Impact of patient positioning on radiotherapy dose distribution: An assessment in parotid tumor

Cite this article as: Sharma S, Goyal S, Muzumdar S, Manigandan D, Sahai P, Biswas A, Subramani V, Chander S. Impact of patient positioning on radiotherapy dose distribution: An assessment in parotid tumor. Int J Cancer Ther Oncol 2016; 4(1):416. DOI: 10.14319/ijcto.41.6 [A part of this research was presented at ASTRO-2009, Annual Meeting, which was held from November 1-5, 2009 in Chicago, USA] © Sharma et al. ISSN 2330-4049 Impact of patient positioning on radiotherapy dose distribution: An assessment in parotid tumor


Introduction
Newer methods and techniques continue to be explored in radiotherapy practice for achieving the goal of maximum tumor dose with sparing of normal structures. The patient position and immobilization plays a pertinent role in external beam radiotherapy planning. Many authors have studied the influence of treatment position on radiotherapy dose distribution. Wilder et al. 1 compared the prostate intrafraction motion in prone and supine position author concluded that prone and supine positions resulted in a similar magnitude of anteroposterior (AP) and superoinferior intrafraction prostate motion (2 mm). Krengli et al. 2 analyzed the dosimetric parameters of patients receiving adjuvant breast radiotherapy (RT) in prone versus supine position and found that prone position is a favorable alternative for irradiation of mammary gland in patients with pendulous breasts. Sharma et al. 3 studied the impact of patient position (supine and prone) on patients of brainstem glioma and concluded that supine and prone positions resulted in almost similar dose distribution. Bakkal et al. 4 evaluated the effect of radiotherapy on testicles with different treatment positions and plans for rectal cancer patients and concluded that supine 4-fields (S4f) external beam radiotherapy for rectal carcinoma allows better testicular dose than prone 3-fields (p3f) and prone 4-fields (p4f). Drzymala et al. 5 also studied the volume of bowel and dose received in the prone and supine positions in patients undergoing pre-operative rectal cancer chemoradiation, results showed that volume of bowel irradiated is not significantly higher in supine position. The supine with head neutral (HN) treatment position is employed for planning radiotherapy in head and neck cancer patients. A pair of angled wedge photon beam is the most common radiation planning technique for parotid tumors. 6,7 Since patients with parotid tumors require unilateral irradiation, a change in position from HN to head tilt (HT) may alter the dose distribution. The purpose of this study was to evaluate the influence of head positioning on dose distribution within the target volume and organs at risk (OARs) in patients with parotid tumor. The plans with 3-D conformal radiation therapy (3-DCRT) and intensity modulated radiation therapy (IMRT) were compared alongside the two different head positions.

Methods and Materials
Three patients with right-sided parotid tumor were treated with radiotherapy. The patients were immobilized using a thermoplastic mould in two positions: supine with head in neutral (HN) and tilted to the contralateral side at 90 degree (HT). Computed tomography (CT) scan of the head and neck region was performed at 3 mm slice thickness in both the head positions. The CT images were transferred to the Eclipse TM (Varian medical system, Palo Alto, CA, Helios version 6.5) treatment planning system (TPS) via DICOM. The clinical target volume (CTV), planning target volume (PTV), and OARs i.e., contralateral parotid, brainstem, spinal cord, oral cavity, cochlea, and mandible were contoured in both scans for all patients. PTV was generated from CTV with a 5 mm margin. PTV was intersected with body contour by 2 mm margin. Two plans with 3-DCRT and IMRT were generated for each of the six CT datasets. The plan with 3-DCRT was generated with a photon wedge pair (WP). Five fields were employed for generating an IMRT plan. The plans were made for Varian CL2300C/D linear accelerator (Varian Medical Systems, Palo Alto, CA, USA) using 6 MV X-rays. The linear accelerator has 40 pairs of multi-leaf collimator (MLC) with a leaf thickness of 1 cm at isocenter.

Dose prescription
A dose of 60 Gy in 30 fractions was prescribed to the PTV. All plans were prescribed at 95% isodose line.

Wedge pair plan
The plan was made with two oblique fields using 45 degree wedge. The beam's-eye-view (BEV) option of the TPS was employed to select the appropriate gantry angle to allow coverage of the PTV while minimizing the dose to OARs. The MLC was geometrically shaped around the PTV with a 7 mm margin for penumbra. A 0.5 cm bolus was added for achieving a dose build-up at surface.

