Target splitting non-coplanar RapidArc radiation therapy for a diffuse sebaceous carcinoma of the scalp: a novel delivery technique
- Jiang Hu†1, 2,
- WeiWei Xiao†1, 2,
- ZhiChun He1, 2,
- DeHua Kang1, 2,
- ALong Chen1, 2 and
- ZhenYu Qi1, 2Email author
© Hu et al.; licensee BioMed Central Ltd. 2014
Received: 10 November 2013
Accepted: 28 August 2014
Published: 16 September 2014
Background and purpose
To compare conventional lateral photon-electron, fixed-beam intensity modulated radiation therapy (IMRT), coplanar and non-coplanar RapidArc for the treatment of a diffuse sebaceous gland carcinoma of the scalp.
Comprehensive dosimetry comparisons were performed among 3D-CRT, IMRT and various RapidArc plans. Target coverage, conformity index (CI), homogeneity index (HI) and doses to organs at risk (OAR) were calculated. Monitor unites (MUs) and delivery time of each treatment were also recorded to evaluate the execution efficiency. The influence of target splitting technique and non-coplanar planning on plan quality was discussed.
IMRT was superior to 3D-CRT concerning targets’ coverage at the sacrifice of larger irradiated brain volumes to low doses. CIs and HIs were better in coplanar RapidArc and non-coplanar RapidArc plans than 3D-CRT and IMRT. Best dose coverage and sparing of OARs were achieved in non-coplanar plans using target splitting technique. Treatment delivery time was longest in the IMRT plan and shortest in the coplanar RapidArc plan without target splitting. The 3%/3 mm gamma test pass rates were above 95% for all the plans.
Target splitting technique and non-coplanar arcs are recommended for total scalp irradiation.
KeywordsTotal scalp irradiation RapidArc Target splitting Non-coplanar Dosimetry
Sebaceous carcinoma of the scalp is rare, with very few cases reported in literature . Radiotherapy has historically been proven an effective method for local treatment of sebaceous carcinoma, especially when surgery is not recommended . However, delivering radiation for total scalp is technically challenging due to the concave shape and the proximity to critical structures. Traditional techniques such as stationary electron-beam fields may cause unacceptable hotspots in the field junctions [3, 4]. Utilizing lateral opposed photon fields matched with lateral electron fields and shifting the junction during the treatment are effective ways to improve dose uniformity at the junction , but target coverage is still unsatisfied.
Intensity modulated radiation therapy (IMRT) is potentially suitable for total scalp irradiation with the ability to produce a concave dose distribution. It has been demonstrated that fixed-beam IMRT can improve the target dose coverage and homogeneity compared to 3D-CRT . Nevertheless, it decreases the brain volume irradiated to high doses at the cost of larger brain volumes irradiated to lower doses . Recently, a rotational IMRT delivery technique, named RapidArc, was reported by Kelly et al. for total dural irradiation . By using case-individualized collimator angle settings, they achieved a much better dose conformity with coplanar RapidArc than 9-field IMRT. This result may suggest that RapidArc can provide a more promising solution over fixed-beam IMRT for total scalp radiotherapy.
The purpose of this study is to explore the feasibility and efficiency of RapidArc in total scalp irradiation. The advantages of target splitting technique and non-coplanar planning were discussed with the aim to acquire an optimal radiotherapy modality for the treatment of a total scalp irradiation-like target volume.
CT simulation and target definition
From 2003, three patients diagnosed as low differentiated sebaceous gland carcinoma of the scalp were treated at our institution. These cases have typical spherical shell-shape tumor target with diffuse infiltration of skull and multiple nodules. Patients were immobilized with head-and-neck thermoplastic masks in a supine position. CT simulation was performed with 3 mm slice thickness and 3 mm slice spacing including the head and neck.
For comparison purpose, different treatment plans were generated by using Varian Eclipse treatment planning system, including 3D-CRT, 9-field IMRT and RapidArc plans. All the treatments were undertaken on a Varian Trilogy linear accelerator (Varian Medical Systems, Palo Alto, CA). Anisotropic Analytical Algorithm (AAA) model was used for dose calculation with a dose grid of 3 mm × 3 mm × 3 mm. Tissue heterogeneity corrections were also included.
