14 Nasopharynx Cancer

Nasopharynx Cancer

12

THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 10/09/2019

Table 4. Sample Dosimetry for a T2 Case

Case 1 Prescription dose: 3.5 Gy Rotterdam applicator

Levendag 2D pointsa

Physical Dose (Gy)

IGBT volumes

Physical Dose (Gy)

EQD2 b (Gy)

Targets

HR-CTV D95 HR-CTV D90 HR-CTV D85 IR-CTV D95 IR-CTV D90 IR-CTV D85

3.2 3.5 3.8 2.4 2.7 3.0 1.1 0 0 0 1.7 1.1 2.3 1.9 2.0 2.1 6.4 5.2

3.5 3.9 4.4 2.5 2.9 3.3 0.9 0 0 0

Na(R)/Na(L) BOS(R)/BOS(L) R

3.8/4.0 2.7/2.6 6.3

Organs-at-risk

Brainstem D2cc Spinal cord D2cc Pituitary D2cc

C P OC

2.5 0.84 0.59 0.54 0.52

Optic chiasm D2cc Retina Right D2cc Retina Left D2cc Clivus D2cc Clivus DMean Atlantoaxial joint D2cc Atlantoaxial joint DMean Soft palate D2cc Soft palate DMean

Re(R) Re(L)

1.6 0.9 2.4 1.9 2.0 2.1 12.0c 8.5

Pa(R) /Pa(L)

4.5/4.9

a Na – nasopharynx (intersection of the Pa-BOS line with the bony outline of the base of skull), P – pituitary (0.5cm from center of sella), OC – optic chiasm (1.5cm ventrally from P), Re – retina (1 cm posterior to the line drawn from the contralateral outer canthus and tragus), C – cord (posterior to R at the posterior border of corpus C1), R – node of Rouviére (ventral part of corpus C1), Pa – palate (junction of soft and hard palate), BOS – base of skull (intersection of the line drawn between the anterior clinoid process and point R and the line drawn from the contralateral outer canthus and tragus),(Levendag,1997). b Per fraction, equivalent dose in 2Gy using αβ ratio of 10 for the tumour, and αβ ratio of 3 for organs-at-risk. c Soft palate doses could be further decreased by incorporation of a lead shield onto the base of the Rotterdam applicator. The lead shield should be insulated to prevent leaching. Care should be taken during insertion to avoid injury to the uvula.

Three-dimensional planning Related to the CTV, dummy sources are loaded into the plastic tubes and their correct position verified with fluoroscopy. Orthogonal radiographs are taken to document the applicator placement and source position, if multiple treatments are intended for a single insertion. One- to three-millimeter CT scan cuts are carried out through the central plane of the sources. MR imaging at diagnosis or prior to external radiotherapy and/or after external radiotherapy may be co-registered in order to aid delineation of target volumes and organs at risk.

inferior meatus. The interstitial technique is used to complement intracavitary techniques to improve coverage of parapharyngeal involvement and entails insertion of sharp plastic needles. Three- dimensional treatment planning is then employed followed byHDR treatment delivery (Figure 17) [Ren 2014, Wan 2014].

9. TREATMENT PLANNING

Benavides Cancer Institute Approach Prescription, optimization and dosimetry

Two-dimensional planning Two-dimensional planning entails acquisition of orthogonal radiographs (anteroposterior and lateral) taken with the applicator and dummy wire markers in-situ, and wire markers taped onto the lateral canthi (Figure 18). The most commonly used system is the Levendag system which defines the following prescription and monitoring points outlined in Table 2. At the Ho Chi Minh Oncology Center, nasopharyngeal ICBT, either as boost in the primary setting or as exclusive salvage treatment for recurrences, is performed using a modified Rotterdam applicator, using 2D planning based on the Levendag system (Table 3).

Dose prescription is to the HR-CTV. Graphical optimization is performed with the following planning objectives HR-CTVD90% ≥100% of prescribed dose (PD)(high priority); IR-CTV D90% ≥75% PD (intermediate priority); and soft palate D2cc <120% PD or as low as possible (intermediate priority). Treatment planning is conducted using Oncentra Brachy version 4.5.3, 2018, Elekta AB, Stockholm, Sweden.