Plan evaluation
All plans were analyzed using DVH. The target coverage was analyzed in terms of minimum, maximum, and mean dose. Coverage index (CI) and heterogeneity index (HI) for PTV were calculated. 8 CI = TVRI/TV where, TVRI = target volume covered by the reference isodose; TV =target volume This index ranges from 0 to 1. The range of CI shows the amount of conformation because a value of 1 is rarely achieved. The volume of adjacent healthy tissues is not taken into account in this index. HI= D5%/ D95% where, D5% = dose delivered to 5% of PTV volume; D95% = dose delivered to 95% of PTV volume It is evident that if the value of heterogeneity index is closer to 1, the better will be the dose homogeneity for PTV.
The maximum and mean dose to brain-stem, contra-lateral parotid gland, mandible, oral cavity, bilateral cochlea was noted. Maximum dose to spinal cord was noted.

Results
The dose distribution for the wedge pair and IMRT plan in neutral and tilted head position is demonstrated in Figure 1.    The doses to OARs with the plans at respective head position are listed in Table 3.

Head neutral versus tilt position with wedge pair plan
A change in head position from HN to HT resulted in a reduction of brainstem Dmax and Dmean. A major variation in the Dmax and Dmean to the contralateral parotid was observed with lesser doses at the HN position (average Dmean 4.54 Gy vs. 8.07 Gy). A difference of 0.9 Gy in the average Dmax to spinal cord was seen. The values of Dmean to mandible, oral cavity, ipsilateral and contralateral cochlea were higher in the HT position.

Head neutral versus tilt position with IMRT plan
A change in head position from HN to HT resulted in a dose reduction in average Dmax to brainstem (22.99 Gy vs. 19.81 Gy). The spinal cord Dmax increased at the HT position by 1.2 Gy. The dose to contralateral parotid and cochlea was comparable in both the positions. However, the Dmean to oral cavity was reduced at HT position.

IMRT versus wedge pair plan at head neutral position
The average Dmean to the contralateral parotid was reduced with the IMRT plan (

Discussion
Various planning techniques have been described for parotid gland tumour. The ipsilateral WP, WP with lateral portal (3-fields), and the mixed electron-photon beam were considered the optimal techniques in a study by Yaparpalvi et al. 9 The WP planning has been conventionally described in supine with HN position. The present study elaborated on dosimetry in tilted head as compared with the neutral position. The target coverage and sparing of normal structures were acceptable with the plans at both head positions. However, an increase in dose to mandible and oral cavity was observed with WP plan at HT position, since the beam entry and exit was unavoidable through these structures in this position. Similarly, IMRT plan in both head positions demonstrated almost same results with respect to the doses to target and OARs.
Nutting et al. 10 compared the conventional radiotherapy, 3-DCRT, and IMRT for patients with parotid gland tumors. A reduction in the radiation dose to critical normal tissues was demonstrated with 3-DCRT compared with conventional radiotherapy. A further reduction in the dose to the cochlea and oral cavity was observed with IMRT. With nine and seven fields, the dose to the contra-lateral parotid gland was increased, but this was avoided by optimization of the beam directions. The benefits of IMRT were maintained with three or four fields when the beam angles were optimized. Bragg et al. 11 compared IMRT with 3-DCRT plans for the treatment of tumors of parotid gland. An IMRT class solution of five fields at 15 0 , 55 0 , 125 0 , 165 0 , and 270 0 was proposed which resulted in improved target dose distribution and sparing of critical structures with an efficient treatment delivery. IMRT reduced the mean dose to the contralateral parotid gland and the maximum doses to the brain and the spinal cord, but increased the ipsilateral lens dose in some cases. A study 12 compared 3D-CRT and IMRT with respect to hearing loss. The mean volume of the inner ear receiving a dose higher than 50 Gy decreased from 14.7% to 1.4%. Likewise, the current study showed a superior sparing of relatively all OARs with IMRT. A considerable reduction in dose to ipsilateral ear was observed which was not achievable with a WP plan (average Dmean 6.06 Gy vs 20.96 Gy in HN position).

Conclusion
A change in head position from neutral to 90°c ontralateral tilt for wedge pair plan in parotid tumor may considerably reduce dose to the brainstem and spinal cord with a modest increase in dose to mandible, oral cavity, contralateral parotid, and bilateral cochlea. The alteration in head position has minimal impact on IMRT planning. Head position and treatment technique may be selected according to the clinical requirement on an individual case basis.