The 3D-CRT plan was planned in three phases: 50 Gy in 25 fractions for the first phase (PTV2), 10 Gy in 5 fractions for the second phase (PTV1) and another 10 Gy in 5 fractions for the third phase (PTV-G). A treatment technique described by Akazawa  was applied in this study, which included two electron and two photon fields. The “skullcap” area was irradiated by parallel opposed 6 MV photon fields with a 1 cm thick wax bolus. The rest of the scalp was treated with two opposed 9 MeV electron beams matched to the upper photon fields. A 0.5 cm thick wax bolus was used in electron beams to build up skin dose and to protect the brain from electron dose.
9-field IMRT plan
Optimization goals and constraints used for IMRT and RapidArc planning
V95% ≥ 100%
V110% ≤ 10%
Dmax < 80Gy
V95% ≥ 100%
V95% ≥ 100%
Dmax < 72Gy
Dmean: <30Gy and as low as possible
V28Gy < 30%
Dmax < 10Gy
Dmax < 10Gy
Dmax < 54Gy
Dmax < 54Gy
Dmax < 55Gy
Dmax < 55Gy
Dmax < 55Gy
Three RapidArc plans were designed using the same dose prescription and optimization constraints as in the IMRT plan, including a standard RapidArc plan (sRapidArc), a split-target volume coplanar RapidArc plan (scRapidArc) and a split-target volume non-coplanar RapidArc plan (snRapidArc).
In the snRapidArc plan, non-coplanar arcs with split-target technique were further evaluated for total scalp irradiation. The non-coplanar plan also included 4 arcs as in the scRapidArc plan, except for the gantry angles which were spatially distributed. The collimator angles were optimized in the three-dimensional directions to provide the best protection for normal brain tissues.
Where VTref was the volume of the target covered by the reference isodose, VT was the target volume and Vref was the volume of the reference isodose.
Where Dx% was the absorbed dose received by x% of the target volume.
Treatment delivery and dose verification
Patient specific dose verification was conducted with a 3D cylindrical diode array (ArcCHECKTM, Sun Nuclear, Melbourne, FL) for both IMRT and RapidArc plans. Hybrid phantom plans were created by re-computing the dose distribution with the QA phantom geometry using the same beam parameters of the patients’ plans. The discrepancies between the measured doses and TPS calculations were analyzed by using gamma-index method with a criterion of 3%/3 mm and a threshold dose of 10% of the maximum dose. MUs and delivery time were also recorded for each plan to evaluate the execution efficiency of different treatment techniques.
Dosimetric results for five treatment plans
Both IMRT and RapidArc plans could offer a better dose coverage and homogeneity for target volumes compared with 3D-CRT. It was found that 95% of the prescription dose covered nearly 100% of PTVs in the IMRT and RapidArc plans. Also, D2% was all within 80 Gy in these plans. Among them, improved dose conformity and homogeneity in the target volume were achieved with the split-target coplanar and non-coplanar RapidArc plans. V110% accounted for less than 1% of PTV-G in the split-target coplanar and non-coplanar RapidArc plans, in contrast to 12.86% and 16.74% in IMRT and standard RapidArc plans.
The brain dose-volume parameters given in Table 2 clearly showed that the use of RapidArc, especially for the split-target coplanar and non-coplanar techniques, could further reduce the high doses delivered to the brain. The D1% of brain was decreased from 69.66 Gy in 3D-CRT to about 66 Gy in the sRapidArc plan, and about 62Gy in both scRapidArc and snRapidArc plans. Beside this, it was found that the irradiated brain volumes from V10Gy to V70Gy in the snRapidArc plan were even smaller than those in 3D-CRT. The protective effectiveness for the brain tissue was thus concluded to be snRapidArc > scRapidArc > sRapidArc.
Dosimetric verification results for five treatment plans
Delivery time (s)
Gamma criteria (%)
The goal of total scalp irradiation is to provide a uniform dose throughout the scalp while keeping the dose to the normal tissues as low as possible. But this therapeutic goal was not easily achieved owing to the concave shape of target and the close proximity of the target to OARs. In this work, a dosimetry comparison of 3D-CRT, fixed-beam IMRT, coplanar and non-coplanar RapidArc treatment techniques was undertaken for the patients diagnosed as low differentiated carcinoma of sebaceous gland in the scalp.
