IMPROVISED BEAM MODIFICATION TECHNIQUES IN TELECOBALT - A Feasibility Study

A comparison between cobalt and linear accelerator-based IMRT treatment plans

Principal Investigator :

Dr. Lokesh Viswanath M.DProfessor & Head of Unit II, Dept of Radiation Oncology

Kidwai Memorial Institute of Oncology, Bangalore

BOG 24June2011 Presentation

Investigating team (Co-investigators) Staff, Dept of Radiation physics Dr. Bindu Venugopal Dr. Padma Dr. Lavanya

post-graduate student, MDRT.KMIO

2015 Developing countries - 10 million CA cases PA 2011 - India 289 radiation therapy centers

270 telecobalt machines 250 linear accelerators

INTRODUCTION

Imaging – 3D rendering Image fusion : CT/MR/PET/othersVirtual simulationTPS Hardware : MLC, Portal Imaging, real time

imaging Mechanics

……. have translated conventional LA into Advanced Teletherapy Machines capable of delivering sophisticated treatments like 3DCRT, IMRT, IGRT etc.

Recent advances in -

3DCRT/IMRT/IGRT 3 DCRT has evolved and established itself

as the Standard of Care In comparison to conventional 2D/ basic

3D field setups, these advanced technologies have demonstrated hitherto unexpected gains in terms of Dose escalation and better normal tissue sparing – which - translate into therapeutic gain

Telecobalt, though a 1970`s technology, continues to be used for Radiotherapy in many developing countries.

These machines are capable of set geometric field shapes and are provided with accessories like universal shielding bocks and wedges.

Most centers use 2D planning system or 3 D plans + conformal blocks

Plans to ensure homogenous dose distribution generally results in unintentional larger volume of normal tissue irradiation

2 D planning systems

2 D plans with unintentional greater normal tissue irradiation

Most often in our routine clinical practice: - we have patients who by virtue of their tumor

location need definitive treatment on LA with 3DCRT/IMRT techniques.

these poor and needy patients cannot afford Linear Accelerator treatment

The treating hospital has cobalt machine alone and due to existing economic situation the centre may not be able to refer the patient to centers with LA with 3D or IMRT facility

How can we help such poor patients ?

RATIONALE FOR THE STUDY This study was initiated to see if improvised

3D treatment plans can be generated for Telecobalt Machines without MLC using locally available material (universal shielding blocks) & other facilities.

To look at the feasibility of being able to deliver such special plans on a Telecobalt machines.

Subjects - absolute indication for IMRTProximity to normal structuresRe-irradiation etc

MATERIALS & METHODS

Mould room techniques Pt Positioning: Supine/Prone Immobilization :

Head rest & base plate thermoplastic material

CT simulation

flat couch Lasers Markers

Ant Rt & Lt Lat

Telecobalt Theratron 780C or E Without MLC Universal Shielding

Block & tray + Cerrobend Shielding

Blocks and Tray

Machine

CT TPS Available 3d or IMRT Computer Treatment

planning system like ECLIPSE with CO 60 beam parameters can be used

Centers which do not have planning system can transfer the patients simulation CT scan data to higher collaborating centers through CD or Internet and obtain the special plan for treatment execution at their respective centers

Virtual Simulation CT Scan Data: DICOM

format ECLIPSE TPS :Ver 8.9 Contouring : Normal

Structures and GTV/CTV/PTV

PTV = Target volume Co 60 Beam Parameters

TPS Planning ProcessOptimal beam positions were selected

taking into consideration the target volume & normal tissue to be spared.

Optimal field placements were chosen for Universal shielding block.

For oblique & lateral fields Customized Shielding tray for each field with cerrobends blocks with locking screws were planned selectively.

3D plans were generated.

Conformal Radiation Two broad subtypes :

Techniques aiming to employ geometric field shaping alone( 3D-CRT)

Techniques to modulate the intensity of fluence across the geometrically-shaped field (IMRT)

Geometrical Field shaping

Geometrical Field shaping with Intesity Modulation

Logical Manual optimal beam technique uses the concept of jaw based IMRT, where in

the resolution of optimization is low compared to MLC/Micro MLC

Since manual forward plan technique was used, a lower resolution of 4 or 9 pixel was adapted

BEV: Blocking of normal structure in the selected quadrant / pixel is adopted

Field in Field Concept was also used to created sub fields with blocked zones

Field Weighting & optimization Calculation was done using PBC ALGORITHM.

Improvisation of 3D plan

Gantry positions selected : to place the available universal shielding blocks

Each field: shielded zones transferred at tray position to OHP sheet for manual setup.

BEV : 4 Quadrant

BEV : 9 Quadrant

BEV : 9 Quadrant

Plan approval Process Target volume TV= PTV: isodose coverage

Cobalt Plans: ~ 90% IMRT : PTV – 95%

Field sizes noted Tracing of fields and shielded areas were done on

separate OHP sheets for each field If there was a chance of universal shielding blocks

sliding on the shielding tray, Slotted tray or perforated tray was used.

cerrobend shielding was prepared and anchored separately.

Case 1

Male 45 yrs multiple myeloma – frontal bone Current status - recurrence in the base

skull and nasal cavity Re-irradiation

Conventional 3 D Plan :Filed AP & 2 Lateral with shield

Improvised 3D Plan for Telecobalt

IMRT Plan: 95% isodose covering PTVCobalt Plan: 90% isodose covering TV

IMRT Plan: Good coverage in other sections

Cobalt Plan: Slight dipping of isodose lines near the lt eye due to fixed shielding block , can be improved with Cerrobent blocks

DVH : Cobalt plan has comparatively similar DVHG to IMRT with considerable normal issue low doses

Comparison of Mean Dose to PTV and adjacent Normal tissue

RT Technique used: Fields, Sub Fields and Shielded Zones: 8 Field

Field Positions: AP(2), RAO, LAO, Rt Lat (2), Lt Lat(2)

Plan Implementation and Treatment Delivery Verification

Setup Time

Exposure time 8 fields - 2.85 minutes Patient positioning, shield placement,

gantry rotation- 8-10 minutesTotal time- 10-12 minutes.

CASE 2

Chondrosarcoma – medial wall of the orbit Status Post Operative with + margin

CONCLUSION 3DCRT/IMRT type of treatment plans are

feasible on conventional telecobalt machine with meticulous planning.

It is possible to generate treatment plans similar to jaw based IMRT with a low resolution on telecobalt without multileaf collimator.

Further study with universal shielding blocks and conformal blocks with necessary QA checks are indicated to improve this technique.

Caution: This technique is not applicable for all patients.