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Volumetric modulated arc therapy improves dosimetry and reduces treatment time compared to conventional intensity-modulated radiotherapy for locoregional radiotherapy of left-sided breast cancer and internal mammary nodes
Volumetric modulated arc therapy improves dosimetry and reduces treatment time compared to conventional intensity-modulated radiotherapy for locoregional radiotherapy of left-sided breast cancer and internal mammary nodes Popescu CC, Olivotto IA, Beckham WA, et al (British Columbia Cancer Agency– Vancouver Island Centre, Victoria, Canada; et al) Int J Radiat Oncol Biol Phys 76:287-295, 2010
Purpose.—Volumetric modulated arc therapy (VMAT) is a novel extension of conventional intensitymodulated radiotherapy (cIMRT), in which an optimized three-dimensional dose distribution may be delivered in a single gantry rotation. VMAT is the predecessor to RapidArc (Varian Medical System). This study compared VMAT with cIMRT and with conventional modified wide-tangent (MWT) techniques for locoregional radiotherapy for left-sided breast cancer, including internal mammary nodes. Methods and Materials.—Therapy for 5 patients previously treated with 50 Gy/25 fractions using nine-field cIMRT was replanned with VMAT and MWT. Comparative endpoints were planning target volume (PTV) dose homogeneity, doses to surrounding structures, number of monitor units, and treatment delivery time. Results.—For VMAT, two 190 arcs with 2-cm overlapping jaws were required to optimize over the large treatment volumes. Treatment plans generated using VMAT optimization resulted in PTV homogeneity similar to that of cIMRT and MWT. The average heart volumes receiving >30 Gy for VMAT, cIMRT, and MWT were 2.6% € 0.7%, 3.5% € 0.8%, and 16.4% € 4.3%, respectively, and the
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average ipsilateral lung volumes receiving >20 Gy were 16.9% € 1.1%, 17.3% € 0.9%, and 37.3% € 7.2%, respectively. The average mean dose to the contralateral medial breast was 3.2 € 0.6 Gy for VMAT, 4.3 € 0.4 Gy for cIMRT, and 4.4 € 4.7 Gy for MWT. The healthy tissue volume percentages receiving 5 Gy were significantly larger with VMAT (33.1% € 2.1%) and IMRT (45.3% € 3.1%) than with MWT (19.4% € 3.7%). VMAT reduced the number of monitor units by 30% and the treatment time by 55% compared with cIMRT. Conclusions.—VMAT achieved similar PTV coverage and sparing of organs at risk, with fewer monitor units and shorter delivery time than cIMRT. Internal mammary node (IMN) irradiation of patients with left-sided breast cancer poses a technical challenge to radiation oncologists. Traditional methods for irradiating IMNs all have dosimetric disadvantages, namely, dose inhomogeneity or increased dose to adjacent normal structures. The MWT technique is a simple, commonly used method that allows for coverage of IMNs without the dose inhomogeneity associated with adding a separate internal mammary field. The primary drawback of the MWT technique is that it increases the volume of heart, ipsilateral lung, and occasionally, contralateral breast, within the highdose tangential fields. In cIMRT, individual fields are divided into hundreds of subfields. Computer optimization is then used to determine where best to deposit the dose so that the physician-specified constraints are met. This technique is an attractive option for covering IMNs while limiting high doses to adjacent normal structures. With cIMRT, the dose is delivered at discrete machine positions. VMAT is an emerging technology that delivers a highly
Breast Diseases: A Year BookÒ Quarterly Vol 21 No 2 2010
modulated dose distribution over a series of 1-2 arcs, rather than at static machine positions. This makes it possible to deliver cIMRT dose distributions while significantly reducing both beam-on and overall treatment times. In this dosimetric comparison, Popescu and colleagues compared plans for left breast and nodal irradiation (including IMNs) using 3 techniques: MWT, cIMRT, and VMAT. As expected, compared with MWT, both cIMRT and VMAT significantly reduced the volume of heart and lung receiving the prescription dose, but they both increased the amount of low-dose radiation to these structures. Compared to cIMRT, VMAT significantly reduced treatment time and also reduced the volume of heart, lung, and contralateral breast receiving low doses (V5 and V10), without compromising target coverage or increasing dose inhomogeneity. While VMAT may be dosimetrically equivalent or superior to cIMRT, there are still some general concerns about the application of IMRT (either with static gantry IMRT or VMAT) to the breast. The sculpting of high doses with IMRT comes at the expense of exposing significantly larger volumes of normal tissues to low-dose radiation. When comparing VMAT to cIMRT, Popescu and colleagues suggest that the decreased lung and contralateral breast dose with VMAT could decrease the risk of a second malignancy. Still, the volume of ipsilateral lung receiving 5 Gy (V5) was significantly higher with both cIMRT and VMAT compared to MWT (91.9%, 70.2%, and 46.8%, respectively). These increased low doses may have both acute and long-term consequences. While there is a lack of data regarding what constitutes a ‘‘safe’’ V5, there is a growing body of literature suggesting that low doses of radiation to large volumes of lung
