the maximum number of MUs for one arc. After upgrading RapidArc to the latest release, we plan to treat these patients on a Novalis Tx unit with a dose rate of 1000 MU/min, which will permit patients to be treated with 2 arcs in less than 6 min beam-on time for a fraction dose of 18 Gy. Shorter treatment time will reduce the likelihood of intrafraction baseline shifts in tumor position [2]. Based on our experience, we expect that a single CBCT set-up will be sufficient if the entire radiation delivery can be completed in 6 min. At present, each CBCTbased set-up procedure, including verification of the automatic co-registration, takes 4–5 min. We are now studying the incidence and extent of baseline shifts in tumor position as a function of the delivery time. Several centers use only 4–6 coplanar beams for SBRT, which allows for a quicker delivery than our 10 non-coplanar beams. However, the use of 10 non-coplanar beams improves the conformity of the high dose region, particularly at the site of the chest wall. This can account for the low reported incidence of rib fractures of only 2% in the first 206 treated patients [1]. This contrasts with a far higher incidence of rib fractures [11] or severe skin reactions [13] in centers using 4–6 coplanar beams. For each patient, RapidArc achieved a better target dose conformity in comparison to our conventional SBRT non-coplanar 10-field approach. Specifically, this leads to improved sparing of the chest wall (Fig. 1), though, at the cost of a minimally larger CI40. Although the minor increase in dose to contralateral lung was not considered significant in these three patients, additional analysis in patients with large tumors located in the lower lobe will be performed to study this issue further. It should be noted,
though, that for these three patients no objectives were used to reduce the lung dose, and that adding such objective can achieve reduced contralateral lung doses. The use of multiple partial arcs could also lead to a lower contralateral lung dose. IMRT could also provide much better dose conformity compared to the conventional 10-field approach, but IMRT is generally not used for most lung SBRT because it increases the delivery time. It should be pointed out that the use of RapidArc for SBRT planning of lung tumors is presently not straightforward as the optimization algorithm is based on convolution of spatially invariant point spread kernels, whereas the final dose calculation by the AAA better takes into account the tissue inhomogeneities. This difference between optimization and dose calculation can lead to a larger dose inhomogeneity in the PTV, though this often falls within our plan acceptance criteria [10]. It is reassuring that dosimetric verification with films in a moving phantom confirmed that the combination of RA (moving leaves) with a moving tumor up to a maximum amplitude of 13 mm did not result in dose inhomogeneity in the PTV due to interplay effects. This study shows that RapidArc allows delivering of hypofractionated doses much faster than conventional SBRT, with the additional advantage that the plans are more conformal compared to those of conventional SBRT. The use of the Novalix Tx with smaller leaf width of the MLC and a higher MU output will further improve these results.
Conflicts of interest statement The VUmc has a research collaboration with Varian Medical Systems.
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