Lateral photon-electron technique was first reported in 1989  and has been used in the treatment of scalp tumors for decades. Matching the photon and electron beams and shifting the match-line during the treatment course might yield acceptable dose coverage of targets and dose sparing of optical organs . However, it could also cause hotspots of greater than 115% of the prescription dose in the fields’ junctions. This has been approved by our 3D-CRT plan, in which only suboptimal target dose coverage was obtained with hotspots in the abutting regions of photon and electron fields.
Substantial dosimetric advantages of IMRT over 3D-CRT have been proved in various cancers [13, 14]. More recently, dosimetric comparisons demonstrated IMRT could get consistent improvements in target coverage for scalp irradiation [6, 15]. We tried 5-field and 7-field coplanar and non-coplanar IMRT plans, but none could meet the dose optimization goal. Thus 9-field coplanar IMRT plan was used for comparison. It was found HIs were significantly decreased in the IMRT plan, compared to 3D-CRT. CIs were also increased from 0.54 to 0.68 for PTV-G, from 0.43 to 0.45 for PTV1 and from 0.59 to 0.74 for PTV2 in our study. These results agreed well with the previous findings reported by Wojcicka et al. . However, as shown by Wojcicka et al.  and our study, IMRT provided little benefits to normal tissues in total scalp irradiation. Compared to 3D-CRT, the IMRT plan slightly decreased the high-dose irradiated volumes of the brain at the cost of larger volumes irradiated to lower doses. In addition, Wojcicka et al.  and our study both found that the 9-field IMRT plan even increased the D1% delivered to optical structures than 3D-CRT.
Rotational therapies may suit to the delivery of scalp irradiation, with beamlets delivering tangentially to the scalp at all points. In Kelly’s study , case-individualized collimator angle of 90° was used to facilitate better shielding of the brain with MLCs. They found the case-individualized RapidArc plan compared favorably with the 9-field conventional IMRT plan. By using similar RapidArc designs, we obtained slightly better values of CI and HI in the sRapidArc plan. Compared to IMRT, the sRapidArc plan decreased the D1% of all the optical structures, and foremost decreased the irradiated brain volumes from V10Gy to V70Gy. However, the brain volumes irradiated to lower doses (from V5Gyto V20Gy) were still larger than 3D-CRT.
Target splitting has been reported to be effective in improving dose distribution, especially in large target volumes adjacent to normal tissues [16–19]. Sahgal et al.  found split-volume treatment planning techniques could significantly improve Cyberknife treatment plan quality for consecutive thoracic vertebral bodies’ irradiation than the standard full-volume technique. Similar results were reported by Seppälä et al.  in craniospinal irradiation. Wurstbauer et al. [18, 19] applied target splitting technique to deliver high dose to lung cancer and achieved a high level of locoregional tumor control and survival times. In this study, we further testified that the split-target technique was also suitable for total scalp irradiation. The scRapidArc and snRapidArc plans provided improved conformality and homogeneity for tumor target than sRapidArc and decreased the D1% and Dmean of brain as well.
For typical spherical shell-shape target as shown in Figure 1, complete shielding of normal brain tissues couldn’t be achieved by using target splitting alone. Non-coplanar arcs may bring dosimetric advantages over static conformal beams especially for large and irregular targets, by allowing for more beam angel selection and more complete avoidance of normal tissue in three dimensional directions [20–23]. Here we applied the non-coplanar technique in the snRapidArc plan. Results showed CIs and HIs were largely the same as in scRapidArc, but D1% and Dmean of the brain were further lower than scRapidArc. To our surprise, irradiated brain volumes from V10Gy to V70Gy were even smaller than 3D-CRT. In a previously published paper, Soisson et al.  also found dosimetric advantage of non-coplanar beam arrangements for treatment of skull-base tumors by reducing the size of low dose volumes of normal brain. Clinically, keeping the low dose volumes to normal brain to minimum might be of significance to limit possible cognitive impairment. As a result, the snRapidArc plan was selected as an optimal solution for total scalp irradiation. Phantom verification results show it can be executed accurately and efficiently.
Considering all the dosimetric indices, target splitting and non-coplanar arcs are recommended for total scalp irradiation, which enable achieving more conformal and homogeneous targets’ dose coverage, lower brain dose, acceptable dose given to optimal structures and enough execution efficiency.
This work was supported by grants: Guangdong Province Natural Science Fund, China, and No. S201210009442, National Natural Science Foundation of China, No.81371710.
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