can increase the risk of radiation pneumonitis. This potential risk should be factored into the costbenefit equation when considering IMRT in patients who have received adriamycin and paclitaxel, 2 agents independently associated with pneumonitis. Studies have also consistently shown an increased cardiac mortality rate at 10 years or more following left-sided breast RT.1 Given the similar 10-year latency period for radiation-induced second malignancy, long-term follow-up will be necessary to determine whether the medium-term benefit of heart avoidance outweighs the long-term risk of second malignancy. One limitation of this study is the use of a single ‘‘standard’’ technique (MWT) for all patients. While this approach simplifies dosimetric comparisons, it does not replicate real-world practice. In the case of the contralateral breast dose, most clinicians would explore various techniques for IMN coverage when such a plan would require treatment of contralateral breast tissue. For comparison purposes, 1 MWT plan included in this study would have
delivered a mean contralateral breast dose of 12.5 Gy. A previous publication by the same authors compared cIMRT with ‘‘best standard techniques,’’ including MWT and direct internal mammary fields.2 In this earlier study, non-IMRT techniques had significantly lower contralateral breast doses than cIMRT, but they had significantly higher dose inhomogeneity. With a judicious modification of technique based on an individual patient’s anatomy, standard treatments can consistently yield lower contralateral breast and ipsilateral lung mean doses than IMRT. This study by Popescu and colleagues is the first step towards the use of VMAT for left-sided breast/IMN RT in particular as well as for breast RT in general. As shown, VMAT is a promising technology that may allow the rapid delivery of highly conformal RT. However, the incorporation of new technologies into clinical practice should be done cautiously. From a dosimetric perspective, the combination of a dynamically changing field aperture and breathing-induced target motion
can result in unanticipated dose distributions at the time of treatment. The impact of target motion on dose distributions delivered with VMAT should be critically evaluated prior to its application in breast RT. The current study is a valuable contribution to our understanding of the dosimetry achievable with VMAT in breast RT. However, prospective studies investigating the impact these techniques may ultimately have on clinical outcomes are needed. V. J. Gonzalez, MD D. K. Gaffney, MD, PhD
References 1. Harris EE. Cardiac mortality and morbidity after breast cancer treatment. Cancer Control. 2008;15: 120-129. 2. Beckham WA, Popescu CC, Patenaude VV, Wai ES, Olivotto IA. Is multibeam IMRT better than standard treatment for patients with left-sided breast cancer? Int J Radiat Oncol Biol Phys. 2007;69:918-924.
Breast Diseases: A Year BookÒ Quarterly Vol 21 No 2 2010
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Purpose.—Volumetric modulated arc therapy (VMAT) is a novel extension of conventional intensitymodulated radiotherapy (cIMRT), in which an optimized three-dimensional dose distribution may be delivered in a single gantry rotation. VMAT is the predecessor to RapidArc (Varian Medical System). This study compared VMAT with cIMRT and with conventional modified wide-tangent (MWT) techniques for locoregional radiotherapy for left-sided breast cancer, including internal mammary nodes. Methods and Materials.—Therapy for 5 patients previously treated with 50 Gy/25 fractions using nine-field cIMRT was replanned with VMAT and MWT. Comparative endpoints were planning target volume (PTV) dose homogeneity, doses to surrounding structures, number of monitor units, and treatment delivery time. Results.—For VMAT, two 190 arcs with 2-cm overlapping jaws were required to optimize over the large treatment volumes. Treatment plans generated using VMAT optimization resulted in PTV homogeneity similar to that of cIMRT and MWT. The average heart volumes receiving >30 Gy for VMAT, cIMRT, and MWT were 2.6% € 0.7%, 3.5% € 0.8%, and 16.4% € 4.3%, respectively, and the
178
average ipsilateral lung volumes receiving >20 Gy were 16.9% € 1.1%, 17.3% € 0.9%, and 37.3% € 7.2%, respectively. The average mean dose to the contralateral medial breast was 3.2 € 0.6 Gy for VMAT, 4.3 € 0.4 Gy for cIMRT, and 4.4 € 4.7 Gy for MWT. The healthy tissue volume percentages receiving 5 Gy were significantly larger with VMAT (33.1% € 2.1%) and IMRT (45.3% € 3.1%) than with MWT (19.4% € 3.7%). VMAT reduced the number of monitor units by 30% and the treatment time by 55% compared with cIMRT. Conclusions.—VMAT achieved similar PTV coverage and sparing of organs at risk, with fewer monitor units and shorter delivery time than cIMRT. Internal mammary node (IMN) irradiation of patients with left-sided breast cancer poses a technical challenge to radiation oncologists. Traditional methods for irradiating IMNs all have dosimetric disadvantages, namely, dose inhomogeneity or increased dose to adjacent normal structures. The MWT technique is a simple, commonly used method that allows for coverage of IMNs without the dose inhomogeneity associated with adding a separate internal mammary field. The primary drawback of the MWT technique is that it increases the volume of heart, ipsilateral lung, and occasionally, contralateral breast, within the highdose tangential fields. In cIMRT, individual fields are divided into hundreds of subfields. Computer optimization is then used to determine where best to deposit the dose so that the physician-specified constraints are met. This technique is an attractive option for covering IMNs while limiting high doses to adjacent normal structures. With cIMRT, the dose is delivered at discrete machine positions. VMAT is an emerging technology that delivers a highly
Breast Diseases: A Year BookÒ Quarterly Vol 21 No 2 2010
modulated dose distribution over a series of 1-2 arcs, rather than at static machine positions. This makes it possible to deliver cIMRT dose distributions while significantly reducing both beam-on and overall treatment times. In this dosimetric comparison, Popescu and colleagues compared plans for left breast and nodal irradiation (including IMNs) using 3 techniques: MWT, cIMRT, and VMAT. As expected, compared with MWT, both cIMRT and VMAT significantly reduced the volume of heart and lung receiving the prescription dose, but they both increased the amount of low-dose radiation to these structures. Compared to cIMRT, VMAT significantly reduced treatment time and also reduced the volume of heart, lung, and contralateral breast receiving low doses (V5 and V10), without compromising target coverage or increasing dose inhomogeneity. While VMAT may be dosimetrically equivalent or superior to cIMRT, there are still some general concerns about the application of IMRT (either with static gantry IMRT or VMAT) to the breast. The sculpting of high doses with IMRT comes at the expense of exposing significantly larger volumes of normal tissues to low-dose radiation. When comparing VMAT to cIMRT, Popescu and colleagues suggest that the decreased lung and contralateral breast dose with VMAT could decrease the risk of a second malignancy. Still, the volume of ipsilateral lung receiving 5 Gy (V5) was significantly higher with both cIMRT and VMAT compared to MWT (91.9%, 70.2%, and 46.8%, respectively). These increased low doses may have both acute and long-term consequences. While there is a lack of data regarding what constitutes a ‘‘safe’’ V5, there is a growing body of literature suggesting that low doses of radiation to large volumes of lung
can increase the risk of radiation pneumonitis. This potential risk should be factored into the costbenefit equation when considering IMRT in patients who have received adriamycin and paclitaxel, 2 agents independently associated with pneumonitis. Studies have also consistently shown an increased cardiac mortality rate at 10 years or more following left-sided breast RT.1 Given the similar 10-year latency period for radiation-induced second malignancy, long-term follow-up will be necessary to determine whether the medium-term benefit of heart avoidance outweighs the long-term risk of second malignancy. One limitation of this study is the use of a single ‘‘standard’’ technique (MWT) for all patients. While this approach simplifies dosimetric comparisons, it does not replicate real-world practice. In the case of the contralateral breast dose, most clinicians would explore various techniques for IMN coverage when such a plan would require treatment of contralateral breast tissue. For comparison purposes, 1 MWT plan included in this study would have
delivered a mean contralateral breast dose of 12.5 Gy. A previous publication by the same authors compared cIMRT with ‘‘best standard techniques,’’ including MWT and direct internal mammary fields.2 In this earlier study, non-IMRT techniques had significantly lower contralateral breast doses than cIMRT, but they had significantly higher dose inhomogeneity. With a judicious modification of technique based on an individual patient’s anatomy, standard treatments can consistently yield lower contralateral breast and ipsilateral lung mean doses than IMRT. This study by Popescu and colleagues is the first step towards the use of VMAT for left-sided breast/IMN RT in particular as well as for breast RT in general. As shown, VMAT is a promising technology that may allow the rapid delivery of highly conformal RT. However, the incorporation of new technologies into clinical practice should be done cautiously. From a dosimetric perspective, the combination of a dynamically changing field aperture and breathing-induced target motion
can result in unanticipated dose distributions at the time of treatment. The impact of target motion on dose distributions delivered with VMAT should be critically evaluated prior to its application in breast RT. The current study is a valuable contribution to our understanding of the dosimetry achievable with VMAT in breast RT. However, prospective studies investigating the impact these techniques may ultimately have on clinical outcomes are needed. V. J. Gonzalez, MD D. K. Gaffney, MD, PhD
References 1. Harris EE. Cardiac mortality and morbidity after breast cancer treatment. Cancer Control. 2008;15: 120-129. 2. Beckham WA, Popescu CC, Patenaude VV, Wai ES, Olivotto IA. Is multibeam IMRT better than standard treatment for patients with left-sided breast cancer? Int J Radiat Oncol Biol Phys. 2007;69:918-924.
Breast Diseases: A Year BookÒ Quarterly Vol 21 No 2 2010
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